snip

However, short of serious wear
in the spindle motor causing the turntable to drop, I can't think of any
wear that would indicate a change in focus bias being needed ? The focus
servo has a pretty wide 'window of lock' anyway to accommodate warped
discs,
so tiny amounts of wear in the motor will not have a lot of effect on the
nominal focus point.

Well, according to what I've read and seen, that's exactly what
happens(1). The traveler wears down as it slides along the rails. The
wear isn't much, but sufficient to impair focus with a warped or
marginal cdrom. However, I don't think that's the only source of
deterioration. A decrease in laser output, filth in the optics, wide
variations in reflectivity from various media, and bad cd burns, all
contribute to read errors.

Disclaimer: I are not an expert on cdrom operation and repair.

(1) Next chance I get, I'll tear apart a traveler from an old and well
used cdrom drive and stuff it under the big inspection microscope at
the local machine shop. It should easily tell me how far out of round
the holes have worn.


Hmmm. That's interesting. I must admit that I hadn't considered wear on the
slide holes or the 'claw' and its runner, if that is what a drive uses on
one side. Most decks that I see are from audio equipment, so do not get the
hammering that you are saying that these of yours did. I think that the wear
would still have to be pretty substantial to significantly affect the focus
bias point on all but the most warped discs. Some of the worst that I've
seen still only cause a deviation of say +/- 1mm of the lens with the
optical block at the outside edge of the disc. Considering that a 'blind'
focus search - i.e. with no disc inserted - results in a lens movement of
probably +/- 2mm, you would have expected that the focus servo would be able
to lock over at least that range. In fact, thinking about it, I know that it
can on many players, because I have held a disc over the top of a deck
that's out of a player, to watch the focus search taking place, and the
focus locks easily and you can then pull the lens up and down as though on
elastic, a considerable amount, before the servo loses lock and the lens
'breaks free'.

Many, although not all, audio lasers use a phosphor bronze sleeve bearing to
run on the slide rod, which is actually *very* hard chrome plated, so I
would not expect them to wear anything like as badly as the ones where it is
just plastic running on the slide rod.

Have you ever tried adjusting tracking (E-F) and focus balance by ear ? It's
actually very easy to do on an audio player, and is remarkably accurate,
when checked against the eye pattern on a 'scope. If you use a scuffed disc,
and turn up the tracking and focus gains a little, the servos will make the
lens suspension 'whistle' as it tries to follow the imperfections on the
disc surface. If you then stick your mark one ear'ole down near the laser,
and tweak the focus and tracking balance controls back and forth a little,
you can hear a very distinct and sharply defined 'null' in the noise. Set
the controls there, and then turn the tracking and focus gains back down to
where they were. Give it a try. You'll be amazed at how simple and accurate
it is.

Arfa