On Mon, 16 Feb 2009 18:39:33 -0000, "Manuel" <mm1001001@...> wrote:

>Thanks for your reply. Actually, the belt has just been replaced
>so I am sure that is not the cause of the speed variation.
>
>Is there a way that I can check the pitch control potentiometer with
>a multimeter to see if that's what's failing?

There is more than a speed pot to the story. When the speed is not locked to a
quartz
crystal, there is still a circuit that has to work. And when the constant is not
a
crystal there is degrading of components to consider.
On older decks that do not rely on the mains frequency to work properly there is
a
way to trim the speed using a small trim pot, these are sometimes accessable
from the
outside of the deck. This may be a passive design, which will deteriorate and
become
unstable as components age. However setting the speed manually and not governing
it
actively is a way to make a cheap deck. Coupled with not making the speed
dependent
on the mains frequency the manufacturer can make the same design using the same
components and procedures for anywhere in the world.
The other approach is to have a governed circuit using feedback of some sort.
On some models Sony implemented a tone head at the edge of the platter, which
had
magnets on it. when the platter turned the magnets generated pulses of
electricity
that were measured and deviations where dealt with.
Technics has some models a simialr approach, feedback and countermeasure for
variations in speed. Within limits more or less power goes to the motor, when
the
sensors pick up less or more speed than is necessary.

What you have to do if find out if the feedback is working. If the governing is
active the countermeasures must also work. I.e. if you find that sensors work,
the
logics receive the signals but the output transistors have gone kaputski then
those
would have to be changed. I am not familiar with this model myself.

I would suggest the cautious approach of examining each component on its own.
And
also clean the pots in question- perhaps soldering in another fresh pot for
testing
to see if the new will make the circuit keep the speed. Over the years the pots
are
not immune to degradtion of the contact surface, but getting the ideal
substitute
will be difficult if, as I mentioned before, these are led to the outside and
knobs
are attached. Lenghts can be cut down to fit. but values thickness of shaft and
so on
must be met, or it may function better but look silly.

I suggest you keep the deck as original as possible. if you are not up to the
task of
performing the surgery have someone else do it or store it and use another until
a
solution materializes. Perhaps you will find a similar deck with a working speed
control. Or perhaps you may find a replacement pot that fits or IC chip or what
ever
is broken/defect.

Passive citrcuits are the most simple as they have less connections and fewer
parts.
But getting a stable speed frm those kinds of circuits is not easy.

Other circuits either measure the turning of the platter or the motor axle. Also
it
would be possible to use a passive circuit for another model or brand as long as
the
voltages to govern are within the same specs. I.e. if a 6V or 12V motor is used
and
it has to turn at an RPM that is within reach of the pot then there is a good
chance
you take out the old circuit and screw in the other. Perhaps extend some leads
to
make it fit. In a pinch you can cover the PCB and components in duct tape and
affix
it to the chassis. If you can pinch a ciruit from a much better model, power to
you.

Finally do not rule out the motor. The motor may be fitted with capacitors that
are
now either out of spec or shorted or open. This can also produce erratic
behaviour,
and also wear in the motor can cause speed fluctuations. Degreasing the motor
and
reoiling it with proper oil may be good idea depending on what you find.

I have to say this: WD40 is not a lubricant, it is a "Water Displacement agent,
formula number 40", indicating at least 39 other formulas that did not displace
water! WD40 is great for rust, covering parts that are in a hostile environment
and
several other uses, especially outdoors or on stuff that was in a dark cellar
for a
long time. It has nothing to do with electronics equipment whatsoever.
Do not use it on switches, pots, trim pots and other stuff in your audio/video
chain.
What it does is pile up particles and help grind them into the surface of the
switch,
or pot, and then you will need to replace that part. Switches are not shelfware
anymore and a matching pot may be difficult to find.

Use proper cleaning agents, use proper lubing and always ventilate accordingly.
Work
switches lot of times while using cleaning agents and then compressed air.

Also if you pinch a circuit from somewhere else, write on it where it came from
- or
it will not match the service manua lfor the unit it is in, and someone else or
even
yourself will be wondering years later what it is and how it is documented.
When you design a solution you are the source of documentation.

-Mikkel