"Mike" wrote in message
o.uk...
I've been given a radio/cassette/cd player with a faulty CD portion to
play with.

The CD spindle does not spin, but the motor operates if given 4+ volts
from a battery (and draws a variable current depending on mechanical
load).

The laser actuators appear to work, because they attempt to hunt for a CD
to read. What I am not clear about is whether the focus process can work
with a stationary CD.

I consulted http://www.repairfaq.org/REPAIR/F_cdfaq*.html which suggests
that if the spindle does not start this may because of failure to focus,
implying the answer to my question is "yes"; but how?

Reading the specification for the "RF Amp + SSP" chip (KA9220C) does not
help me much either. However it does say "The KA9220C is designed for 3-
spot type optical pick-up assembly. ... The photo detector A,B, C and D
detect audio modulation signal on the disc and generate focus error
signal." That suggests to me that the disc needs to be rotating for focus
to happen. But if the above answer were "no" we would have a Catch-22
situation!

Can anyone explain, please?

Mike.


It is perfectly normal for the spindle servo not to be turned on until the
laser has obtained focus. When the spot is out of focus at the disc surface,
it will be eliptical, so the reflected beam will not illuminate all four
central pickup diodes equally (ABCD). The focus servo will move the lens
until this situation is corrected, indicating that the spot is now properly
round and in focus. The spindle motor is started up at this time. The
tracking servo is turned on once the disc is rotating (roughly the right
speed but not yet servo-locked), and makes use of the two other beams which
run equally either side of the 'groove'. If the central beam is off to
either side, the two side beams will be off position also, illuminating the
E-F pickup diodes unevenly. The tracking servo bends the lens to one side or
the other to correct this and keep the central beam running right over the
groove. The spindle servo is turned on as soon as valid data is being
recovered. The data recovery rate is used to lock the spindle servo to
control the rotational speed of the disc. This is constantly altering as the
disc plays. As the laser moves to the outer edge of the disc, the angular
velocity changes, so the disc is progressively slowed by the servo to
maintain a constant data rate.

If you want to get yourself a real in-depth understanding of the subject,
have a read of

https://books.google.co.uk/books?id=...tems& f=false

Not sure how the post will cope with a link that long, but I'm sure that you
will work it out :-)

How the beams within the optical block are used to obtain the various servo
locking signals is explained around about page 46 or so.

If focus is not obtained during the initial attempts - which are usually
three and bust - some players will 'kick' the disc to rotate it a little so
that a different disc area is exposed to the 'hunting' beam. This is in case
the disc is dirty or damaged at the initial try-out point. Again, this is
usually a 'three and bust' sequence. Some players do spin the disc
immediately the laser has checked that it is home, and focus is attempted on
the rotating disc. This is not common, however, as it introduces another
potential error term for the focus servo to cope with whilst trying to lock
in the first place, if the disc is warped or the turntable is not flat. Far
better that focus is obtained on a stationary disc.

If you are able, look under the disc and watch the lens. It should move up
and down just once and stop at some roughly midway position, indicating that
focus has been obtained. The FOK signal within the player will be asserted
at this time. If the lens moves up and down several - probably three - times
and then stops, then the laser did not obtain focus. By far the most
common - although not the only - reason for this is a defective laser. This
can be because it is simply worn out, or because of dust / film on the
surface of the lens, or dust / film on the critical-angle mirror or pickup
diode array inside the optical chamber. If the lens surface is clean, you
can try holding the lens gently to one side with a scalpel tip, and using an
air duster to blast a shot of air down the gap. Sometimes works ...

Most of the common lasers - Sony KSS series, Sanyo SPF101 series etc, are
incredibly cheap now, so usually worth a punt if you suspect a bad laser.
Most modern players are fully self-adjusting for servo gain and balance etc,
so a replacement laser job is pretty much just a case of 'drop it in'. If
the player does have adjustments, these are extremely easy to carry out with
a scope on the RF test point, or in the absence of a scope, are nearly as
easy to adjust with the benefit of a scratched disc and the mk 1 ear-'ole
.... :-)

If you have never replaced a laser, and end up giving it a go, you should
note that there is a blob of solder across a pair of shorting points on the
back of the little pcb on the laser. This protects the laser diode from
static damage when it is not connected to anything, and needs to be removed
when the laser is installed.

Arfa