"William R. Walsh" m
wrote in message news:jU0mh.334794$1i1.210997@attbi_s72...
Hi!

Lots of adjustments are critical in various electronics.

You'd get no argument from me. I know the critical nature of adjustments
in
electronic devices, and even understand why some of them are not made to
be
adjusted. But that's not really answering what I asked. I suppose it is
possible that I'm asking a question that only CD player manufacturers
could
answer. :-)

I'm not wanting to come across as impolite. I'd simply like to know why
this
is done.

William


Your question is not impolite, and I'm sure would not be taken as such by
anyone on here. Asking is how you find out.

The manufacturing tolerances of the optical block itself, are subject to an
amount of spread. These are compensated for when the block is manufactured,
by putting it into a test jig that sets certain conditions that should
result in a certain range of drive currents to produce a specific level of
laser diode output. The pot which sets the drive current is adjusted to
achieve what the manufacturers consider to be ideal for that particular
laser diode in that particular model of optical block. Once this setting has
been made, the pot is very firmly sealed, not just to make it vibration
proof, but to indicate that it should not be adjusted from its set position.
This is the case for almost all optical blocks. Other characteristics that
may result from mechanical tolerances of the deck that the optical block
becomes fitted to, or tolerances in the circuitry that the block has to work
with, are often provided with adjustments which are intended to be moved as
part of a servicing procedure.

Examples of these are focus offset, where the zero point of the servo is set
to match the average mechanical zero focus point, which should of course be
the mechanical resting point of the lens suspension, but may not be if the
turntable height is not set exactly correctly. Likewise, tracking offset
does the same to compensate for mechanical errors in the tangential or tilt
directions. There may also be adjustments which set the gains of the servos
to take into account spreads in the servo circuitry. However, many players
from the last 10 years or so, have no service physical adjustments at all.
With these players the chipsets used have fully electronic internal
adjustment systems.

As I said in the previous post, one notable exception to the "don't touch it
!" rule, is Pioneer players. The laser power pot is a minuscule little thing
located on the laser flexiprint, and is adjusted as part of the setup
procedure. Pioneer provide a set of testpoints in a single row on their
players, and detailed instructions in their manuals, on how to adjust the
entire pot-set, which is often a total of eight pots, and which includes the
power pot. Interestingly, Pioneer lasers very seldom fail or wear out. I
think in 30 odd years, I have only ever replaced perhaps two of the original
types ( there were several, but all pretty similar looking ) and maybe two
or three of the later ones which have a couple of tiny pots on their PCB.

Arfa