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

You have the two correct areas. The way to test this, is to get the parts
and try them. Over an email, it is only possible to go by your description
for evaluation.

--

Jerry G.
=====


"Jonathan H N Chin" wrote in message
...
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

"Jerry G." writes:
You have the two correct areas. The way to test this, is to get the parts
and try them. Over an email, it is only possible to go by your description
for evaluation.

Excellent. It's always nice to have a second opinion.

Playing with the machine some more last night, the power off pin is
definitely being set low, so it looks like it is the servo chip that's
broken. I now have one on order (I'm amazed they are still available).
All that's left is to hope I can remember how to use a soldering iron!


-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

I wrote:
Playing with the machine some more last night, the power off pin is
definitely being set low, so it looks like it is the servo chip that's
broken. I now have one on order (I'm amazed they are still available).
All that's left is to hope I can remember how to use a soldering iron!

For the benefit of anyone googling for this subject in the futu

Yes, it was the servo amp ic (IC601, CX20084, on the corner of the
board next the battery compartment) that was broken. Replacing it
has fixed the problem.

I had some private correspondence with people who serviced this model
in the past. Their recollection was that it was usually this ic that
broke if power was connected with the wrong polarity.

In addition, I discovered that the ground of the external power-in is
connected to the system ground only by a black-covered wire. A diode
can be placed in series with the wire (cathode to the pcb, anode to
the wire) and will protect against wrong polarity. It is not even
necessary to cut any pcb traces. The diode is not in circuit when the
machine is being powered from battery and so it will have absolutely
no effect on battery life.


-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