Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives

Preface

Author and Copyright

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Copyright © 1994-2006
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Introduction

Compact discs and the digital audio revolution

Most of us take all of this for granted rarely giving any thought to the amazing interplay of precision optics and complex electronics - at least until something goes wrong. The purpose of this document is to provide enough background on CD technology and troubleshooting guidance so that anyone who is reasonably handy whether a homeowner, experimenter, hobbiest, tinkerer, or engineer, can identify and repair many problems with CD players and possibly laserdisc players, CDROM drives, and optical storage drives as well.

Even if you have trouble changing a light bulb and do not know which end of a soldering iron is the one to avoid, reading through this document will enable you to be more knowledgeable about your CD player. Then, if you decide to have it professionally repaired, you will have a better chance of recognizing incompetence or down right dishonesty when dealing with the service technician. For example, a bad laser is not the most likely cause of a player that fails to play discs - it is actually fairly far down on the list of typical faults. A dirty lens is most likely. There - you learned something already!

Scope of this document

The primary differences between these types will relate to how the disc is loaded - portables usually are top loaders without a loading drawer or tray:

However, as a result of the level of miniaturization required for portables and to a lesser extent, CDROM drives, everything is tiny and most or all of the electrical components are surface mounted on both sides of an often inaccessible printed circuit board with the entire unit assembled using screws with a mind of their own and a desire to be lost.

For other types:

• Laserdisc players and optical disk storage units have much in common with CD players with respect to the mechanical components and front-end electronics. Therefore, the information contained in this document can represent a starting point for their troubleshooting as well. However, they may include additional servo systems (optical pickup tilt, for example), as well as additional and/or different signal processing subsystems.

• DVD (Digital Versatile - or Video - Disc) players (which are now widely available and will completely replace CD players in a few years), suffer from many of the same problems as CDs and Laser Discs. Thus, a familiarity with the operating and repair of current technology will give you a head start on the amazing wonders (and similarly amazing problems) to come. There is a great deal of information on DVD technology in the DVD FAQ. Electronics Now, December, 1997, has a nice article by Steven J. Bigelow covering everything from the DVD format to installing and using a DVDROM drive in your PC.

• BD (Blu-Ray) players and recorders. Stay tuned for future developments. However, much of the information on CD and DVD technology applies to these next generation high capacity video and computer storage devices as well.

Note that throughout this document, the term 'CD player' is used most often. However, it should be understood that in most cases, the information applies to CDROM drives, game machines using CDs like the Sony Playstation, laserdisc players, MiniDisk players/recorders, DVD players, and other types of optical disk systems. Also see the document specifically devoted to these other technologies: "Notes on the Troubleshooting and Repair of Optical Disc Players and Optical Data Storage Drives". Also, where I remember, the term 'disc' is used to denote a read-only medium (e.g. a regular audio CD or LD) while 'disk' is used for one that is recordable (e.g., CD-R or MiniDisk).

Note: Links to all the diagrams and photographs referenced from this document can be found in Sam's CD FAQ Files.

Types of problems found in CD players

1. Mechanical - dirt, lubrication, wear, deteriorated rubber parts, dirty/bad limit switches, physical damage. A dirty lens (coated with dust, tobacco smoke residue, or condensed cooking grease) - easily remedied - is probably the number one cause of many common problems: discs not being recognized, seek failure, audible noise, and erratic tracking, sticking, or skipping.

   Even many professionals may mistake (either accidentally or on purpose) these symptoms being due to much more serious (and expensive) faults. Don't be fooled!

   Cleaning of the lens and any other accessible optical components (usually only the turning mirror, if that) and a mechanical inspection should be the first things done for any of these problems (and as periodic preventive maintenance especially if the equipment is used in a less than ideal environment). See the section: General inspection, cleaning, and lubrication.

2. Electrical Adjustments - coarse tracking, fine tracking, focus, laser power. However, some CD players no longer have some of these adjustments. The servo systems are totally digital - they either work or they don't.

3. Power problems (mostly portables) - weak batteries, inadequate, defective, or improper AC wall adapter.

4. Bad connections - broken solder on the pins of components that are stressed like limit or interlock switches, or audio or power jacks, internal connectors that need to be cleaned and reseated, broken traces on flexible cables, or circuit board damage due to a fall.

5. Electrical Component Failure. These are rare except for power surge (storm and lightning strike) related damage which if you are lucky will only blow out components in the power supply. (Or, plugging a 3 V portable into the 12 V of your automobile. You can probably forget about this even being a CD player again.)

6. Incompatible geographic location. :-) This doesn't really apply to CD players but may be a factor with equipment like Sony PlayStations and very likely with DVD players. In their infinite wisdom (or greed), manufacturers are including 'country codes' on the discs so that a game or movie sold in one place cannot be used in another. So, if you bought a disc on the other side of the world and it doesn't work at home, thank the lawyers.....

You can often repair a CD player which is faulty due to (1) or (2) except for laser power which I would not attempt except as a last resort without a service manual and/or proper instrumentation if needed - improper adjustment can ruin the laser. If discs are recognized at all or even if the unit only focuses correctly, then laser power is probably ok. While the laser diodes can and do fail, don't assume that every CD player problem is laser related. In fact, only a small percentage (probably under 10%) are due to a failure of the laser diode or its supporting circuitry. Mechanical problems such as dirt and lubrication are most common followed by the need for electrical (servo) adjustments.

The solutions to category (3) and (4) problems are obvious - but it may take a conscious effort to remember to check these out before assuming that the fault is due to something much more serious.

Category (5) failures in the power supply of component (AC line powered) CD players can also be repaired fairly easily.

Most other electrical failures will be difficult to locate without the service manual, test equipment, and a detailed understanding and familiarity with audio CD technology. However, you might get lucky. I have successfully repaired problems like a seek failure (replaced a driver chip because it was running excessively hot) and a door sensor failure (traced circuitry to locate a bad logic chip). Since so much of the intelligence of a CD player is in the firmware - the program code inside the microcontroller, even the schematic may be of only marginal value since I can pretty much guarantee that the firmware will not be documented. The service manuals rarely explain *how* the equipment is supposed to work - and then perhaps only in poorly translated Japanese!

You can pretty much forget about repairing electrical problems in portable equipment other than perhaps bad connections (usually around the audio or power jacks, internal connectors, interlock switch (since it is stressed), or elsewhere due to the unit being dropped). Nearly everything in a portable (and most CDROM drives for that matter though this is not quite as bad) is itty-bitty surface mount components. There is generally only minimal useful information printed on the circuit board. Tracing the wiring is a nightmare. Even the test points and adjustments may be unmarked!

Repair or replace?

Therefore, unless you really do need a 250 disc CD changer with a remote control that has more buttons than a B777 cockpit and 2000 track programmability, a 10 year old CD player will sound just as good and repair may not be a bad idea. Many older CD players are built more solidly than those of today. Even some new high-end CD players may be built around a mostly plastic optical deck and flimsy chassis.

If you need to send or take the CD player or CDROM drive to a service center, the repair could easily exceed the cost of a new unit. Service centers may charge up to $50 or more for providing an initial estimate of repair costs but this will usually be credited toward the total cost of the repair (of course, they may just jack this up to compensate for their bench time). Parts costs are often grossly inflated as well - possibly due to a deliberate effort on the part of manufacturers to discourage repair of older equipment. However, these expensive parts do not really fail nearly as often as is commonly believed - the laser is not the most likely component to be bad! Despite this, you may find that even an 'authorized' repair center will want to replace the expensive optical pickup even when this is not needed. I do not know how much of this is due to dishonesty and how much to incompetence.

If you can do the repairs yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Thus, it may make sense to repair that bedraggled old boombox after all.

CD Digital Audio Technology

General Introduction to CD Technology

CD-Rs - recordable CDs use a slightly different construction. CD-R blanks are prestamped with a spiral guide groove and then coated with an organic dye layer followed by a gold film, resin, and label. The dye layer appears greenish and deforms upon exposure to the focused writing laser beam to form pits and lands.

The newest variation - DVDs or Digital Versatile Disks (or Digital Video Disks depending on who you listen to) - implement a number of incremental but very significant improvements in technology which in total add up to a spectacular increase in information density - almost 10:1 for the same size disc. These include higher frequency laser (670 or shorter visible wavelength), closer track spacing, better encoding, and a double sided disc. According to early reports on the final specifications, DVDs will be able to store 8 times the audio of current CDs at a higher sampling rate and bit resolution, 2 hours of MPEG encoded high quality movies, and all kinds of other information. Raw data capacity is somewhere between 5 and 10 GBytes. See the section: Comparison of CD and DVD Specifications for additional information.

On the near horizon is the "Blu-Ray" standard using a blue-violet laser to achieve even higher capacity for multimedia and computer storage applications. See the section: Comparison of CD, DVD, BD Specifications

CD information storage and playback

That is followed by extremely sophisticated coding of the resulting 16-bit two's-complement samples (alternating between L and R channels) for the purpose of error detection and correction. Finally, the data is converted to a form suitable for the recording medium by Eight-to-Fourteen modulation (EFM) and then written on a master disk using a precision laser cutting lathe. A series of electroplating, stripping, and reproduction steps then produce multiple 'stampers', which are used to actually create the discs you put in your player (more below).

Of course, it is possible to create your own CDs with a modestly priced CD-R recorder (which does not allow erasing or re-recording). Now, re-writable CD technology with fully reusable discs enables recording and editing to be done more like that on a cassette tape

Like a phonograph record, the information is recorded in a continuous spiral. However, with a CD, this track (groove or row of pits - not to be confused with the selections on a music CD) starts near the center of the CD and spirals (counterclockwise when viewed from the label side) toward the outer edge. The readout is through the 1.2 mm polycarbonate disc substrate to he aluminized information layer just beneath the label. The total length of the spiral track for a 74 minute disc is over 5,000 meters - which is more than 3 miles in something like 20,000 revolutions of the disc!

The digital encoding for error detection and correction is called the Cross Interleave Reed-Solomon Code or CIRC. To describe this as simply as possible, the CIRC code consists of two parts: interleaving of data so that a dropout or damage will be spread over enough physical area (hopefully) to be reconstructed and a CRC (Cyclic Redundancy Check) like error correcting code. Taken together, these two techniques are capable of some remarkable error correction. The assumption here is that most errors will occur in bursts as a result of dust specs, scratches, imperfections such as pinholes in the aluminum coating, etc. For example, the codes are powerful enough to totally recover a burst error of greater than 4,000 consecutive bits - about 2.5 mm on the disc. With full error correction implemented (this is not always the case with every CD player), it is possible to put a piece of 2 mm tape radially on the disc or drill a 2 mm hole in the disc and have no audio degradation. Some test CDs have just this type of defect introduced deliberately.

Two approaches are taken with uncorrectable errors: interpolation and muting. If good samples surround bad ones, then linear or higher order interpolation may be used to reconstruct them. If too much data has been lost, the audio is smoothly muted for a fraction of a second. Depending on where these errors occur in relation to the musical context, even these drastic measures may be undetectable to the human ear.

Note that the error correction for CDROM formats is even more involved than for CD audio as any bit error is unacceptable. This is one of many reasons why it is generally impossible to convert an audio CD player into a CDROM drive. However, since nearly all CDROM drives are capable of playing music CDs, much can be determined about the nature of a problem by first testing a CDROM drive with a music CD.

CD (disc) construction

Each byte of the processed information is converted into a 14 bit run length limited code taken from a codebook (lookup table) such that there are no fewer than 2 or more than 10 consecutive 0s between 1s. By then making the 1s transitions from pit to land or land to pit, the minimum length of any feature on the disc is no less than 3*p and no more than 11*p where p is 0.278 um. This is called Eight-to-Fourteen Modulation - EFM. Thus the length of a pit ranges from 0.833 to 3.054 um.

Each 14 bit code word has 3 additional sync and low frequency suppression bits added for a total of 17 bits representing each 8 bit byte. Since a single bit is 0.278 um, a byte is then represented in a linear space of 4.72 um. EFM in conjunction with the sync bits assure that the average signal has no DC component and that there are enough edges to reliably reconstruct the clock for data readout. These words are combined into 588 bit frames. Each frame contains 24 bytes of audio data (6 samples of L+R at 16 bits) and 8 bits of information used to encode (across multiple frames) information like the time, track, index, etc:

                Sync                    (24 + 3).
                Control and display     (14 + 3).
                Data                    (12 * 2 * (14 + 3).)
                Error correction        ( 4 * 2 * (14 + 3).)
                                        --------------------
                                        588 total bits/frame

Information on a CD is recorded at a Constant Linear Velocity - CLV. This is both good and bad. For CD audio - 1X speed - this CLV is about 1.2 meters per second. (It really isn't quite constant due to non constant coding packing density and data buffering but varies between about 1.2 and 1.4 meters per second). CLV permits packing the maximum possible information on a disc since it is recorded at the highest density regardless of location. However, for high speed access, particularly for CDROM drives, it means there is a need to rapidly change the speed of rotation of the disc when seeking between inner and outer tracks. Of course, there is no inherent reason why for CDROMs, the speed could not be kept constant meaning that data transfer rate would be higher for the outer tracks than the inner ones. Modern CDROM drives with specs that sound too good to be true (and are) may run at constant angular speed achieving their claimed transfer rate only for data near the outer edge of the disc.

Note that unlike a turntable, the instantaneous speed of the spindle is not what determines the pitch of the audio signal. There is extensive buffering in RAM inside the player used both as a FIFO to smooth out data read off of the disc to ease the burden on the spindle servo as well as to provide temporary storage for intermediate results during decoding and error correction. Pitch (in the music sense) is determined by the data readout clock - a crystal oscillator usually which controls the D/A and LSI chipset timing. The only way to adjust pitch is to vary this clock. Some high-end players include a pitch adjustment. Since the precision of the playback of the any CD player is determined by a high quality quartz oscillator, wow and flutter - key measures of the quality of phonograph turntables - are so small as to be undetectable. Ultimately, the sampling frequency of 44.1 K samples per second determines the audio output. For this, the average bit rate from the disc is 4.321 M bits per second.

Tracks are spaced 1.6 micrometers apart - a track pitch of 1.6 um. (This is the nominal specification but may vary somewhat and will be less on those CDs that contain more than 74 minutes of music or 650 MB of data. However, unlike LPs, the pitch is not affected in the slightest by the content.) Thus a 12 cm disc has over 20,000 tracks for its 74 minutes of music. Of course, unlike a hard disk and like a phonograph record, it is really one spiral track over 3 miles long! However, as noted above, the starting point is near the center of the disc. The width of the pits on a track is actually about 0.5 um. The focused laser beam is less than 2 um at the pits. Compare this to an LP: A long long playing LP might have a bit over 72 minutes of music on two sides or 36 minutes per side. (Most do not achieve anywhere near this much music since the groove spacing needs to vary depending on how much bass content the music has and wide grooves occupy more space.) At 33-1/3 rpm, this is just over 1,200 grooves in about 4 inches compared to 20,000 tracks on a CD in a space of just over 1.25 inches! The readout stylus for an LP has a tip radius of perhaps 2 to 3 mils (50 to 75 um).

CD (disc) manufacturing

An LP is pressure pressed using a solid vinyl biscuit. A CD, on the other hand, is not manufactured in this manner. CDs are replicated through injection molding, where molten polycarbonate is injected into a mold under high pressure. CDs *must* be manufactured in strict clean room environments. On a side note, when LaserDiscs were released to market by MCA DiscoVision in 1978, this requirement wasn't recognized, or ignored by MCA Corporate in an attempt to keep manufacturing costs of these silver platters down. The first discs were manufactured in an environment similar to an LP plant. As a result, the finished product, while looking visibly okay when observed casually, had major problems playing reliably on many LaserDisc players. Now, of course, we know better, although Pioneer recognized these requirements far more quickly than MCA. Even RCA's Videodisc plant for their needle-in-grove CED (SelectaVision Videodisc) format recognized these requirements better than MCA! CED's market introduction in 1981 did not start as catastrophically like LaserDisc did as a result.

And you thought driving on a narrow winding country road was tough!

At a constant linear velocity of about 1.2 meters per second, the required tracking precision is astounding: Proper tracking of a CD is equivalent to driving down a 10 foot wide highway (assuming an acceptable tracking error of less than +/- 0.35 um) more than 3,200 miles for one second of play or over 14,400,000 miles for the entire disc without accidentally crossing lanes! Actually, it is worse than this: focus must be maintained all this time to better than 1 um as well (say, +/- 0.5 um). So, it is more like piloting a aircraft down a 10 foot wide flight path at an altitude of about 12 miles (4 mm typical focal length objective lens) with an altitude error of less than +/- 7 feet! All this while the target track below you is moving both horizontally (CD and spindle runout of 0.35 mm) 1 mile and vertically (disc warp and spindle wobble of up to 1 mm) 3 miles per revolution! In addition, you are trying to ignore various types of garbage (smudges, fingerprints, fibers, dust, etc.) below you which on this scale have mountain sized dimensions. Sorry for the mixed units. My apologies to the rest of the world where the proper units are used for everything).

The required precision is unbelievable but true using mass produced technology that dates to the late 1970s. And, consider that a properly functioning CD player is remarkably immune to small bumps and vibration - more so than an old style turntable. All based on the reflection of a fraction of a mW of invisible laser light!

Of course, this is just another day in the entertainment center for the CD player's servo systems. Better hope that our technological skills are never lost - a phonograph record can be played using the thorn from a rosebush using a potter's wheel for a turntable. Just a bit more technology is needed to read and interpret the contents of a CD!

And, for a DVD with its narrower longer track (0.74 um compared to 1.6 um and more than twice as long), it's even more of a challenge!

Why does focus need to be so precise?

Wrong.

First, laser light that remains precisely parallel - doesn't diverge - only can be found in bad Sci-Fi. Laser light still obeys the laws of physics and in order to get the required spot size on the disc - about 1 micrometer (um), 1,000th of a mm, 1,000,000th of a meter, it needs to be focused precisely at the disc surface. Due to manufacturing tolerances for disc flatness (warp), the surface may move up-and-down as much as 100 times this amount. And disc height from player to player isn't that precise either. Large diameter laser beams can be kept quite parallel but a beam 1 um in diameter would diverge at about a 60 degree angle. The lens in the CD player has a focal length of about 4 mm and focuses the light from a beam a couple millimeters in diameter to a 1 um spot on the disc surface and because of the small depth of focus, the distance needs to be kept constant to 1 or 2 um. For DVD systems, the required precision is even greater.

Laser printers DO have focusing optics with correction for the flat paper surface. They don't need to be quite as precise because the spot size is much larger than for a CD or DVD player - a 1,200 dpi printer would have a spot on the order of 50 um. Therefore, the lens can be quite far away from the paper and the depth of focus is much larger. Thus, no active focusing mechanism is needed.

CD optical pickup operating principles

This design is typical of older optical pickups (though you may come across some of these). Newer types have far fewer individual parts combining and eliminating certain components without sacrificing performance (which may even be better). Additional benefits result is lower cost, improved robustness, and increased reliability. However, operating principles are similar.

The purpose of the optical pickup in a CD player, CDROM drive, or optical disk drive, is to recover digital data from the encoded pits at the information layer of the optical medium. (With recordable optical disks, it is also used to write to the disk medium.) For CD players, the resulting datastream is converted into high fidelity sound. For CDROMs or other optical storage devices, it may be interpreted as program code, text, audio or video multimedia, color photographs, or other types of digital data.

Most of the basic operating principles are similar for single-beam CD pickups and for pickups used in other digital optical drives.

It is often stated that the laser beam in a CD player is like the stylus of a phonograph turntable. While this is a true statement, the actual magnitude of this achievement is usually overlooked. Consider that the phonograph stylus is electromechanical. Stylus positioning - analogous to tracking and focus in an optical pickup - is based on the stylus riding in the record's grooves controlled by the suspension of the pickup cartridge and tone arm. The analog audio is sensed most often by electromagnetic induction produced by the stylus's minute movements wiggling a magnet within a pair of sense coils.

The optical pickup must perform all of these functions without any mechanical assistance from the CD. It is guided only be a fraction of a mW of laser light and a few milligrams of silicon based electronic circuitry.

Furthermore, the precision involved is easily more than 2 orders of magnitude finer compared to a phonograph. Sophisticated servo systems maintain focus and tracking to within a fraction of a micrometer of optimal. (1 um is equal to 1/25,400 of an inch). Data is read out by detecting the difference in depth of pits and lands of 1/4 wavelength of laser light (about 0.15 um in the CD)!

• The laser beam is generated by a solid state laser diode emitting at 780 nm (near IR). Optical power from the laser diode is no more than a couple of mW and exits in a wedge shaped beam with a typical divergence of 10x30 degrees in the X and Y directions respectively.

  Note that despite what some people believe, the laser diode in a CD or DVD player is a true laser and not just a glorified LED. It has a gain medium (the semiconductor), mirrors (on the cleaved parallel ends of the crystal), and an means of excitation (electric current). Its nearly monochromatic single spatial mode (TEM00) beam can be focused to a spot less than 2 um in diameter. No LED or other non-laser light source is capable of this kind of performance.

• A diffraction grating splits the beam into a main beam and two (first order) side beams. (The higher order beams are not used). Note that the diffraction grating is used to generate multiple beams, not for its more common function of splitting up light into its constituent colors. The side beams are used for tracking and straddle the track which is being read. The tracking servo maintains this centering by keeping the amplitude of the two return beams equalized.)

• Next, the laser beam passes through a polarizing beam splitter (a type of prism or mirror which redirects the return beam to the photodiode array), a collimating lens, a quarter wave plate, a turning mirror, and the objective lens before finally reaching the disc.

• The collimating lens converts the diverging beam from the laser into a parallel beam.

• The quarter wave plate converts the linearly polarized beam into a circularly polarized beam. The purpose of this obscure step will become clear below.

• A turning mirror (optional depending on the specific optical path used) then reflects the laser light up to the objective lens and focus/tracking actuators.

• The objective lens is similar in many ways to a high quality microscope objective lens. It is mounted on a platform which provides for movement in two directions. The actuators operate similarly to the voice coils in loudspeakers. Fixed permanent magnets provide the magnetic fields which the coils act upon. The focus actuator moves the lens up and down. The tracking actuator moves the coil in and out with respect to the disc center.

• The collimated laser beams (including the 2 side beams) pass through the objective lens and are focused to diffraction limited spots on the information - pits - layer of the disc (after passing through the 1.2 millimeters of clear polycarbonate plastic which forms the bulk of the disc).

• The reflected beams retrace the original path back through the quarter wave plate and polarizing beam splitter. However, the reflected circularly polarized beam in passing through the quarter wave plate gets converted back into a linearly polarized beam but at an orientation 90 degrees to the original! So, the polarizing beam splitter is able to divert it to the photodiode array. To repeat in different words: The polarizing beam splitter passes the (horizontally polarized) laser beams straight through. However, two passes (out and back) through the quarter wave plate rotates the polarization of the return beam to be vertical instead and it is reflected by the polarizing beam splitter toward the photodiode array. Sorry, an explaination of why quarter wave plates and polarized laser beams behave this way is somewhat beyond the scope of this document.

The return beams from the disc's information layer are used for servo control of focus and tracking and for data recovery.

• A cylindrical lens slightly alters the horizontal and vertical focal distances of the resulting spot on the photodiode array. The spot will then be perfectly circular only when the lens is positioned correctly. To close or to far and it will be elliptical (e.g., elongated on the 45 degree axis if too close but on the 135 degree axis if too far).

  The central part of the photodiode array is divided into 4 equal quadrants labeled A,B,C,D. Focus is perfect when the signal = (A+C)-(B+D) = 0.

  The actual implementation may use a thick beam splitter mirror (which adds astigmatism) or an astigmatic objective lens rather than a separate cylindrical lens to reduce cost but the effect is the same. Since the objective lens is molded plastic, it costs no more to mold an astigmat (though grinding the original molds may have been a treat!). It is even possible that in some cases, the natural astigmatism of the laser diode itself plays a part in this process.

• The side beams created by the diffraction grating are positioned forward and back of the main beam straddling the track of pits being followed (not directly on either side as shown in the diagram - but that was easier to draw!).

  Segments on either side of the photodiode array designated E and F monitor the side beams. Tracking is perfect when the E and F signals are equal.

• The data signal is the sum of A+B+C+D.

In essence, the optical pickup is an electronically steered and stabilized microscope which is extracting information from tracks 1/20 the width of a human red blood cell while flying along at a linear velocity of 1.2 meters per second!

See the sections: "Parts of a CD Player or CDROM Drive" and "Startup Problems" for more information on the components and operation of the optical pickup and descriptions and photos of some typical laser diodes, optical pickups, and optical decks.

The Laser Fundamentals Page has an interactive tutorial (requires JAVA) illustrating the operation of an optical pickup in very simplified form. It doesn't really have much detail but if the explanation above makes no sense, it may be worth viewing.

Optical pickup complexity

• Generally, the most complex types are also the oldest. With these, there were individual optical elements for each stage in the beam path and completely separate laser diode and photodiode array packages. In short, while details varied, the overall construction was very similar to the diagram and description given in the section: CD optical pickup operating principles. These also had several optical adjustments - which in some cases needed frequent attention.

  An example of this type is the Sony KSS110C Optical Pickup. Most components perform individual functions and it is larger and heavier than more modern designs.

• The most common types still have a separate laser diode and photodiode array but may have eliminated the cylindrical and collimating lenses and perhaps the polarizer and quarter wave plate. There are few if any adjustments.

  The Sony KSS361A Optical Pickup is typical of these mainstream designs. With very minor variations (mostly in mounting), various models may be found in all types of CD players and CDROM drives manufactured by Sony, Aiwa, and others.

  Another similar design is used in the Sanyo K38N Optical Pickup which is somewhat newer and more compact.

  For a diagram and detailed description of these mainstream pickups, see the section: Sony KSS series optical pickups.

• Some manufacturers have gone to a combined laser diode/photodiode (LD/PD) array package which looks like a large LD but with 8 to 10 pins. Aside from the objective lens assembly, the only other part may be the turning mirror, and even this is really not needed. Such a pickup can be very light in weight (which is good for fast-access drives) and extremely compact.

  Eliminating the components needed to separate the outgoing and return beams should result in substantial improvement in optical performance. The only disadvantage would be that the beams are no longer perfectly perpendicular to the disc 'pits' surface and this may result in a very slight, probably negligible reduction in detected signal quality - more than made up for by the increased signal level.

  The CMKS-81X Optical Pickup and Optical Pickup from Philips PCA80SC CDROM are typical of these modern designs.

  The smallest ones such as the Optical Pickup from the Philips CR-206 CDROM are only about 1/2" x 5/8" x 3/4" overall - just about the size of the lens cover! For this single-beam pickup, there are absolutely NO additional optical elements inside. A three-beam pickup would have a diffraction grating in front of the laser diode.

  Some of these use what are known as "hologram lasers" (a designation perhaps coined by Sharp Corporation). With these, the functions previously performed by multiple optical components. can be done by a "Holographic Optical Element" or HOE. The HOE can simply be a diffraction grating replacement or can be designed to perform some more complex beam forming. A variety of hologram lasers (as well as conventional laser diodes and photodiode arrays) are listed under Sharp Laser Diode Products. The typical Sharp hologram laser (versions for CD, DVD, and other types of optical storage devices) eliminate the normal diffraction grating in the three-beam pickup as well as the polarizing beam splitter and associated components making for a very simple, compact, low cost unit. DVD Laser Holographic Optical Element shows the HOE glued to the front of a DVD laser diode assembly.

  For a diagram and detailed description of this type of pickup, see the section: Super simple optical pickups.

For more information on CD technology

Philips/Magnavox used to have a very nice on-line introduction to a variety of consumer electronics technologies. Although their site has disappeared - and even people who work for them have no clue - I have now recovered several of the articles including those on TVs, VCRs, camcorders, satellite reception, and connections. See the Introductory Consumer Electronics Technology Series.

Also check out:

• A Fundamental Introduction to the Compact Disc Player is a somewhat more theoretical discussion of compact disc audio technology with diagrams and even some equations. If it doesn't put you to sleep, you will find quite a bit of interesting information in this article. In either case, it may prove of value.

• The University of Washington has at least one course in consumer electronics which varies from year to year. Last year (2000) it was on CD technology but most of that info has disappeared. However, there is an on-line tutorials which appears to be an excellent resource providing easy to understand discussions with many diagrams:
  • Audio Compact Disc - An Introduction.

• The How Stuff Works Web site has some really nice introductory material (with graphics) on a variety of topics relating to technology in the modern world. Of relevance to this document are articles on CD technology, motors, remote controls, power adapters, etc.

• The Cimram International, Inc Web site has some information on CD and DVD technology, formats, mastering, and manufacturing.

The following sites have a variety of information on CD and DVD technology:

• CD Page
• MediaWorld

A site with CD-R specific information including some repair tips is:

• Andy McFadden's CD-R FAQ.

An extensive amount of information on other optical disc/k technologies with many useful links can be found at:

• Leopold's Audio/Video Page
• MiniDisc, DAT, CD-R, DVD-RAM, etc..
• The DVD Page.
• DVD Central at E/Town.
• DVD Demystified (Includes the DVD FAQ).

CD Player Placement, Preventive Maintenance, and CD Care

General CD player placement considerations

• Locate the CD player in a cool location. While the CD player is not a significant heat producer, keeping it cool will reduce wear and tear on the internal components and assure a long trouble free life.

• Don't locate CD players in dusty locations or areas of high (tobacco) smoke or cooking grease vapors. I cannot force you to quit smoking, but it is amazing how much disgusting difficult to remove brown grime is deposited on sensitive electronic equipment in short order from this habit.

• Make sure all audio connections are tight and secure to minimize intermittent or noisy sound.

• Finally, store CDs away from heat. The polycarbonate plastic used to mold CDs is quite sturdy but high temperatures will eventually take their toll. Return them to their jewel cases or other protective container when not being played.

Preventive maintenance

An occasional internal inspection and cleaning is not a bad idea but not nearly as important as for a VCR. Realistically, you are not going to do any of this anyway. So, sit back and enjoy the music but be aware of the types of symptoms that would be indications of the need for cleaning or other preventive or corrective maintenance - erratic loading, need to convince the CD player to cooperate and play a disc, audio noise, skipping, sticking, and taking longer than usual to recognize a disc or complete a search.

If you follow the instructions in the section: A HREF="#cdgicl">General inspection, cleaning, and lubrication, there is minimal risk to the CD player. However, don't go overboard. If any belts are in good condition (by appearance and stretch test), just clean them or leave them alone. Except for the Sony drawer loading mechanism, belts are rarely as much of a problem in CD players as in VCRs.

Of course, acute symptoms like refusal to play or open the door is a sign of the need for emergency treatment. This still may mean that a thorough cleaning is all that is needed.

CD lens cleaning discs

I generally don't consider CD lens cleaning discs to be of much value for preventive maintenance since they may just move the crud around. However, for pure non-greasy dust (no tobacco smoke and no cooking grease), they may not hurt and could even do a good enough job to put off a proper cleaning for a while longer.

However, it's also possible they will ruin the lens. Consider that the worst thing to do to a precision optical surface is to wipe it with a dry cloth as this is likely to scratch the surface as it rubs the dust over it. To the lens, a speck of dust is like a boulder. Once the lens is scratched, replacement of the entire optical pickup is the only remedy. And, since there are absolutely no sorts of standards for these things, it is possible for a really poorly designed cleaning disc to damage the lens even if the dust itself is non-abrasive. In addition, if the cleaning disc doesn't look like a CD to the optical pickup or disc-in sensor, the lens it may not even spin. So, the drawer closes, the drawer opens, and NOTHING has been accomplished! (But at least no damage will be done.)

As if this isn't enough, NEVER put one into a high-X CDROM (DVD player or DVDROM drive). The high speed rotation may cause the cleaning disc and/or player/drive to self destruct. And, don't try a cleaning disc on an automotive CD player that sucks in the disk - it will get stuck.

CD protection and handling

It is important that the label side be protected from major scratches which could penetrate to the information layer. Even with the sophisticated error correction used on the CD, damage to this layer, especially if it runs parallel to the tracks, can make the CD unusable.

The CD is read by focusing a laser beam through the bottom 1.2 mm of polycarbonate. As a result of the design of the optical system used in the pickup, at the bottom surface, the beam diameter is about 1 mm and thus small scratches appear out of focus and in many cases are ignored and do not cause problems.

At the information layer with the pits, the beam diameter has been reduced to under 2 um. Still, scratches running parallel to the tracks are more problematic and can cause the optical pickup to get stuck repeating a track, jumping forward or back a few seconds, or creating noise or other problems on readout. In severe cases, the CD may be unusable especially if the damage is in the directory area.

This is why the recommended procedure for cleaning a CD is to use soap and water (no harsh solvents which may damage the polycarbonate or resin overcoat) and clean in a radial direction (center to edge, NOT in the direction of the tracks as you would with an LP). While on the subject of CD care, CDs should always be returned to their original container for storage and not left out on the counter where they may be scratched. However, if there is a need to put one down for a moment, here are some considerations:

• The label side is probably to be preferred since minor scratches have no effect on performance as long as they do not penetrate to the storage layer below (in which case the CD is probably history). Protectors are available to prevent damage to the label side of the disc. Personally, I think this is taking care to an excessive level but, hey, if you use your CDs as frisbies, go for it!

• However, the opposite argument may apply as well: Slight damage to the readout-side will be ignored by the optical system or corrected for by the decoding process And, there are ways of dealing with scratches should they occur.

Thus, I won't offer a hard and fast rule other than to avoid leaving CDs out where the dog can get to them. :)

Never apply sticky labels to the readout-side of a CD or to the label-side unless they are specifically designed for this application. And, if a label was stuck on despite the warnings, don't attempt to remove it (or at least exercise the utmost care) as the lacquer layer and some of your valuable bits may come away with it. This is especially critical for CD-Rs (and maybe CD-RWs) which seem to be more fragile than normal CDs. I've seen samples of CD-Rs literally self destruct due to slight stress on the label side.

CD cleaning

Use a soft cloth, tissue, or paper towel moistened with water and mild detergent if needed. Wipe from center to edge - NOT in a circular motion as recommended for an LP. NEVER use any strong solvents. Even stubborn spots will eventually yield to your persistence. Washing under running water is fine as well.

Gently dry with a lint free cloth. Do not rub or use a dry cloth to clean as any dirt particles will result in scratches. Polycarbonate is tough but don't expect it to survive everything. Very fine scratches are not usually a problem, but why press your luck?

Should I really worry about cleaning my CDs?

Very severe errors - long bursts - will result in audible degradation including noise and/or muting of the sound. Even this may not always be detectable depending on musical context.

Shorter runs of errors will result in the player interpolating between what it thinks are good samples. This isn't perfect but will probably not be detected upon casual listening.

Errors within the correcting capability of the CIRC code will result in perfect reconstruction.

Not all players implement all possible error handling strategies.

Therefore, it is quite possible for CD cleaning to result in better sound. However, a CD that is obviously clean will not benefit and excessive cleaning or improper cleaning will introduce fine (or not so fine) scratches which can eventually cause problems.

Can a dirty CD or dirty lens damage my player?

The claim made at one major chain was that dirt or dust on the laser eye would cause heat build-up that would burn out the mechanism. This is different from a dirty disc. The cleaner he was pushing was a little brush attached to a CD that brushed off the lens as it played.

This is total rubbish. The power of a CD laser is less than 1 mW and is not concentrated at the lens. And, as noted elsewhere, those cleaning CDs with the little brush are next to useless on anything but the smallest amount of dry dust.

There are a lot of suckers out there. Save your money.

The worst that can happen is the CD will not play properly. There may be audible noise, it may fail to track properly, abort at random times, or not even be recognized. The electronics will not melt down.

It is just about impossible for a dirty CD to do any damage to the player. A dirty lens will only result in disc recognition or play problems similar to those caused by a dirty CD. The laser will not catch fire.

The only way damage could occur is if you loaded a cracked CD and the crack caught on the lens.

You do not need any fancy CD cleaners in any case - soap or mild detergent and water and a soft cloth are all that are required. If the CD looks clean, it probably will be fine. If there are serious smudges or fingerprints, then cleaning could make a significant difference in performance.

For further information, see the sections "CD cleaning" and "General inspection, cleaning, and lubrication".

Rental or library CD considerations

(From: Bart Wessel (wessel@home.nl).)

There seems to be a new risk in playing CDs or CD-ROMs borrowed from a public library.

New, because of the fact that (at least at our library) they have a small metallic strip attached to the top of the CD, apparently as a measure against theft. The strip can be activated/deactivated at the counter, just like the system in use in most department stores.

The risk comes from the fact that these strips can come off if you happen to have a CD-ROM player that plays at speeds higher than 40X. There is a warning on the box not to use plates over 40X but who reads the warnings!

Can a CD player damage CDs?

• If the lens moves too high while attempting to focus and the mechanical stop does not prevent it from hitting the disc, scratches can occur. On some players, the objective lens can easily go this high if focus is not found on the first pass. Note that in most cases, the lens will not suffer since it is protected by a raised ridge which is what actually scratches the disc.

• Mechanical misalignment of the spindle motor or plastic cabinet parts can result in the disc touching the bottom or top of the disc compartment and this can leave scratches. This could be the result of poor or cheap design, shoddy manufacturing, or damage from a fall or other abuse.

• If the control logic gets confused, it may allow you to eject a disc while it is still spinning and not fully supported by the spindle platter. A dirty disc that resulted in failure of the CLV servo to lock can result in a disc speed runaway condition with some players. If the drawer is then opened too soon, the disc will still be spinning because the controller has no way of knowing its present status and will not have provided enough reverse torque to stop the spindle motor - or too much and it will be spinning in reverse.

The likelihood of any of these is increased with dirty, smudged, warped, or previously damaged discs.

Minor scratches may not result in a serious problem and there are products to polish them - don't know how well they work. However, if these scratches can be proven to be a direct consequence of a defective player still under warranty, you should try to get some compensation from the manufacturer for any seriously damaged and now unplayable CDs.

The one thing that is extremely unlikely is that the laser beam itself is damaging the disc. Although this IS in principle possible IF the disc is stationary AND the laser is on and focussed properly, AND laser power were high enough, at most what would happen is that the information layer would have a microscopic hole blown in it (and this would be taken care of by the error correction processing). However, this really is extremely improbable in a normal CD player or CDROM drive with normal CDs, especially if the unit is working otherwise since the disc starts spinning as soon as focus is established. Forget it. Mechanical causes of damaged discs are about a zillion times more likely! :-)

Thus, there is absolutely no way for a software command to the CDROM drive to affect the contents of the disc in any way. The laser power is simply too low to affect the CD and there is no way to boost it, even for an instant. Anything you've heard to the contrary it total rubbish. However, a faulty CD-R or CD-R/W writer could indeed result in damage to CD-R and CD-R/W media from its higher power laser but that's another story.

Repairing a scratched CD

Now it sounds like a poor excuse for a 78 rpm record. What to do?

There seem to be about as many ways of fixing scratches on CDs as producing them in the first place. However, they fall into 3 classes of techniques:

1. Mild abrasives: plastic or furniture polish, silver polish, toothpaste. These will totally remove minor scratches. DO NOT use anything that contains solvents including petrolium distillates - if it smells like kerosene, it will likely attack the polycarbonate plastic of the CD rendering it useless. Brasso(tm), a popular product may fall into this category but I've also heard that it works fine without damaging the CD.

2. Fillers: turtle wax, car wax, furniture wax. Apply over the whole disc and buff out with a lint free cloth. Filling larger scratches should be fairly effective but the disc will be more prone to damage in the future due to the soft wax.

3. Motorized buffing wheel. Sort of a miniature version of what is used for polishing your car. :) See the details below.

4. Blowtorch. At least one person who claims to have worked for several years in used CD store swears by this technique. Supposedly, he uses a pencil-type pocket butane torch and with great dexterity fuses the surface layer of the readout side of the disc so that all of those scratches and unsightly blemishes-well-melt away. Obviously, there are dangers in using fire on plastic and this is likely a last resort. I would assume that you are rolling with hysterical laughter at this point. In any case, I would not take this approach too seriously :-).

For (1) and (2), as with cleaning a CD, when applying or rubbing any of these materials, wipe from the center to the outside edge. A CD player can generally track across scratches that are perpendicular to its path reasonable well, but not those that run the parallel to the tracks.

A mild abrasive will actually remove the scratch entirely if it is minor enough. This is probably more effective where the surface has been scuffed or abraded rather than deeply scratched.

Wax-like materials will fill in the space where the scratch is if the abrasive was not successful. Even deep scratches may succumb to this approach.

A combination of (1) and (2) may be most effective.

Exorbitantly priced versions of these materials are available specifically marketed for repair of CDs. However, the common abrasives and waxes should work about as well.

I cannot comment on the use of the blowtorch or how many years of practice is required to get you CD repair license with this technique. However, I am highly skeptical that this works at all and suspect that destruction of the CD is the most likely outcome - totally melting, warping, or cracking or shattering from the thermal stress. In other words, I don't recommend trying the Blowtorch approach unless you have a stack of AOL or MSN CDs to sacrifice and you have sufficient accident insurance!

Even some of other solutions may make the problem worse or destroy the CD entirely if not done correctly or if the wrong materials or technique is used. So, test any method on a CD you don't care about first.

An alternative to CD home repair are companies specializing in this service. A couple of these are: Aural Tech CD and CD Repairman. I do not have information as to their effectiveness or cost. However, if you have a very special irreplaceable CD that someone used as a skateboard, one of these may be worth considering.

(From: Shawn Stopper (shstop@prodigy.net).)

In the CD repair process, I use a 1/4 horse electric motor, cotton buff, 2 hose clamps, 2 washers, a screw, and brown tripoli rouge. The motor should be mounted to a surface for permanent use. The first hose clamp should be mounted about halfway back on the motor shaft. A shaft about 4 inches in length will be necessary for this application. after mounting the first hose clamp, apply a washer, the buff, another washer, and the final hose clamp. Mount a screw about 1/2 inch above the motor shaft where the outer clamp can be twisted around the screw to keep the buff spinning. When buffing cds, start out using brown tripoli rouge and slowly move the cd from inside to out. Do not apply too much pressure on the CD because this will cause the CD to "splinter", and it will be ruined. Patience is the key to CD buffing. The first few you do may take longer than you expect, but the more you do the better you get at it. At this time, I can buff about 3 to 4 CDs in five minutes. Once again, practice is the key!

Repairing top-side problems on CDs

What if the aluminum (or gold) reflective layer has come off with no damage to the plastic underneath? First of all, I don't know how this could occur unless you were attempting to clean them with a strong solvent. Any physical damage which removed the mirror coating will also damage the pits and recoating will be useless.

(Note that I have unintentionally removed the gold coating on a CD-R using a solvent similar to what is in Liquid Wrench(tm). I was actually trying to remove the label but went a little too far! The solvent apparently dissolved the greenish coating or binding underneath allowing the gold film and label to just flake off - very strange behavior. Most of the green layer was still intact. I now have a nice greenish somewhat transparent plastic coaster.)

Some discs may still work on some players or drives without the aluminum coating. However, this isn't that likely. How to replace it? Ideally, vacuum deposition is needed. The problem isn't only the reflectance but the micro structure - the original coating was vacuum deposited to conform to the pits and lands of the information layer. It is perfectly uniform below the resolution of the laser beam. Modeling (silver or gold colored) paint is amorphous and rough at these feature sizes and floppy disk write protect stickers or other adhesive backed reflective films don't even come close to contacting the information layer consistently. Mirror paint may work but is a long-shot.

Air purifiers and rubber parts

The type of air cleaner that's most likely to be a problem are probably those that supposedly emit negative ions and/or ozone (activated oxygen, O3). These are typically more expensive and some models show up on eBay and in up-scale catalogs.

See, for example, Electronic Design, September 29, 2005, under "Bob's Mailbox" (Javascript must be enabled.)

CD Player and CDROM Drive Troubleshooting

SAFETY

• Electrical: There may be a few exposed electrically live parts from the power line, usually around the power cord entrance, power transformer, and on/off switch. If there are, tape them over or cover them somehow so you need not be concerned with a low tech shock! Unless you are troubleshooting a primary side power supply problem, there will be no need to go near the AC line.

• Laser (CD): The laser in a CD player is infra red, near IR - 780 nm - border of visible range but for all intents and purposes invisible. However, it is very low power (generally under 1 mW at the lens) and due to the optics, extremely unlikely that you could be in any danger. Nonetheless, don't go out of your way to look closely into the lens while the unit is on!

  As long as the lens is intact, the beam is highly divergent and at anything beyond a few inches, especially at an oblique angle, is quite safe. The only possibility of risk would be if the lens fell out and you were looking directly into a collimated beam from above. While the power is less than that of most laser pointers, there would be no aversion reflex to the nearly invisible IR. And, yes, some models of CD players are known to drop their lenses!

  CAUTION: There is usually a very low intensity (in appearance) emission from an IR laser which appears deep red. It will be visible as a spot the size of the period at the end of this sentence when the lens is viewed from an oblique angle. For a photo of how it actually looks in the dark, see Appearance of IR (780 nm) Laser in CD Pickup (Original photos courtesy of Gag Helfront (ie.dunster@ukonline.co.u).) This is just your eye's response to the near IR energy of the main beam. (Some people apparently cannot see this at all.) Do not be mislead into thinking that the laser is weak as a result of how dim this is. The main beam is up to 10,000 times more intense than it appears! It's power output is generally around 1 mW - comparable to a laser pointer. Take care. However, the red dot is an indication that the laser is being powered and probably functional, though it is no guarantee of the latter. You really need a laser power meter or at least an IR detector to confirm the existence of an IR laser beam.

  Whenever a full size (5-1/4") CD is in place, there is absolutely no danger of exposure to the laser beam. Reflections of laser light at these power levels are harmless. However, if you are testing with a 3-1/2" 'single' or homemade cut-down test CD (see the section: Useful ways to mangle CDs, avoid staring into the lens if there is any chance the laser is powered.

  If you don't want to take even the minimal risk of looking into the lens at all, project the beam onto a piece of paper held close to the lens. In a dark room, it should be possible to detect a red spot on the paper when the laser is powered.

• Laser (CD-R, CD-RW, and other writeable optical drives): These may use IR laser diodes producing 10s of mW. A typical CD-R drive sets the laser power at 3 to 5 mW for read and 25 to 30 mW for write. While the basic precautions are the same, much greater care must be taken due to the much higher power when writing.

• Laser (DVD): The DVD laser pickup is very similar to that of the CD player except that the wavelength is in the 635 to 650 nm range which is a very visible red - about the same as the color of newer laser pointers or a helium-neon laser (632.6 nm). The power is still under 1 mW and everything else above is still applicable.

  One note: If the DVD player is of the dual pickup variety with a separate laser for CDs, that one is IR like a normal CD player and the precautions listed above will apply. Take care because it may not be obvious ahead of time which one (or if both) will be powered!

Troubleshooting tips

When attempting to diagnose problems with a CDROM drive, start by trying to get it to play an audio CD. Data readback is more critical since the error correction needs to be perfect. However, with audio playback functional, all of the optical pickup and most of the servo systems and front-end electronics must be working. A CDROM drive which cannot even play a music CD will have no chance of loading Windows 95.

If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous and mostly non-productive (or possibly destructive).

Whenever working on precision equipment, make copious notes and diagrams. You will be eternally grateful when the time comes to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not always. Apparently identical screws may be of differing lengths or have slightly different thread types. Little parts may fit in more than one place or orientation. Etc. Etc.

Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly.

Select a work area which is well lighted and where dropped parts can be located - not on a deep pile shag rug. Something like a large plastic tray with a slight lip may come in handy as it prevents small parts from rolling off of the work table. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage.

Another consideration is ESD - Electro-Static Discharge. The electronic components - especially the laser diode - in CD players, CDROM drives, and similar devices, are vulnerable to ESD. There is no need to go overboard but do take reasonable precautions like not wearing clothing made of wool that tends to generate static. When working on component CD and laserdisc players, get into the habit of touching a ground like the metal chassis before touching any circuit components. The use of an antistatic wrist strap would be further insurance especially if the optical pickup assembly needs to be unplugged for any reason.

A basic set of precision hand tools will be all you need to disassemble a CD player and perform most adjustments. However, these do not need to be expensive. Needed tools include a selection of Philips and straight blade screwdrivers, needlenose pliers, wire cutters, tweezers, and dental picks. A jeweler's screwdriver set is a must particularly if you are working on a portable CD player or CDROM drive.

For making servo adjustments, non-metallic fine tip jeweler's screwdrivers or alignment tools will be essential as some of the front-end circuitry may be sensitive to body capacitance - contact with the slot may alter the behavior of the player (for better or for worse). In a pinch, wrapping electrical tape around the part of a normal jeweler's that you grasp will probably provide enough isolation. However, with a tool with a blade made out of an insulator, you will be less likely to accidentally short things out as well

Note that low level signals from the optical pickup like the data (RF) and other photodiode outputs are extremely sensitive to interference picked up from a finger on or near the flex cable, a disconnected ground strap, or possibly even a nearby broadcast antenna. Thus, when the optical deck isn't fully mounted and connected, there may be unusual behavior - this is probably normal. Just be aware of this and don't panic, and adjustments should be made with the unit as close to fully assembled as possible.

You should not need any CD specific tools except in the unlikely event you get into optical alignment in which case the service manual will detail what tools and special rigs are needed.

A low power fine tip soldering iron and fine rosin core solder will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components.

CAUTION: You can easily turn a simple repair (e.g., bad solder connections) into an expensive mess if you use inappropriate soldering equipment and/or lack the soldering skills to go along with it. If in doubt, find someone else to do the soldering or at least practice, practice, practice, soldering and desoldering on a junk circuit board first! See the document: Troubleshooting and Repair of Consumer Electronic Equipment for additional info on soldering and rework techniques.

For thermal or warmup problems, a can of 'cold spray' or 'circuit chiller' (they are the same) and a heat gun or blow dryer come in handy to identify components whose characteristics may be drifting with temperature. Using the extension tube of the spray can or making a cardboard nozzle for the heat gun can provide very precise control of which components you are affecting.

Basic cleaning supplies include Q-tips (you may know them as cotton buds), lint free cloths or paper towels, water, and isopropyl alcohol (preferably 91 percent medicinal grade or better). Note that isopropyl alcohol also goes by the names isopropanol, 2-propanol, and propan-2-ol. They are all the same thing.

For info on useful chemicals, adhesives, and lubricants, see Troubleshooting and Repair of Consumer Electronic Equipment as well as other documents available at this site.

Test equipment

A DMM or VOM is necessary for checking of power supply voltages and testing of sensors, LEDs, switches, and other small components. This does not need to be expensive but since you will be depending on its readings, reliability is important. Even a relatively inexpensive DMM from Radio Shack will be fine for most repair work.

For servo and other electronic problems, an oscilloscope will be useful. However, it does not need to be fancy. A 10 to 20 MHz dual trace scope with a set of 10X probes will be more than adequate for all but the most esoteric troubleshooting of CD players and CDROM drives.

To determine if the laser diode is working properly, a laser power meter is very useful. Such a device is expensive but is often essential to properly and safely adjust laser power on many CD players and CDROM drives. However, for many problems, simply knowing that an IR laser beam is being emitted is enough. For this, the simple device described in the section: IR detector circuit is more than adequate. Alternatively, an inexpensive IR detector card or even some camcorders can perform the same function.

A stereo amplifier and loudspeakers is essential to allow your most important piece of audio test equipment to function effectively - your ears. A lot can be determined by listening to the audio output to distinguish among dirt, lubrication, servo, control, and other mechanical or electronic problems. I would caution against the use of headphones as a sudden burst of noise could blow your eardrums and spoil your entire day.

For testing of optical pickups, some additional equipment will be needed. However, this will be detailed in the section: Testing of Optical Pickup Assemblies.

Test CDs

Keep those old demo CDs or even obsolete CDROM discs - they can be used for testing purposes. Where an optical deck has a servo problem, the disc will end up spinning out of control. Stopping this suddenly may result is the CD scraping itself against the drawer or or base of the deck and getting scratched. Therefore, some 'garbage' discs are always handy for testing purposes.

To evaluate tracking and error correction performance, any CD can be turned into a test CD with multiple width strips of black tape, a felt tip marker, or even a hand drill! In fact, some professional test discs are made in exactly this manner.

Also see the sections: "Comments on test discs" and "Custom test CDs using CD-Rs".

Useful ways to mangle CDs

• For portable CD players where the designers in their infinite wisdom put some of the servo adjustments *under* the spinning disc, a 3-1/2" CD 'single' is extremely handy. A normal CD can be cut down as well - to whatever size you need as long as enough actual tracks are left so that the directory and a few minutes of music/data remain - this could be as little as about 2-1/2" to gain access to the adjustments on some models. This surgery is best done on a band saw with a narrow fine tooth blade. However, tiny cracks may grow in from the edge (overnight, even) if the disc is subjected to any heating or stress from cutting or smoothing. Perhaps some annealing is needed to prevent these from getting started.

  Note that the lower mass (actually the lower moment of inertia for you purists) of the small CDs may alter the servo response somewhat. Putting a heavy metal ring or washer on top should help. However, this is still much much better than continually having to remove a normal CD to get at the adjustments, incrementally moving them one way or another, and then replacing the CD to see how you made out. One can grow old doing this! The little CDs will enable you to monitor the test points as the adjustments are made which is also a definite advantage :-).

  The RCA RP-7903A Portable CD Player is an example of a design where this type of modified CD is invaluable for testing.

• A handy special miniature CD can be made to permit viewing of the focusing action on any CD player or CDROM drive as long as you can get to the top of the deck while testing. Using a band saw, cut a garbage disc down so as to leave only a 1-1/2" diameter center hub with a 1/2" by 1/2" tab sticking out from it. This can then be positioned by hand to just cover the lens while it is supposed to be doing its focus search.

• An alternative that will permit you to view both the laser output (from a safe distance) and the focusing action is to create a window in a garbage CD by removing the label and aluminum layers from an area of the CD at the inner tracks - at least a square inch worth. Lacquer thinner (nail polish remover, with adequate ventilation) will probably work to remove the label. Fine sand paper or steel wool will remove the aluminum and information pits/lands (grooves). Then polish with a buffing wheel or old rag.

CAUTION: when using any of these cut-down or windowed test CDs, or 3-1/2" 'singles', avoid staring into the lens when the laser is powered. See the section: SAFETY.

Getting inside a CD player or CDROM drive

A CD player still under warranty should probably be returned for service for any covered problems except those with the most obvious and easy solutions.

On the other hand, it is possible that you will do a better job than some repair shops. You will probably have a better understanding of the basic theory and will certainly be able to spend much more time on the problem. And, of course, hobbiest/handyman's time is cheap - as in free.

• Component CD players. It is generally very easy to remove the top cover on most CD players. There are usually some very obvious screws on the sides and possibly back as well. These are nearly always Philips head type - use the proper screwdriver. Once all the screws are out, the top cover will lift up or slide back and then come off easily. If it still does not want to budge, recheck for screws you may have missed.

  Once the top cover is removed, the optical deck and electronics board will usually be readily accessible.

  In rare cases, removing the bottom cover will provide access to the solder side of the electronics board. However, with most CD players, the bottom is solid sheet metal and the entire board would need to be unmounted. On some, the electronics board is mounted upside-down so there is full access to the wiring side once the cover is removed.

• With most single play designs, the entire optical deck can be lifted out after removing 3 or 4 screws. One screw may have a grounding contact under it. Replace this in exactly the same position. There may be fragile flexible cables. Be careful so as not to damage any. Usually, these cables plug in to connectors on the electronics board and permit the entire optical deck to be easily replaced if needed (not very common, however, despite what you may have heard).

• For changers, details will depend on the particular model but in general, it is more likely that removal of the entire changer mechanism will be more involved. However, this is usually not needed unless there is an actual mechanical problem with it. With Pioneer cartridge changers, for example, the optical deck is easily removed with just 4 screws.

• For portables, the bottom plate or top cover usually comes off after removing several very tiny screws - use the proper size Philips blade jeweler's screwdriver and don't lose them. Then, you either have access to the bottom of the mainboard or the top of the mainboard blocked mostly by the optical deck. With the RCA RP-7903A Portable CD Player, it is the latter and the pickup and/or normal size CD conveniently block all access to servo adjustments and test points (which as is often the case, are ummarked in this RCA unit). These types of CD players are usually quite a pain to troubleshoot! Of course, there are also many components including most of the large multilegged ICs surface mounted on the *bottom* side of the mainboard which makes for even more fun should probing be required! You can easily see all the 'stuff' packed into a box just slightly larger than a CD!

• For CDROM drives, both top and bottom covers may be removable depending on model. These are more wide open than portables, especially the newer models where everything has been shrunk to a tiny optical pickup and circuit board with a few large ICs. Unfortunately, adjustments (if any) and test points are even less likely to be labeled on CDROM drives. All testing will also require a working PC unless your model has built-in audio play capability.

Make notes of screw location and type and immediately store the screws away in a pill bottle, film canister, or ice cube tray.

When reassembling the equipment make sure to route cables and other wiring such that they will not get pinched or snagged and possibly broken, or have their insulation nicked or pierced, and that they will not get caught in moving parts. Replace any cable ties that were cut or removed during disassembly and add additional ones of your own if needed. Some electrical tape may sometimes come in handy to provide insulation insurance as well. (This applies mostly to portables and CDROM drives - component CD players are very wide open.

CD enhancers

For more amusement, see the section: Totally worthless gadgets for CD enthusiasts.

Along the same lines, some apparently knowledgeable people (knowledgeable in what you might ask!) have asked if offers of software to turn a CDROM drive into a CD-R writer should be believed! This is just utter and total nonsense and what's more likely to happen if you fall for such a SCAM is to become the new owner of some nasty computer virus! Besides, this must be impossible since there is no place for a red "write" LED on a CDROM drive! :)

What about DVD player and DVDROM drive servicing?

In any case, eventually all things break, and DVD equipment will be no exception. Fortunately for us, the similarities between CD and DVD technology are much more significant than the differences. The inside of a DVD player looks pretty much the same as the inside of a CD player and, for the most part, the same problems are likely to occur. Here are some things to look out for:

• DVD discs look virtually identical to CDs. They are the same size and thickness with the same large center hole and will fit perfectly well into a CD player or CDROM drive - but of course won't do much there. Despite the tracks and pits being closer together, the rainbow/diffraction effects are about the same so a casual glance isn't enough to tell them apart but there is always that DVD logo! However, the information layer is precisely in the center of a sandwich of polycarbonate (each half 0.6 mm thick). (The DVD standard supports a double sided DVD. This, of course leaves virtually no room for the label!) More info on DVD media, storage capacity, and so forth can be found in the section: Comparison of CD and DVD Specifications.

• The laser in a DVD based device is a very visible red (a wavelength around 650 nm compared to the nearly invisible IR 780 nm laser used for CDs). This is about the same wavelength as used in newer red laser pointers. It is still low power (1 mW or less at the objective lens) but the laser will appear very bright when it is powered and working correctly. As far as hazards to vision, the same recommendations apply - don't look into the beam directly but viewing from an oblique angle at 12 inches or more from the objective lens should be safe. The eye's aversion reflex will prevent damage in any case - you will blink or turn away from a the bright light. CAUTION: Some (probably older) DVD equipment may also have a standard CD pickup to be able to read CDs. For these, obviously, all the IR laser precautions apply.

• It should be easier to determine if the laser is bad by brightness alone though a laser power meter and the manufacturer specifications will still be needed in marginal cases.

• As a result of the march of technology, the optical pickup is likely to of simpler design than that in older CD based devices with even fewer or no adjustments possible. However, there could be additional complexity due to the need to handle DVDs and CDs in the same equipment. The use of combined laser diodes and photodetector arrays is likely to be quite common if not pervasive. However, pickups may also use separate laser diodes and photodiode arrays possibly sharing some of the optics or at least part of the beam path in the optical block. In some pickups, objective lenses for the CD and DVD lasers can be rotated into position as required by applying enough current of the appropriate polarity to the fine tracking coils..

• The same basic functions need to be performed by the front-end electronics including amplification of the photodetector array output, and focus, tracking, and spindle servos. Much this is likely to be done inside large chips with no service adjustments.

• More functions will be incorporated into fewer surface mount chips. Fortunately, failures of the large scale integrated circuits themselves are not nearly as common as simple mechanical problems. Better hope so in any case as troubleshooting of things like an MPEG decoder is way beyond what could be done without a complete service manual, sophisticated test equipment, and probably a whole lot more!

• More plastic and less metal is likely to be used - lighter, flimsier, less likely to be serviceable at all.

So, the bad news is that if something breaks inside a large chip, accept defeat and send the unit in for service. The good news is that most problems will still be mechanical - dirt, dust, gummed up grease, bad motors, abuse. From our experience with CD repair, we should be well equipped to deal with these!

Hopefully, manufacturers have learned from their experience with CDs to make a more reliable robust product but that may be wishful thinking where the bottom line is involved. It's still too early to tell.

CD Player and CDROM Drive Fundamentals

Parts of a CD player or CDROM drive

Power supply

Usually, at least three voltages are needed: logic power (e.g. +5 Vcc) and a pair of voltages for the analog circuitry (e.g., +/- 15V). However, some designs use a variety of voltages for various portions of the analog (mainly) circuitry.

Electronics board

The optical deck

• Loading drawer - Most portable and many lower cost CD players or CDROM drives lack this convenience. Most are motor driven. However, some must be pushed in or pulled out by hand.

  Common problems: loose or oily belt causing drawer to not open or close, or to not complete its close cycle. There can be mechanical damage such as worn/fractured gears or broken parts. The drawer switch may be dirty causing the drawer to decide on its own to close. The motor may be shorted, have shorted or open windings, or have a dry or worn bearing.

• Spindle, spindle table, or spindle platter, (we will use these names more or less interchangeably) - When the disc is loaded, it rests on this platform which is machined to automatically center it and minimize runout and wobble.

  Common problems: Dirt on table surface, bent spindle, dry or worn bearings if spindle not part of motor but is belt driven, loose spindle.

• Spindle motor - The motor that spins the disc. Most often the spindle platform is a press fit onto the spindle motor. Two types are common: The first is a miniature DC motor (using brushes) very similar to the common motors in toys and other battery operated devices. The second type is a brushless DC motor using Hall effect devices for commutation. If there are more than 2 wires attached to the motor or if it uses exposed coils and control board, it is likely of the brushless type. In very rare cases, a belt is used to couple the motor to the spindle but most are direct drive - the spindle is the motor shaft.

  Common problems: partially shorted motor, shorted or open winding, dry/worn bearings, defective electronics. The brushless type are much less likely to have electrical problems.

• Clamper - Usually a magnet on the opposite side of the disc from the spindle motor which prevents slippage between the disc and the spindle platform. The clamper is lifted off of the disc when the lid or drawer is opened. Alternatively, the spindle may be lowered to free the disc.

  Common problems: doesn't engage fully permitting disc to slip on spindle due to mechanical problem in drawer closing mechanism.

• Sled - The mechanism on which the optical pickup is mounted. The sled provides the means by which the optical pickup can be moved across the disc during normal play or to locate a specific track or piece of data. The sled is supported on guide rails and is moved by either a worm or ball gear, a rack and pinion gear, linear motor, or rotary positioner similar to what is in a modern hard disk drive - in increasing order of performance.

  Note that a single-beam optical pickup can be used with either a linear or rotary mechanism. However, a three-beam pickup will not work with a rotary positioner because the angle of the pickup changes with radial position. Functionally, neither type is fundamentally superior but most manufacturers seem to use the three-beam type. Philips/Magnavox (and their other brand names) appear to be the principal exceptions.

  Common problems: dirt, gummed up or lack of lubrication, damaged gears.

• Pickup/sled motor - The entire pickup moves on the sled during normal play or for rapid access to musical selections or CDROM data. The motor is either a conventional miniature permanent magnet DC motor with belt or gear with worm, ball, or rack and pinion mechanism, or a direct drive linear motor or rotary positioner with no gears or belts.

  Common problems: partially shorted motor, shorted or open winding, dry or worn bearings.

• Optical pickup - This unit is the 'stylus' that reads the optical information encoded on the disc. It includes the laser diode, associated optics, focus and tracking actuators, and photodiode array. The optical pickup is mounted on the sled and connects to the servo and readback electronics using flexible printed wiring cables.

  Common problems: hairline cracks in conductors of flexible cable causing intermittent behavior.

Typical optical decks

Note: The resolution of the optical deck photos is 37.5 dpi unless otherwise noted. All other photos include a scale indicator.

The first 4 are from consumer grade CD players:

• The Pioneer CD Player Optical Deck shows a typical sled-type using a PM motor driven screw. This uses a three beam pickup.

  This model (or one similar to it) can be found in both Pioneer single (e.g., PD5100) and changer (e.g., PDM500) type CD players. In the latter case, the assembly is mounted upside-down with the clamper on the bottom.

• The Sony D-2 CD Player Optical Deck shows another common sled-type with a gear driven rack. This model (and as far as I know, all others from Sony) use three-beam pickups.

  This deck (or one similar to it) can be found in the Sony Model D2 and other portable CD players. (The flex cable, a common failure item, has been removed to provide unobstructed views.)

  It uses the Sony KSS220A optical pickup which is virtually identical to the Sony KSS361A Optical Pickup.

• The Sony D-14 CD Player Optical Deck is also uses a gear driven rack. It has a three-beam pickup.

  This deck is from a very old D-14 portable CD player, possibly only the second portable model manufactured by Sony.

  The Sony KSS110C Optical Pickup it uses is distinctly different than other more modern Sony models. In addition to being larger, the optics include a beam splitter prism, a negative lens in the return path, and the objective lens is mounted on a shaft enabling it to slide up and down (for focus), and rotate (for tracking).

• The Philips CD Player Optical Deck provides an example of a unit using a rotary type voice coil tracking actuator and uses a single-beam pickup.

  This one came from a front loading (flip down see-through door) Magnavox Model AH197M37 Modular Stereo System (includes dual cassette, AM/FM radio, and turntable).

  CD players and some CDROM drives manufactured by Philips (this includes the Magnavox and Sylvania brand names) seem to be the only ones still using rotary actuator technology in consumer products. In older versions, parts of the optical pickup (like the laser diode) were pluggable and easily replaced.

The three below are from CDROM drives:

• The Sony CDU-31/33A CDROM Optical Deck is typical of the mechanism found in lower performance models that use a screw drive for sled positioning. The pickup used is a three-beam KSS360A which is very nearly identical to the Sony KSS361A Optical Pickup (only the shape of the mounting bracket differs). Like its consumer CD player counterpart, everything is glued in place at the time of manufacture - there are no adjustments.

  The CDU-31A 1X, CDU-33A 2X, and other CDROM drives using this deck were probably the most popular models in the early 1990s. The CDU-31/33A used the Sony proprietary interface (also available on some sound cards) and were certainly nothing to write home about in the speed department. These drives used a high quality brushless DC motor for the spindle while other similar performance CDROM drives of the era had cheap permanent magnet DC motors that were prone to failure. However, they were the only popular front loading CDROM drives to NOT have the convenience of a motorized drawer mechanism - just a solenoid release. Of course, there was less to break down!

• The Sony CDU-8001 CDROM Optical Deck provides an example of a unit using a direct drive linear motor for the coarse tracking actuator. The pickup is a three-beam Sony KSS180A - quite similar to the Sony KSS361A Optical Pickup but appears to be more solidly constructed with at least one additional optical element that may be a collimating lens. Unlike most consumer grade pickups, the KSS180A is not totally glued together and some adjustment of optical alignment is possible.

  This deck came from a Sony CDU-8001 CDROM Drive Unit - a speedy 1X drive (aren't you impressed?) used with a SCSI interface for an Apple MacIntosh computer. The NEC Model CDR-82 CDROM Reader and others of the same vintage also use the same Sony KSS180A pickup.

  These were of the cartridge loading type (loading mechanism removed). The spindle motor is a high quality DC brushless type.

  Some component CD players by Technics (Matsushita) and others (in addition to Sony) also used linear motor technology as early as 1983 (possibly even before) to provide fast (under 1/2 second) music seek times which is better performance than some of the early CDROM drives using screw or gear type actuators.

• The Philips CR-206 CDROM Optical Deck views provide an example of a drive using a rotary actuator for both coarse and fine tracking. This uses a single-beam pickup where the laser diode and photodiode are apparently combined into one package which is mounted in a very simple compact optical assembly.

  This deck came from an inexpensive Philips CR-206 2X CDROM drive (vintage 1994). Note how much smaller this assembly is compared to the Philips CD player optical deck, above, which dates from around 1990.

Interestingly, most common popular higher performance CDROM drives (e.g., 4X, 12X, even 16X or more) do not use linear motors or rotary positioners to achieve rapid seek times. They use a screw or gear drive powered by a cheap permanent magnet DC motor! However, they do all use high quality brushless DC motors for the spindle since these high-X drives put a lot of stress on this component (especially those which are the true CLV type and vary speed based on track location). Although the optical pickups themselves have been simplified and have reduced mass, and the drive mechanism had been speeded up compared to the typical cheap portable CD player, this type of implementation is still far from optimal. Therefore, while the transfer rate may be pretty good (see the section: CDROM drive speed - where will it end? for why this really isn't assured even with a 32X unit), seek times may be mediocre - 250 ms full stroke being typical.

The next two are nearly complete CDROM drives of this type:

• The Philips PCA80SC CDROM Drive Optical Deck is a relatively modern design typical of low cost high spin-rate units. This one is an 8X model. The Optical Pickup from Philips PCA80SC CDROM appears to be a three-beam type.

  Apparently, many manufacturers used this basic mechanism. I have an Aztech CDA-268-01A CDROM drive (2X) which has the same pickup and a very similar optical deck.

• The Teac CD-532S CDROM Drive is another popular design used in late model (1998) low cost high spin-rate units. This one is a 32X (Max) model with a SCSI interface. The 32X (Max) rating really means that it spins at constant speed roughly equivalent to a 13X rate and the 32X spec is only achieved for data located near the outer edge of the disc.

  The Sony KSS575B three-beam pickup used in this drive is quite compact but of the more complex design using a separate laser diode and photodiode array with beam splitter. The optical path is equivalent to that of that of the Sony KSS361A Optical Pickup. (See the section: Sony KSS series optical pickups.) The guts are located in a central box-like object about 1.5 cm on a side. However, the pickup is mostly made of plastic - gone are the days of the cast metal optical block! While this does make it weigh less, the difference would hardly seem to be significant for access speed given the primitive screw drive.

  The Sanyo K38N Optical Pickup used in the earlier (like all of 3 months!) Teac model, the 16X CD516s, is substantially similar to this but of more solid construction. Teac CDROM drives from 6X (and possibly below) through this 32X unit appear virtually identical mechanically.

  Also notice how little electronics there is in this unit - nearly all the circuitry is on the single small circuit board on the left side of the bottom view. On all the other CDROM drives, the logic board occupied all the space (and more in some models) above or below the optical deck!

Finally, here are photos of DVDROM drives: <