Exploring CD optical unit problem
 

Well probably the problem - intermittant failure to play or even spin up
('no disc' error).
According to, as far as servo and optics system, a different model, same
make, service manual
the optics power supply pass transistor output in Play should be 4.9V.
Whether working or not working this voltage is 3.5V, main 5V supply line is
not loaded down.
Removing the optics assembly and powering off a bench supply
of 3.1V (minus ammeter drop) gives curent drain of 74mA, at 3.6V then 102mA
ie
above the rating of the pass transistor.
Waving CD, mirror or bright torch onto the laser beam does not vary this
consumption.
Changing the SM power level preset just varies marginally this laser current
..
Putting a 10K across the receive photodiode ( that is coupled to the main
laser as distinct from the
signal receive array diodes) cuts off the beam and the current drain so the
feed forward
3 SM pnp tranny circuit works.
Can the response of the receive diode decrease over time - ten years old. ?
Completely isolating this photodiode and putting a DVM across, one way
gives, an off the
scale ohms reading. Bringing a bright torch exactly on beam will drop it to
of order 1Meg.
Any ideas if I 'repair' by putting a high value preset across this
photodiode
to bring the auto power reduction feed-forward circuit into operation.?
Also does anyone know what the normal in use range of laser current would be
expected in general?

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse