Thirteen years ago, I plugged an external power adaptor that had
the wrong polarity into my walkman D6C. I have finally been stupid
enough to do it again. 8^(

The last time I did it, the servo amp ic blew up. The symptoms
were that the playback speed varied depending on the input voltage.
Replacing the IC was sufficient to cure the problem.

This time round, playback speed is unaffected. In fact, everything
on the machine still appears to work fine except for one, really
annoying, thing: The motor never shuts off.

Tracing the electronics in the service manual, and using my very
rusty memory of what little electronics I ever knew, I guess that
one of two things is causing the problem:

a) I have again broken the servo amp ic, but this time only the
power off input has been affected (the chip cannot be completely
broken since speed tune still works). Or,

b) The auto-off circuitry is broken and so the power off signal
to pin 7 of the servo chip is never being sent.

Have I missed any other possible cause?

My gut feeling is that the correct answer is (a) but how would I go
about testing (b)? It looks as though I ought to be simply to short
pin 7 to ground, and if that causes the motor to power off, then
that would indicate that the servo ic is not broken, but that the
problem is (b), the auto-off circuitry. Does that sound plausible?
(Of course, if the pin is already at zero, then that would indicate
that the servo ic input is faulty and no further tests are required.)


-jonathan

--
Jonathan H N Chin, 1 dan | deputy computer | Newton Institute, Cambridge, UK
| systems mangler | tel/fax: +44 1223 767091/330508

"respondeo etsi mutabor" --Rosenstock-Huessy