Monday, 4 January 2010

The First Repair

So, your CD player won't read or play discs anymore?
Well my little Denon UD-M31 hasn't for a while.  Now that I've got my office and workspace finished and equipped, I thought it's about time that this old guy had a new optical pick-up (laser).  So, it promptly became the first repair conducted in this new space.  A number of symptoms can be apparent when the laser is wearing out in a player:

  1. On loading the disc nothing at all happens.
  2. As 1 above with the usual spin-up "shhh" sound from the mechanism.
  3. As 1 above with a quiet "tick tick tick" sound from the mechanism.
  4. As 1, 2 or 3 with "no disc" appearing on the display.
  5. As 1, 2 or 3 with zeros appearing on the display instead of the TOC information.
  6. The machine takes longer that normal to read the disc contents.
  7. The machine reads the disc contents but then won't play.
  8. The machine is very selective with which of your discs it will or won't play.
  9. The machine won't play or has difficulty playing CD-R discs (older machines and some commercial players aren't designed to play these).
  10. The laser jumps and skips whilst playing the CD and you hear audible ticks or noise during play.
  11. Playback stops part-way through the disc.
Beware of dirty or damaged discs as they will also cause some of the faults described above, so if in doubt check the disc for the obvious first.  This is a major problem that I have encountered with commercial establishments over the years.  Despite the fact that the CDs are all scratched up because they have been all over the bar and on the floor naked, and/or they have a nice coating of stale beer, which by now is growing a few cultures, the owner still wants to blame the machine!  I think it's something to do with that "general knowledge" that you can't damage a CD (there could be more about this in another posting).

Back to the repair
Some of these "All-in-One" units can be annoyingly difficult to get in and at the pick-up, so beware if you're going to attempt laser replacement yourself.  With this machine, the top has to come off first followed by the front panel.  Behind and at the bottom of the front panel is an incredibly delicate ribbon cable which you won't know is there until you've broken it, unless you have the service manual and you've studied it first!  The mechanism is shrouded in printed circuit boards, so there's some unplugging of more incredibly delicate cables and the removal of the PCB sitting on top.  Unless you've had the experience, it's best to take the mech. out to change the laser, this means more unplugging and more PCBs.  It's usually a fairly straightforward mechanical disassembly to get the laser out of the assembly, but this one came complete with the sub-chassis, so it's very easy.  Just one or two more delicate cables to remove out of the way!

It's great when a laser comes as a complete unit like this.  It means you're also changing most of the moving mechanical parts, including the spin and sled motors, together with the pick-up.  These are items that may well also be worn and ready to cause you more problems.  So, from a preventative maintenance point of view, it's good practice to fit the whole thing whenever you can.

So, having got this far, the rest is easy - reassemble in the reverse order.  Just hope that you can remember where all the bits all came from!  Fitting the ribbon cable back in the rear end of the laser is usually fiddly at the best of times.  With this machine, you need hands that are the size of a new born baby's and fingers that work like octopus tentacles!

Once reassembled, you're not finished yet.  Some makes/models will be OK with no further action, but this and several other machines I have come across still may not work properly yet.  There is a calibration procedure to be carried out (not all lasers perform identically).  With some units, this is done using laboratory instruments like an oscilloscope and a signal generator.  Measurements and corresponding adjustments have to be made to the machine's electronics.  But, with this player, the calibration is automated within electronics and software.  First you have to access the service program and then you need to follow a set pattern of key entries, check for responses indicated on the display and ultimately perform the auto-calibration.  Clearly, without the service manual, there is no way of knowing how to do any of this, or even if you have to.

If this hasn't put you of doing it yourself, there are only a few other minor issues to consider:

  • Some lasers have several variants, you'll need the exact one.
  • Some lasers are incredibly difficult to identify without experience and information.
  • Similarly some have been replaced with an equivalent.
  • Try to only fit original laser types, a lot of the after market replacements (particularly those for obsolete models) are very fussy and some don't work at all.  There are a few machines in which I now won't replace the laser because the original is no longer available and the replacements are too unreliable.
  • Laser pick-ups are sensitive to static electricity and unless you are properly grounded whilst undertaking the procedure, the unit could be damaged by a discharge.  Worse, this might not show up straight away.
  • Some machines require special tools or software, as well as the laboratory equipment, to make the calibration adjustments.  These are often only available to trained dealers from the manufacturer.
  • Shock hazard - be aware of danger.  There can be exposed mains voltages inside the machine, make sure you know where they are especially if you intend to operate the machine partially disassembled.
  • Blindness hazard - be aware of danger.  Never look directly into the laser when it is operating, you could damage your eyesight.  The laser light is at the infra-red end of the spectrum and is invisible to us, so you won't know if it's there or not.  The faint red glow that you may see is only a visible component of the total output and is not an indication that the laser is worn out.
  • Trying to fix cheap supermarket CD players and CD-ROM drives is futile - they are very inexpensive to replace.  Don't waste your time!
  • Even if the machine exhibits any of the symptoms mentioned at the beginning of this post, don't assume that the laser is to blame.  In my experience of recent machines, it probably is, but there are many components in the "front-end" circuitry which can give the same response.  A more scientific approach to prove it is often beneficial, without this your new laser may have a short lifetime.
Why did it stop working properly anyway?
This is a question I am asked frequently.  There are numerous reasons, some I have theorised myself.

The answer you really want to hear is "old age".  I think a good analogy, which most people understand, is lasers are a bit like a light bulb, they start wearing out from the first time you switch on.  Most lasers seem to have a lifetime of around 5,000 hours.  In domestic terms this equates, quite often, to 7 years plus (with average use).  With commercial use, this would be reduced to 2 or 3 years - a lot of venues are now open 25 hours a day, 8 days a week; the laser isn't going to last forever!  This is all providing (in my experience) that it's a decent machine to start with, and there are exceptions that live longer.

Lifetime also has a lot to do with cleanliness.  In the past, I have handled many machines where I have felt the laser pick-up has suffered premature failure.  Sometimes this is down to design and other times it is some external contribution.

Traditionally, a Hi-Fi tower system had the turntable at the top.  When CD came along in 1982, the tower system had become known as a "midi" system and the optional CD player was placed underneath the whole stack.  Then CD players became a must and they were built into the bottom and the optional turntable was placed on top.  This all seemed fine until eventually the turntable was deemed a non-requirement and the CD player migrated to the top of the box.  That's when the trouble started.  Right or wrong, I have deduced the following:  All the heavy power consuming, heat generating components are in the bottom of the unit - mains transformer, power supplies, the power amplifiers with heatsinks and so-on.  Ventilation is provided in the rear or sides and draws in cool air.  The cool air warms up inside the machine and rises due to convection, eventually exiting via the position of the CD player.  Simple.  Except, along with the cool air, contaminants like dust, pet hair and cigarette smoke are also drawn into the box.  The rising warm current of air does two things to the CD player before venting back into the room - heats it up and deposits a good deal of the contaminants into the workings.  The components can be cleaned, re-lubricated, etc., but the problem with a laser is that you can't get to the internal bits - the laser diode, mirrors and lenses.  Once a laser is contaminated in this way, in my book it's useless.  The only cure is replacement.  Any amount of cleaning of the laser which may get it working again is usually a temporary solution.  I never pass a machine as fixed when this is the case.  It may be OK as a temporary situation so the customer can find something else, or maybe to buy some time whilst funds allow for a new laser (which might be expensive), but usually these machines will come back within a short period of time.

Out of all CD players that I have had for repair or investigation over the last 10 years or so, about 20% were serious Hi-Fi separates.  The vast majority of these were suffering effects of age.  Of the remaining 80%, the vast majority were all-in-one units, midi, and (now) micro systems with failed lasers.  More than half of these showed physical evidence of dust, pet hair, cigarette smoke, talcum powder, occasionally human hair and  small (but significant) proportion of cat urine!

I think that there are a number of significant reasons why the HI-Fi separates last longer.  First, generally, they are better designed and built.  Then there are no high power components and little heat is generated, therefore no, or little ventilation is required and, unless you leave the tray open, little in the way of contamination can get in.

A word about CD lens cleaners...
If you want a definitive answer, I'm not really sure if they do any good or not!  If the type of contamination we're talking about is light dust then they can't do any harm and may even extend the life of your player.  But you must use one frequently.  Anything heavier and the best they are going to do is move it around.  Chances are you'll buy one and forget about it until you have a problem.  No good reaching for it at this stage, it's already too late - if your player won't read a CD it's not going to play the cleaner either.  Time for a new laser!

If you want to know more about how CD players work look here.

I hope that this has been useful, informative or just plain interesting.  As always your comments are invited, as are your faulty CD players!

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