cd6500 unable to propery adjust laser mV (and other issues)

[TK]

Here’s my latest unresolved issue – the usual “Replace 2103 with a new capacitor” trick has failed to get a CD6500 back to reading CDs.  I’ve looked at most of the simpler tests but I’ve been unable to get it into specification.  So before I give up, I thought I’d see if anyone else has any thoughts on a fix that I have not tried.

Some background:

• The 5 voltage inputs on Plug 33 check out OK, both at idle and under operation
• 2103 has been replaced with a Vishay 47uF capacitor.  I’ve also tried a second one and a 33uF Vishay just to see if I could get lucky and have it be a bad capacitor. I have successfully used this exact batch of caps to repair other units.
• No visible cold solder joints on the servo board.  The board had two other 220uF caps out of spec which I replaced.
• Laser light is visible, and the arm does pivot easily.  The servo adjustment works under certain circumstances – see below

Symptoms

• Passes service mode 1
• In service mode 2 I’m unable to get the laser adjustment to go beyond 18mV- the dial is all the way over clockwise
• In the extreme clockwise setting for laser power, at 18mV, it passes service mode 2 focus test
• Service mode 3 has the CD drive spinning excessively fast – much faster than a similar good unit I have for comparison
• Service mode 4 fails immediately, and continues blinking.  This points to a failure of the radial motor servo, from the manual.

One other symptom: under load, resistor R3142 gets quite hot when seen by a thermal camera- much hotter than it does in a comparable system. This is sitting on the output of the focus motor amplifier, which does appear to work under the circumstances (I can visually inspect the lens moving as it attempts to focus, and it passes Service 2, but only with the laser level adjustment hard-over at its 18mV maximum).  With this, I was hoping that perhaps I had a bad 6104.  Do these tend to fail?  Also of note, the focus motor and radial motor both register 22 Ohm resistance when disconnected, which had given me hope that it was the board at fault.

I’ve put in a replacement servo board just to test if there was an issue with the old board, but I get the same result- too low of a laser current, which again cant exceed 20 mV at its extreme.  So I’m imagining that there is something wrong with the laser unit itself, which is depressing.

Any stones left un-turned?

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