OBERHEIM SYNTH

When it's suddenly failed, I'd make sure that there aren't bad trimpots in the bender section. If it's in tune before the autotune, make sure it's actually in concert pitch, not just OK in relative pitch, and make sure the octaves and bender functions are correct.
I also recently saw an Xa where everything was appearing right, but the autotune was totally messing the tuning. It was a TL084 with a weird offset. I spent HOURS finding it. There was also another bad 84, so I swapped them all out, since they were all the same date code and the failures were so unpredictable. This one had to be shipped about 700 miles and I didn't want to risk a return over some 50 cent parts...

Sent from my iPad

On Jul 25, 2015, at 6:37 AM, Bob Grieb bobgrieb@yahoo.com<mailto:bobgrieb@...> [oberheim] <oberheim@yahoogroups.com<mailto:oberheim@yahoogroups.com>> wrote:



Hi,

I would agree that that waveform has some issues.

The signal "oscmux" is the output of an op amp fed by a sum of the voices.
It will be swinging rail to rail with whichever voice is enabled during tuning.

That signal feeds into a circuit that only takes the positive part of the op amp
output, and feeds it into a schmitt trigger. This is A119, a 4049 and several
resistors.

The output of that circuit should be what you were probing. Looks like it
also goes to a connector. Not sure if it's also used somewhere else? If so,
that circuit could be loading it down. But first I would replace A119, or at least
scope the signals in that area. And set your scope to capture a complete period
of the input signal, not 1 uSec/div. That was useful to see the oscillation on
the signal, but not so much for all of the other photos. I would also try to measure
with your DMM if the resistors around A119 are OK and hooked up to the chip
properly. You can measure right at the pins of A119, then you will know they are
hooked up and also see their values. If you have a decent meter, A119 won't affect
the measurement much.

If you decide to change out A119, cut all of the leads next to the body of the chip,
remove the body, then remove the pins one at a time with the soldering iron and tweezers.
Then clean the bd, and install a socket and a new chip.

Of course if you were replacing a hard-to-find chip and you were not sure it's bad,
then you would do it differently to save the old chip.

Bob

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On Sat, 7/25/15, richtrix@...<mailto:richtrix@...> [oberheim] <oberheim@yahoogroups.com<mailto:oberheim@yahoogroups.com>> wrote:

Subject: Re: [oberheim] Re: Autotune on OBXa suddenly shutting down all voices [18 Attachments]
To: oberheim@yahoogroups.com<mailto:oberheim@yahoogroups.com>
Date: Saturday, July 25, 2015, 1:15 AM























[Attachment(s) from richtrix@...<mailto:richtrix@...>
[oberheim] included below]







Thanks yet again Bob for your help. You are a treasure in
this Yahoo! community.
So the results are strange. I scoped pin 10
(clock, according to the internet) of U118, a 4021B, and
when unstimulated it simple read 5 volts DC. If ever I hit a
key and triggered a note a whole slew of strange signals
would ensue. When hitting the autotune button I would get
approximately the same resulting signals. I've tried to
capture some of the madness in a series of screenshots
attached. Notice these shots are mostly less than one second
apart.
Any thoughts? Is this normal? Doesn't seem
normal.
Sincerely,Rich Clarke