OB-8 Flakiness

[Magnus Danielson]

From: Tim Curtis <vampope at swbell.net>
Subject: OB-8 Flakiness
Date: Sat, 25 Mar 2000 04:31:41 -0600

Hi Tim!

OK, I'm out on the thin ice here, but a few comments anyway:

> 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?

Hmm... without having an OB-8, an OB-8 service manual or even had the
possibility to dig into one it sounds like maybe the culprit is that this
board somehow got damaged and now corrupts the bus when connected. I don't
know what the named ICs are, but if they are hooked to the bus I think it
could explain for the peculiareties that you are experiencing.

If it is a shared bus for these cards and some of the troubled ICs are
connected to the bus, then anything sent over that bus may be damaged
(data-wise).

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

Seems feasable.

> 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?

What ICs are there? If it is normal TTL they should not vary that much, but
it really depends on which values they have and how they are interconnected.
It may be that you need to replace those ICs.

Cheers,
Magnus