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


  16.32) Impress your friends with the power of your CD player

The laser output of the typical CD player optics is less than 1 mW but since
the beam is focussed to a diffraction limited spot of less than 2 um the
resulting power density is truly impressive:

(Portions of the following from: James Carter (jcarter@photon-sys.com)).

Intensity is related to power by the 'area' of the beam.  For a Gaussian laser
(as most semiconductor lasers are), the 'area' of the beam is related to the
area of the intensity contour (usually an ellipse for these guys) representing
1/e^2 or approx. 13.5% of peak intensity (at the centroid).

Thus the peak intensity occurs at the centroid and equals

              2 * Po
    Io  =  ------------
           pi * Wx * Wy

Wx and Wy are the beam semi-diameters for the 1/e^2 contour.

The beam size at the facet of a semiconductor laser can be as small as 1.5 by
3.5 microns.  The high power density at the facet represents the cause for most
common failure modes in laser diodes.  For a 5 mW laser diode, the resulting
power sensity on this facet can be in excess of 600 MW (that is mega-watts)
per square meter!  Sounds impressive, doesn't it?

At the CD, the spot is even smaller which for the same power would result in
even higher densities.  However, this is more than offset by the fact that
a significant fraction of the original power is lost in the optics so the the
power density might be only - 300 MW per square meter.  I still would not
recommend hanging out at the focal point!

Note that while these numbers are impressive, conduction and other losses
generally prevent any actual damage from occurring to most common materials.
However, in a CD-R recorder using a laser diode with a power output of similar
magnitude, the temperature rise at the disc even while spinning at 4X or
greater speed is sufficient to blast holes in the intermediate (green)
information layer.  Watch out!

Chapter 17) Exploration and Tinkering

  17.1) Interesting CD player signals

Poking around inside a working CD player makes an excellent exercise for
the student.  Component CD players very often have clearly marked test
points for RF, focus, tracking, and audio data.  With care, there is
little risk of damaging anything as long as you are not tempted to try
your hand at tweaking any of the internal adjustments.

If you have nothing better to do and you have your CD player open, try
to locate the test points for data, fine tracking, and focus.  They may be
labeled something like TP.DTA (or TP.RF), TP.FO, TP.TR.

TP.DTA or TP.RF is the data coming off of the disc having gone through only the
photodiode segment combiner and preamp (probably).  Using a 10:1 probe
set the scope for a horizontal sweep of around .5 us/div.  Try a vertical
sensitivity of .2 V per division to start and adjust for a full screen
display.  Use internal positive triggering.  While playing a disc, you should
see the classic 'eye' pattern used in the communication world to characterize
channel quality.

  17.2) The CD player 'eye' pattern

The 'eye pattern' depicted below results from the characteristics of
the run length limited 8-14 modulation coding used on the CD where
there are no fewer than 3 and no more than 11 clock cycles per symbol.
You should be able to make out the fact that the minimum distance between
channel bits is 3 with the smallest distance between bit transitions of
about 3*232 ns.  The readout clock is 1/(232 ns) or about 4.321 MHz.

A 'good' eye pattern will be clean, symmetric, and stable with clear
visibility in the cross hatched areas.  Its amplitude is typically in the
.75 to 2 V range p-p when measured at the RF test point.  This waveform may
be viewed using an oscilloscope of at least 5 MHz bandwidth.

Some typical RF amplitude specifications:

* Aiwa: 1.3 to 1.4 V p-p.
* Sony full size: 1.2 V p-p, auto and portable: 0.85 V p-p.

This diagram shows the general form of the eye pattern present while playing
a musical track or reading data from a CDROM.

        /      \   \   \   \   \/  \/  \/  \/  \/  \/  \/  \/
       /        \   \   \   \  /\  /\  /\  /\  /\  /\  /\  /\
      /          \   \   \   \/  \/  \/  \/  \/  \/  \/  \/  \/
                  \   \   \  /\  /\  /\  /\  /\  /\  /\  /\  /\
                   \   \   \/  \/  \/  \/  \/  \/  \/  \/  \/

                         |<---- 1 us ---->| (approximately)

Examination of the eye pattern would be the first measurement that would be
performed to determine the condition of the CD player optics and electronics.
A good eye pattern eliminates most of the parts of the optical pickup from

Note that the eye pattern observed while the player is accessing the following
areas of the disc may not be well formed as in the diagram above:

    * Disc directory (Table of Contents or TOC).
    * Before the start of the first track (Track 1, time less than -0:01).
    * Between tracks of distinct selections (where there is silence).
    * After the end of the last track.

This is because there is no musical data at these locations on the disc (but
probably a constant value like 0) and the TOC and/or time display is obtained
from the Q bit.  The Q bit is part of the Control and Display byte that is
present once per frame (14 EFM coded bits out of 588 total bits per frame).
See the section: "CD (disc) construction".  This funny looking eye pattern
has much more low frequency content and thus does not exhibit the nice cross
hatched area as will be present with the highly variable audio data.

selections (tracks) will look strange.  This is because the digital info
for the TOC is obtained from the Q bit (I think) which is present once
per frame (588 bits on the disc).  The audio data can be anything.  Same
goes for inter-track data.  I don't know if this will be consistent on
all discs.

  17.3) Focus and tracking drive or error signals

TP.FO or TP.FE is the focus voice coil error signal.  Monitoring this with a
disc in good condition will show what looks like noise - the more or less
random fluctuations in actuator current necessary to maintain proper focus
within +/- .5 um of the disc surface.  On a warped disc you will see the DC
level of this signal varying at the disc rotation rate.  On a damaged disc,
you will see higher frequency variations in the level depending on what
kind of defects are present.  Gently tapping the optical deck should
evoke a visible effect on this signal as well as the servos correct
for your mischief.

TP.TR or TP.TE is the fine tracking voice coil error signal.  As with TP.FE,
this will show a noise waveform with a good disc.  On a disc with runout, you
will see a periodic level variation at the spindle rotation frequency.
Note how the DC value of this signal gradually changes as the voice coil
actuator maintains lock on the track while the track spirals outward.

Eventually, this error becomes great enough to trigger the coarse tracking
motor to jog the pickup a fraction of a mm and recenter it on the track at
which point the signal you are watching will suddenly shift its DC level.

On a disc with scratches, there will be higher frequency deviations which will
be readily visible on a scope trace.  Gently tap the optical deck from various
points and observe the effects on this signal.

For both focus and tracking, you can actually hear the voice coil actuators as
they compensate for minute defects or just the normal data pattern.  This is
the 'gritty' sound one hears from the CD audio or CDROM transport when
it is operating correctly and is an indication that the laser and focus
(at least) are most likely functioning properly.  If you listen carefully,
you can actually hear various defects by the effect they have on this
gritty sound but there will be no corresponding effect in the audio outputs
as there would be with an LP.

  17.4) Focus, tracking, and error correction performance

If you have a test CD (or use your regular CD), put your scope on one
of audio outputs.  Put some thin pieces of tape or mark with a (water
soluble) felt tipped pen radially on the bottom surface of the disc
to create some 'defects'.  Play some tracks which have constant pure
tones or silence.  For widths less than the error correcting capability
of your CD's LSI chipset, there should be no detectable signal degradation.
See what happens as you increase the width of your 'defects'.  Put your
finger on the spindle or even gently touch the disc as it is rotating.
Note that unless you really press hard, the disc will continue to play
normally without any change in pitch.  This is due to the servo control
and extensive buffering of the data - unlike an LP turntable where the
instantaneous speed is what determines pitch.

Other experiments are left as exercises for the student.

  17.5) IR detector circuit

This IR Detector may be used for testing of IR remote controls, CD player
laser diodes, and other low level near IR emitters.

Component values are not critical. Purchase photodiode sensitive to near
IR - 750-900 um or salvage from optocoupler or photosensor.  Dead computer
mice, not the furry kind, usually contain IR sensitive photodiodes. For
convenience, use a 9V battery for power.  Even a weak one will work fine.
Construct so that LED does not illuminate the photodiode!

The detected signal may be monitored across the transistor with an

 Vcc (+9 V) >-------+---------+
                    |         |
                    |         \
                    /         /  R3
                    \ R1      \  500
                    / 3.3K    /
                    \       __|__
                    |       _\_/_  LED1 Visible LED
                  __|__       |
        IR ---->  _/_\_ PD1   +--------> Scope monitor point
          Sensor    |         |
        Photodiode  |     B |/ C
                    +-------|    Q1 2N3904
                    |       |\ E
                    \         | 
                    / R2      +--------> GND
                    \ 27K     |
                    /         |
                    |         |
       GND >--------+---------+

  17.6) Laser diode fundamentals

Note: This is a summary.  For additional information on using laser diodes,
see the document: "Laser diodes and Helium Neon Lasers".

Typical CD laser optics put out about .1-1 mW at the objective lens though
the diodes themselves may be capable of up to 4 or 5 mW depending on type.
The laser diodes for CD players are infra red - IR - usually at around 780 nm.
Visible laser diodes are also readily available from many sources.  The most
common wavelength is 670 nm which is deep red but 630 nm diodes are also
available - red orange and appear much brighter (and more expensive at the
present time).  Inexpensive (well relatively) laser pointers use visible
laser diodes with power outputs up to about 5 mW.  This is enough power to risk
permanent retinal damage if you look into the beam especially when well
collimated as is required for a pointer.  Don't.

Typical currents are in the 30-100 mA range at 1.7-2.5 V.  However, the power
curve is extremely non-linear.  There is a lasing threshold below which
there will be no output.  For a diode rated at a threshold of 80 mA, the
maximum operating current may be as low as 85 mA.  This is one reason why all
actual applications of laser diodes include optical sensing (there is a built
in photodiode in the same case as the laser emitter) to regulate beam power.
You can easily destroy a laser diode by exceeding the safe current even for an
instant.  It is critical to the life of the laser diode that under no
circumstances do you exceed the safe current limit even for a microsecond!

Laser diodes are also extremely sensitive to electrostatic discharge, so use
appropriate precautions.  Also, do not try to test them with a VOM which
could on the low ohms scale exceed their safe current rating.

While only a few hundred mW at most is dissipated by the laser diode, a good
heat sink is also important for long life and stability.  The optical pickup
is usually a metal casting partially for this reason.  Remember that the
active diode chip is only about .1 mm on a side.  However, some optical blocks
are now made of plastic so this must not be as important as in the past.

It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed, you can easily exceed the ratings.

One approach that works for testing is to use a 0-10 VDC supply (preferably
a linear supply - a switching supply may put out laser diode destroying
pulses) with say, a 100 ohm resistor in series with the diode.  Slowly bring
the current up until you get a beam. Use an IR detector for this!  If you get
the polarity backwards or are actually measuring across the internal
photodiode, the voltage across the diode will go above 3 volts or will be
less than 1 V.  Then, turn power off and reverse the leads.  Note: some
laser diodes will be destroyed by reverse voltage greater than 3 V - a spec
sheet will list the reverse voltage rating.  The ones I have tried out of
CD players were fine to at least 5 V in the reverse direction.

Without a laser power meter, however, you will have no way of knowing when
the limit on safe beam power (safe for the laser diode, that is) is reached.
If you have the data sheet for your laser diode, then the best you can do
is limit the current to specified maximum rating.  Also, there is usually
a weakly visible emission which appears red (for IR laser diodes) present when
powered.  Do not be fooled into thinking that the laser diode is weak as 
a result of this dim red light.  The main beam is IR and invisible - and
up to 10,000 times more intense than it appears.

The beam from the raw laser diode is emitted in a broad wedge typically
10 x 30 degrees.  A convex lens is needed to collimate the beam (make
it parallel).  For optimal results, this needs to be anamorphic - unequal
horizontal and vertical focal lengths - to correct the astigmatism of
the beam.  The mass produced optical pickups used in CD players include
this as well as other sophisticated optics.

For an actual application, you should use the optical feedback to regulate
beam power.  This usually takes the form of a simple current controlled
power supply with extensive capacitive filtering and a regulated reference.
It is possible to modulate the beam power by tapping into the feedback
circuits - as long as you guarantee that the maximum current specification
will never be exceeded.  Laser diodes do not behave like LEDs and cannot
be pulsed for higher peak power - they turn into DEDs - Dark Emitting Diodes.

Single chips are available from a number of manufacturers for driving laser
diodes in both CW and modulated modes.

For additional information, see the document: "Laser Diode and HeNe Laser

  17.7) Laser diode life

For all intents and purposes, laser diodes in properly designed circuits do
not degrade significantly during use or when powered on or off.  However, it
doesn't take much to blow them (see the section: "Laser diode fundamentals").
I have seen CD players go more than 10,000 hours with no noticeable change
in performance.  This doesn't necessarily mean that the laser diode itself
isn't gradually degrading in some way - just that the automatic power control
is still able to compensate fully.  However, this is a lower bound on possible
laser diode life span.

Laser diodes that fail prematurely were either defective to begin with or,
their driver circuitry was inadequate, or they experience some 'event'
resultling in momentary (> a few nanoseconds) overcurrent.

As noted elsewhere, a weak laser diode is well down on the list of likely
causes for CD player problems.

Of course, in the grand scheme of things, even LEDs gradually lose brightness
with use.

CW Laser Light (reverse engineered from commercial unit).

This circuit was traced from a commercial CW laser light.  Errors
may have been made in the transcription.  The type and specifications
for the laser diode assembly (LD and PD) are unknown.  The available
output power is unknown but the circuit should be suitable for the
typical 3-5 mW visible or IR laser diode (assuming the same polarity of
LD and PD or with suitable modifications for different polarity units.)

If you do build this or any other circuit for driving a laser diode,
I suggest testing it first with an LED and discrete photodiode to
verify current limited operation.  Them with the laser diode in place,
start with a low voltage supply rather than 9V until you have determined
optimal settings and work up gradually.  Laser diodes are very unforgiving.

Note the heavy capacitive filtering.  Changes would be needed to enable
this circuit to be modulated at any reasonable rate.

+9 >------|>|-------+------------+-----------------+-----+--------+
         1N4001     |            |                 |     |        |
         Reverse    |            |      Pwr Adj    |    _|_     __|__
        Protection  |            /      R3 10K (2) | PD /_\  LD _\_/_
                    |        R2  \     +----+      |     |        |
                    |       560  /     |    V      +-----|---||---+
                    |            \     +---/\/\--+-------+   C4   |
                    |            |     |         |          .1 uF |
                    |+           |     |         +----||----+     |
                  __|__          |     |       __|__  C2 (1)|     /
               C1 -----          |     |     E /   \  100 pF|     \
              10 uF | -          +-----|------' Q1  '-------+     / R4
                    |            |     |    BC328-25 (5)    |     \ 3.9
                    |            |     |       (PNP)        |     |
                    |            |     |                    |     |
                    |        +---+     |                    |   |/ Q2
                    |        |_ _|_i   |                    +---|  BD139 (NPN)
                    |     VR1  _/_\_   |                   +|   |\ (5)
                    |    LM431   |     |              C3  __|__  E|
                    |    2.5 V   |     |           10 uF  -----   |
                    |    (3)     |     |                   -|     |
         R1 3.9     |            |     |                    |     |
GND >----/\/\/\-----+------------+-----+--------------------+-----+


1. Capacitor C4 value estimated.
2. Potentiometer R3 measured at 6K.
3. LM431 shunt regulator set up as 2.5 V zener.
4. Supply current measured at 150 mA (includes power on LED not shown).
5. Transistor types do not appear to be critical.

  17.8) Use of a CD, CDROM, CD-R, or DVD disc as diffraction grating

You have no doubt been impressed by the neat and nifty rainbow patterns
seen in the reflection off of a compact disc.  This is due to the effect
of the closely spaced rows of pits acting like a diffraction grating.

How good is it?

I tried an informal experiment with both a normal music CD and a partly
recorded CD-R (using the label side of the CD-R as the green layer on the back
is a great filter for 632.8 nm HeNe laser light!).

Both types worked quite well as reflection gratings with very sharply defined
1st and 2nd order beams from a collimated HeNe laser.  There was a slight
amount of spread in the direction parallel to the tracks of the CD and this
was more pronounced with the music CD, presumably caused by the effectively
random data pits.

If you can figure out a non-destructive way of removing the label, top lacquer
layer, and aluminum coating, the result should be a decent transmission type

Note that there is usually no truly blank area on a normal CD - the area
beyond the music is usually recorded with 0s which with the coding used, are
neither blank nor a nice repeating pattern.  The CD-R starts out pregrooved so
that the CD-writer servo systems can follow the tracks while recording.  There
is no noticeable change to the label-side as a result of recording on a CD-R.

Track pitch on a CD is about 1.6 um or about 15,875 grooves per inch, quite
comparable to some of the commercial gratings from Edmund Scientific or
elsewhere.  For a 1 mm HeNe spot, the curvature of the tracks is totally
inconsequential.  However, for larger area beams, this will have to be taken
into account - using outer tracks will be better.

Most other optical media can be used as diffraction gratings as well.  DVDs
(Digital Versatile Discs) in particular should be even better at this as their
tracks are much closer together than those on CDs :-).

Chapter 18) Service Information

  18.1) Advanced CD troubleshooting

If the solutions to your problems have not been covered in this document,
you still have some options other than surrendering your CD player to the
local service center or the dumpster.

When tackling electronic faults, a service manual with schematics will
prove essential.  Many manufacturers will happily supply this for
a modest cost - $10 to $50 typical.  However, some manufacturers are not
providing schematics - only mechanical and alignment info.  Confirm
that a schematic (not just a block diagram) is included if you need one
before purchasing the manual.

Howard Sams publishes Sams Photofacts service data for almost every model
TV that has ever been sold but their selection of CDfacts is nearly if not
totally nonexistent.

Test point locations, important signals, and power supply voltages are
often clearly labeled on the electronics board.  In this case, quite
a bit of troubleshooting can be done without the schematic.  There is a
good chance that the problem can be isolated to a particular subsystem
by just following the signals using this information.

Whatever the ultimate outcome, you will have learned a great deal.
Have fun - don't think of this as a chore.  Electronic troubleshooting 
represents a detective's challenge of the type hat Sherlock Holmes
could not have resisted.  You at least have the advantage that the
electronics do not lie or attempt to deceive you (though you may
beg to differ at times).  So, what are you waiting for?

  18.2) Web resources

Tandy (Radio Shack) has a nice web resource and fax-back service.  This is
mostly for their equipment but some of it applies to other brands and there
are diagrams which may be useful for other manufacturers' VCRs, TVs, CD
players, camcorders, remote controls, and other devices.

 http://support.tandy.com/                          (Tandy homepage) http://support.tandy.com/audio.html                (Audio products) http://support.tandy.com/video.html                (Video products)

Since Tandy does not manufacture its own equipment - they are other brands
with Realistic, Optimus, or other Radio Shack logos - your model may actually
be covered.  It may just take a little searching to find it.

  18.3) Suggested references

There are a variety of books dealing with all aspects of CD player repair.
While not as common as books on VCR repair, there are more of these than you
might think.  Your local public library may have some in the electronics
section - around 621.38 if your library is numbered that way.  Technical
bookstores, electronics distributors, and the mail order parts sources
listed in this document carry a variety of these texts.

1. Troubleshooting and Repairing Compact Disc Players
   Homer L. Davidson
   TAB Books, A Division of McGraw Hill, Inc., 1989
   Blue Ridge Summit, PA 17294, USA
   ISBN 0-8306-9107-3 (hardcover), ISBN 0-8306-3107-0 (paperback)

   Includes several complete CD player schematic diagrams which are quite
   interesting in their own right.)

2. Compact Disc Troubleshooting and Repair
   Neil Heller and Thomas Bentz
   Howard W. Sams & Company, A Division of Macmillan, Inc., 1988
   4300 West 62nd Street
   Indianapolis, Indiana 46268, USA
   ISBN 0-672-22521-2

3. The Compact Disc Book - A Complete Guide to the Digital Sound of the Future
   Bryan Brewer and Edd Key
   Harcourt Brace Jovanovich, Publishers, 1987
   Orlando, FL 32887
   ISBN 0-15-620050-3 (paperback)

   Includes a variety of high level information but no details.

4. The Complete Guide to Digital Audio Tape Recorders including
     Troubleshooting TIps
   Erik S. Schetina
   P.T.R. Prentice Hall,
   Englewood Cliffs, NJ 07632
   ISBN 0-13-213448-9

   Mostly directed to digital audio tape recording but also includes some
   information on digital sampling and CIRC coding.

5. DAT - The Complete Guide to Digital Audio Tape
   Delton T. Horn
   TAB Books, Inc., 1991
   Blue Ridge Summit, PA 17294-0214
   ISBN 0-8306-7670-8 (hardcover), ISBN 0-8306-3670-6 (paperback)

   Includes a chapter on the compact disc.

6. The Compact Disk
   Ken C. Pohlmann

7. All Thumbs Guide to Compact Disc Players
   Gene B. Williams
   TAB Books, Inc., 1993
   Blue Ridge Summit, PA 17294-0214
   ISBN 0-8306-4179-3 (paperback)
   This one is very basic but does cover the most common problems and has
   illustrated instructions for hookup, cleaning the lens, cleaning and
   lubricating the mechanism, simple electronic problems, etc.

  18.4) Rubber belts in CD players

The type of belts used in CD players for drawer loading and sometimes
elsewhere is nearly always a type with a square cross section.  Obtaining
an exact replacement belt may be difficult and not really necessary.

Measure the old belt and select one from a parts supplier like MCM Electronics
which is as close as possible - equal or slightly greater thickness and an
inside circumference (this is how they are measured) such that it will be
tight but not so tight as to slow the motor or cause damage to the bearings.
This usually means about 5 to 10 percent less than the old (stretched) belt.

  18.5) Interchangeability of electronic and mechanical components

The question often arises: If I cannot obtain an exact replacement or
if I have a CD, VCR, or other equipment carcass gathering dust, can I
substitute a part that is not a precise match?  Sometimes, this is simply
desired to confirm a diagnosis and avoid the risk of ordering an expensive
replacement and/or having to wait until it arrives.

For safety related items, the answer is generally NO - an exact replacement
part is needed to maintain the specifications within acceptable limits with
respect to line isolation, X-ray protection and to minimize fire hazards.
However, these components are rare in CD players.

Although only a few manufacturers produce most of the components in
CD players and CDROM drives, don't expect a lot of readily interchangeable
parts other than the common electronic ones listed below.  In their never
ending search for cost reductions and technology improvements, manufacturers
are constantly tweaking their designs.  More and more circuitry is finding
its way into custom VLSI chips.  Fortunately, these do not fail too often.

The only parts that are fairly standardized aside from the electronic
components are motors.  Often, if the motor is physically interchangeable,
then it will work as a replacement.  Electronic components and entire
circuit boards (if identical models and production run) can often be
substituted without difficulty though servo alignment will probably be
needed due to slight unavoidable differences between apparently identical
pickups or electronic components.

For common components, whether a not quite identical substitute will work
reliably or at all depends on many factors.  Except for the optical pickup,
non-custom components in CD players are fairly standard.

Here are some guidelines:

1.  Fuses - exact same current rating and at least equal voltage rating.
    I have often soldered a normal 3AG size fuse onto a smaller blown 20 mm
    long fuse as a substitute.

2.  Resistors, capacitors, inductors, diodes, switches, potentiometers,
    LEDs, and other common parts - except for those specifically marked as
    safety-critical - substitution as long as the replacement part fits
    and specifications should be fine.  It is best to use the same type - metal
    film resistor, for example.  But for testing, even this is not a hard
    and fast rule and a carbon resistor should work just fine.

3.  Rectifiers - replacements should have at equal or better PRV and Imax
    specifications.  For power supply rectifiers, 1N400x types can usually
    be used.

4.  Transistors - substitutes will generally work as long as their
    specifications meet or exceed those of the original.  For testing,
    it is usually ok to use types that do not quite meet all of these as
    long as the BVceo and Ic specifications are not exceeded.  However,
    performance may not be quite as good.  For power types, make sure to
    use a heatsink.

5.  Motors - small PM motors may be substituted if they fit physically.
    Brushless DC spindle motors are not usually interchangeable.

6.  Sensors - many are sufficiently similar to permit substitution.

7.  Power transformers - in some cases, these may be sufficiently similar
    that a substitute will work.  However, make sure you test for compatible
    output voltages to avoid damage to the regulator(s) and rest of the

8.  Belts - a close match should be good enough at least to confirm a
    problem or to use until the replacements arrives.

9.  Mechanical parts like screws, flat and split washers, C- and E-clips,
    and springs - these can often be salvaged from another unit.

10. Optical pickups - see the section below: "Interchangeability of components in the optical pickup".

The following are usually custom parts and substitution of something from
your junk box is unlikely to be successful even for testing: microcontrollers,
other custom programmed chips, display modules, and entire optical pickups,
optical decks, or power supplies unless identical.

  18.6) Interchangeability of components in the optical pickup

Once you have located a problem in the optical pickup, what should you do?
The quick answer is: probably nothing.  In the end any such attempts may
simply prove too time consuming and frustrating.

For parts like laser diodes and photodiode arrays, there are probably
too many variables to consider and the labor and risks involved - even
for the do-it-yourselfer - would likely be unacceptably high.  As an example,
the laser diode, which is an expensive component you might be tempted to
attempt replacing with one from another pickup (1) may not fit physically,
(2) may have different polarity laser diode and photodiode inside the case,
(3) may have a very different threshold current and safe operating current,
and (4) may have a different optical alignment with respect to any index
marks.  Any of these would likely make the interchange virtually impossible.
Even replacement with an identical laser diode would prove challenging without
the optical alignment jigs and specialized test equipment.

The only breakdown below the pickup level that I would consider as having a
reasonable chance of success would be to swap the lens assembly including
focus and tracking coils between identical pickups.  The optical alignment
is not supercritical at this point.  However, servo alignment might be needed
after this exchange.  See the section: "Aligning the lens assembly after replacement".

One style of lens assembly found in many (Sony) pickups is mounted with two
tiny Torx style screws from the top of the optical block.  Pop the black
plastic cover and you will see these at the end opposite the lens.  A small
straight blade screwdriver or .7 mm hex wrench may work in place of the Torx.
Unsolder the four connections for the focus and tracking coils and the entire
lens assembly can be removed without disturbing anything else.  (Yeh, right,
like anyone would actually go to all this trouble!).  The lens assembly may be
mounted on a platform that is fastened with three screws - two which affect
optical alignment from the bottom and a spring loaded screw from the top.  Once
the alignment is set at the factory, the lens assembly is fixed in place with
adhesive.  It should not need to be touched.

Thus, interchange of these lens assemblies is possible but expect to spend a
lazy afternoon or more :-(.  However, you will probably wish you had that
friendly unemployed Swiss Watchmaker for your assistant.

If you have narrowed the problem down to the pickup and you have an identical
pickup which you believe to be functional, the best bet is to exchange the
entire pickup as a unit.  Only minimal servo system alignment would likely be
needed after such a replacement.  The only optical adjustment needed might be
the setting making the beam perpendicular to the disc surface - possibly a
hexagonal nut on the bottom of the deck.  Be careful with respect to static
discharge which could destroy the laser diode.  Sometimes, the cable carrying
the laser drive voltage has a pair of solder pads to short while handling the
pickup not connected to the electronics board.  Take care not to rip any of
the fine ribbon or other electrical cables and avoid damaging the delicate
lens assembly.  One other risk is that the laser power adjustment may be set
too high for your new pickup - especially if you had turned it up in an effort
to revive a weak laser diode.

Better yet is to replace the entire optical deck as a unit.  This is a lot
less work and there is no risk of optical alignment problems at all.
Then, only (probably minor) servo alignment may be needed.

If you are lucky, the design of your player will even permit you to twiddle
the servo adjustment screws while attempting to play a disc (with all the
wiring in place) - which is really handy.  Also see the section: "Test CDs".

  18.7) Aligning the lens assembly after replacement

Should you need to remove the lens assembly from a Sony or other optical
pickup, it will need to be replaced in *precisely* the same position, accurate
to .1 mm or better.  Unless it is keyed in place to begin with, this will
require monitoring of the return beam and maximizing the amplitude of the
sum of the photodiodes A,B,C,D from a mirror or disc.

First of all, hope you never have to deal with this!

Second, it may be fundamentally impossible to accomplish with a disc in place
unless you are the size of a dust mite and can fit between the CD and the

Finally, a minor miracle may also be required and it is best to arrange for
this ahead of time :-).

If you get mostly one type of pickup, then you can build a test device which
would power the laser and and provide a test point to monitor the combined
photodiode current.  In principle, it is simple.  In practice you will most
likely need a custom device for each type of pickup.

With some CD players, you can do this in test mode and monitor the RF while
adjusting the alignment.

  18.8) Recommended parts suppliers

For general electronic components like resistors and capacitors, most
electronics distributors will have a sufficient variety at reasonable
cost.  Even Radio Shack can be considered in a pinch.

However, for consumer electronics equipment repairs, places like Digikey,
Allied, and Newark do not have the a variety of Japanese semiconductors
like ICs and transistor or any components like flyback transformers or
even degauss Posistors.

The following are good sources for consumer electronics replacement parts,
especially for VCRs, TVs, and other audio and video equipment:

* MCM Electronics                 (VCR parts, Japanese semiconductors,
  U.S. Voice: 1-800-543-4330.      tools, test equipment, audio, consumer
  U.S. Fax: 1-513-434-6959.        electronics including microwave oven parts
                                   and electric range elements, etc.)
  Web: http://www.mcmelectronics.com/

* Dalbani                         (Excellent Japanese semiconductor source,
  U.S. Voice: 1-800-325-2264.      VCR parts, other consumer electronics,
  U.S. Fax: 1-305-594-6588.        Xenon flash tubes, car stereo, CATV).
  Int. Voice: 1-305-716-0947.
  Int. Fax: 1-305-716-9719.
  Web: http://www.dalbani.com/

* Premium Parts                   (Very complete VCR parts, some tools,
  U.S. Voice: 1-800-558-9572.       adapter cables, other replacement parts.)
  U.S. Fax: 1-800-887-2727.

* Computer Component Source       (Mostly computer monitor replacement parts,
  U.S. Voice: 1-800-356-1227.      also, some electronic components including
  U.S. Fax: 1-800-926-2062.        semiconductors.)
  Int. Voice: 1-516-496-8780.
  Int. Fax: 1-516-496-8784.

Also see the documents: "Troubleshooting of Consumer Electronic Equipment" and
"Electronics Mail Order List" for additional parts sources.

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