While CD players and CDROM drives started out and still have much in common, they are diverging. The optical pickups remain similar but the data processing and servo systems needed to support 16X speed CDROM technology are much more sophisticated than those needed for 1X speed CD audio. Therefore, should you peak inside your shiny new CDROM drive, you may see parts that differ considerably from those in a old Discman.
In component stereos units, there are normally linear supplies and thus very reliable but easy to repair as well. In portables, they are likely to be switching supplies, possibly sealed in a shielded can (or at least all surface mount components), and difficult to troubleshoot and repair. 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.
This contains the microcomputer controller, servos, readback electronics, audio D/A(s) and filters. Most servo adjustment pots will be located here. In many cases they are clearly marked but not always. DO NOT turn anything unless you are sure of what you are doing - and then only after merking their original positions precisely.
This subsystem includes all of the components to load and spin the disc, the optical pickup, and its positioning mechanism. Refer to the section: "Typical optical decks" for photos of some common models. * 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 provide 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 principle 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.
Some examples of common optical decks are shown in the following 3 sets of photos. Note: The disc loading components and clampers are not shown. Note: The resolution of the optical deck photos is 37.5 dpi. 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!
All the parts described below are in the optical pickup. As noted, the optical pickup is usually a self contained and replaceable subassembly. The actual complement and arrangement of parts depends on the specific pickup design - a number of popular variations on the basic arrangement are used. Thus, should you actually end up dismantling a dead optical pickup, it will probably not match this description exactly. While the relatively old Sony KSS110C Optical Pickup has most of the same components as described below, the very common newer Sony and Sanyo optical pickups combine multiple functions into fewer elements. Typical examples are found in the Sony KSS361A Optical Pickup and Sanyo K38N Optical Pickup. The even simpler CMKS-81X Optical Pickup and Optical Pickup from Philips PCA80SC CDROM combine the laser diode and photodiode array into single package and eliminate all of the other optical components except for the diffraction grating and turning mirror (and the latter could be eliminated where space permits below the deck). The resulting designs are much cheaper to manufacture, more robust and reliable, and should have better performance as well since there are fewer intermediate optical components to degrade the beam. Also see the section: "CD optical pickup operating principles". Despite its being a precision optomechanical device, optical pickups are remarkably robust in terms of susceptibility to mechanical damage. * Laser Diode - This is Infra Red (IR) emitting usually at 780 nm - near IR, just outside the visible range of 400-700 nm. The power output is no more than a few milliwatts though this gets reduced to .25-1.2 mW at the output of the objective lens. A photodiode inside the laser diode case monitors optical power directly and is used in a feedback loop to maintain laser output at a constant and extremely stable value. The photos below show some of the types of laser diodes you may encounter in CD players, CDROM drives, laser printers, and bar code scanners: - A Variety of Small Laser Diodes (CD, laser printer, bar code scanner) - Closeup of Typical Laser Diode (from a laser printer) - Closeup of Laser Diode from the Sony KSS361A Optical Pickup (seen 'actual size' in the upper left corner of the group photo, above.) On an increasing number of pickups, the laser diode and photodiode array are combined into a single package. These are recognizable by their 8 or 10 lead package. See the section: "Optical pickup complexity". Common problems: bad laser diode or sensing photodiode resulting in reduction or loss of laser output. * Photodiode array - This is the sensor which is used to read back data and control beams. These are usually integrated into a single chip with a clear plastic cover. On an increasing number of pickups, the laser diode and photodiode array are combined into a single package. These are recognizable by their 8 or 10 lead package. See the section: "Optical pickup complexity". The photodiode array for a three-beam pickup has 6 segments - 4 in the center (A,B,C,D) and 1 on either side (E,F). Only the center segments are used in a single-beam pickup. However, there are some CD players and CDROM drives are fitted with complete three-beam pickups, but don't take advantage of the side beams - the E and F segments of the photodiode array are simply grounded! So, the blurb for these models may say "Featuring three-beam pickup" when only a single-beam is used! Isn't marketing wonderful? :-). Common problems: bad photodiode(s) resulting in improper or absence of focus and weak or missing RF signal. A missing bias voltage to the photodiode array would also result in lack of output. * Collimating lens - This converts the wedge shaped beam of the laser diode into nearly parallel rays. Not present in many (newer) designs. * Diffraction grating - In a 'three-beam pickup', this generates two additional lower power (first order) beams, one on each side of the main beam which are used for tracking feedback. It is absent in a 'single-beam pickup'. * Cylindrical lens - In conjunction with the collimating lens, this provides the mechanism for accurate dynamic focusing by changing the shape of the return beam based on focal distance. Modern pickups may actually combine this function into an astigmatic objective lens and/or take advantage of the natural astigmatism of the laser diode itself. * Beam splitter - Passes the laser output to the objective lens and disc and directs the return beam to the photodiode array. There will be no beam splitter (and related optics) if the laser diode and photodiode are combined in a single package. * Turning mirror - Redirects the optical beams from the horizontal of the optical system to vertical to strike the disc. Where space permits under the pickup, there is no need for a turning mirror as everything can be vertically oriented. Common problems: dirty mirror. Unfortunately, this may be difficult to access for cleaning. Note: the turning mirror is probably not silvered but is coated to reflect IR so do not be surprised if you can see through it. The previous five items are the major components of the fixed optics. Outside of damage caused by a serious fall, there is little to go bad. Better hope so in any case - it is usually very difficult to access the fixed optics components and there is no easy way to realign them anyhow. Fortunately, except for the turning mirror, it is unlikely that they would ever need cleaning. Usually, even the turning mirror is fairly well protected and remains clean. Depending on the design of the pickup, many of the components of the optical system listed above may be missing or combined into a single unit. In fact, the most modern pickup designs combine the laser diode and photodiode into a single package with 8 to 10 leads. With this approach, there is no need for a beam splitter or related optical components as the outgoing and return beams take nearly the same path. The overall manufacturing process is simplified, performance is improved, the cost is reduced, and reliability and robustness are enhanced. See the section: "Optical pickup complexity". The following items are associated with focusing the laser beam down to a microscopic point and maintaining it precisely on the CD's tracks: * Objective lens - High quality focusing lens, very similar to a good microscope objective with a numerical aperture (N.A.) of .45 and focal length of 4 mm. (Should you care, the N.A. is defined as the sine of the angle from the optical axis to the edge of the objective, as seen by the object. An N.A. of .45 implies a very fast high quality lens.) If you examine CD player objective lenses closely, you will also note that they are aspheric - the surface is not shaped like the surface of a sphere (as is the case with most of the small lenses you are likely to encounter) but its radius of curvature changes from center to edge (it is somewhat pointed). Because the light source (laser diode) is coherent and monochromatic, a low cost single element plastic molded lens with an antireflection coating (the blue tinge in the central area) can produce a diffraction limited spot (less than 2 um in diameter) at the disc information (pits) layer. An expensive multielement lens system would be required if the light source were not coherent and monochromatic. Of course, the entire technology would not be practical in this case! There is usually a ridge around its periphery to prevent the polished surface from being scratched should the assembly accidentally contact the spinning disc. Note: Some objective lenses (e.g., Philips/Magnavox) have a perfectly flat front surface. This would appear to be more susceptible to damage but perhaps a mechanical stop prevents contact even at the extreme upper limit. The lens is suspended to permit movement in two directions: up and down (focus) and toward and away from the spindle (tracking). Common problems: dirty lens, dirt in lens mechanism, scratched lens, damage from improper cleaning or excessive mechanical shock. * Focus actuator - Since focus must be accurate to 1 micron - 1 um, a focus servo is used. The actuator is actually a coil of wire in a permanent magnetic field like the voice coil in a loudspeaker. The focus actuator can move the objective lens up and down - closer or farther from the disc based on focus information taken from the photodiode array. Common problems: broken coil, damaged suspension (caused by mechanical shock or improper cleaning techniques). * Tracking actuator - Like focus, tracking must be accurate to 1 um or better. A similar voice coil actuator moves the objective lens from side-to-side (relative to the tracks - toward or away from the spindle) based on tracking feedback information taken from the photodiode array. Note: On pickups with rotary positioners, there may be no separate tracking coil as its function is subsumed by the positioner servo. The frequency response of the overall tracking servo system is high enough that the separate fine tracking actuator is not needed. These are also always of the single-beam type since the angle of the pickup changes with radial position and three-beam tracking control cannot be used. Common problems: broken coil, damaged suspension (caused by mechanical shock or improper cleaning techniques).
While there are a semi-infinite number of distinct things that can go wrong with a CD player, any set of symptoms can be classified as a hard failure or a soft failure: 1. Hard failure - door opening/closing problems, disc is not recognized, no sound, unit totally dead. 2. Soft failure - skips, continuous or repetitive audio noise, search or track seek problems, random behavior. Both of these types of problems are common with CD players and CDROM drives. The causes in both cases are often very simple, easy to locate, and quick and inexpensive to repair.
While it is tempting to blame the most expensive component in a CD player or CDROM drive - the laser - for every problem, this is usually uncalled for. Here is a short list of common causes for a variety of tracking and audio or data readout symtoms: * Dirty optics - lens, prism, or turning mirror. * Drawer loading belts - worn, oily, flabby, or tired. * Sticky mechanism - dirt, dried up/lack of lubrication, dog hair, sand, etc. * Broken (plastic) parts - gear teeth, brackets, or mountings. * Need for electronic servo adjustments - focus, tracking, or PLL. * Intermittent limit or interlock switches - worn or dirty. * Bad connections - solder joints, connectors, or cracked flex cable traces. * Motors - electrical (shorted, dead spot) or mechanical (dry/worn bearings). * Laser - dead or weak laser diode or laser drive (power) problems. * Photodiode array - bad, weak, or shorted segments or no power. * Bad/heat sensistive electronic components. * Bad or missing optical pickup shield ground. The following two areas cover the most common types of problems you are likely to encounter. For any situation where operation is intermittent or audio output is noisy, skips, or gets stuck, or if some discs play and others have noise or are not even recognized consistently, consider these FIRST: * Dirty lens - especially if your house is particularly dusty, the player is located in a greasy location like a kitchen, or there are heavy smokers around. Cleaning the lens is relatively easy and may have a dramatic effect on player performance. * Mechanical problems - dirt, dried up lubrication, damaged parts. These may cause erratic problems or total failure. The first part of a CD may play but then get stuck at about the time location. If your CD player has a 'transport lock' screw, check that it is in the 'operate' position before breaking out the heavy test equipment!Go to [Next] segment
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