OB-8 Flakiness

[Tim Curtis]

Okay - my OB-8 is experiencing some pretty strange problems - hopefully
this will make sense to someone here...

First - the original symptom is that it has had a problem with auto tune
for a few years now.  I can calibrate the oscillators according to the
manual, but auto tune always has problems with a few of them.

So, I tried again to adjust it and tried to calibrate the filters as
well.  As far as I can tell everything is dead on calibrated.  I can
turn the OB-8 on and the voices sound in tune, but as soon as I try auto
tune several of them go out.

So I run through the tests in the manual and I find that when I try to
run the LED test only a handful of the LEDs on the front panel will
light, then the OB-8 locks up.  If I power down and run the test again
it will be a different handful of LEDs, then it will lock up again.

I tried disconnecting the power and ribbon cables to the various boards,
and when I disconnect the top voice board the LEDs light up normally.  I
figure this is probably a pretty good indication that it's a problem
somewhere on that board.

So I start checking the various ICs and I find something that seems (to
me, at least) peculiar.  U17, U18, U19, and U20 seem to have different
voltages on some pins on the upper board than they do on the lower
board.  Not all pins, but several.  The thing is, it seems like they're
consistently about a half a volt higher.  Does this sound normal?  These
ICs are right where the Ribbon cable connects to the voice board.  Could
this account for my LED test trouble?

Also, could it account for part of my auto tune difficulties?

Can anyone give me a good idea where to look next?  What could this
indicate?  Or is it normal that the IC voltages would vary from board to
board?  I would think it would be clone theory, but I'm wrong a lot...

Thanks for any help.

Tim