[sdiy] Oberheim OB-XA autotune procedure questions

[Gene Stopp]

I have some dim recollection of how the OB-8 does autotune, which may be
similar to the OBXa...

The autotune button is pressed. The microprocessor sets all VCO's to pulse
wave, then sets all pulse widths to beyond 100% (silence). It then  sets the
pulse width on each VCO, one at a time, to an audible setting. The filters
are opened up all the way, and the final mono audio output is monitored by
the microprocessor through a comparator via an I/O pin (or some such thing).
A set of test CV's are set up, the output frequency is monitored, and the uP
builds up a scaling table for each VCO.

If the feedback path from the output back to the uP is opened somehow, all
autotune will fail.


- Gene

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Roy J. Tellason
Sent: Friday, April 29, 2005 12:24 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Oberheim OB-XA autotune procedure questions


On Thursday 28 April 2005 05:44 pm, doof wrote:
> Hi all,
>
> I've just finished replacing the battery in my OB-XA (along with
installing
> new bushings and cleaning some problematic J-wires) and I've found that
all
> voices now fail the autotune procedure.  When I press the button, it
cycles
> through all 8 voices, then the 8 LEDs flash for about 2 seconds.  All 8
> voice cards are perfectly functional.
>
> I have both versions of the service manual, and neither address the
> autotune function other than using it in calibration routines.  How
> autotuning is implemented and any specific troubleshooting/calibration
> strategies are completely neglected.
>
> So I'm wondering what could possibly be contributing to this problem. 
> Autotune worked before the battery replacement, although tuning wasn't
> perfect.  Does something within the circuit rely on battery power to
retain
> memory for the autotune algorithm?
>
> At first I thought bad tuning might be causing the problem.  Turning the
> voice disable feature off with the test2 switch, I've started calibrating
> the individual voice boards to achieve better tuning, yet reagardless of
> the more accurate initial tuning the autotune function still does not want
> to cooperate.
>
> Looking back at the AH archives, it seems a few people have experienced
> this exact problem, though no answer ever surfaced.
>
> ...Anyone?
>
> Thanks.

I've little direct experience with that brand,  but this sounds awfully 
familiar.  I suspect that what you're running into is something in the
memory 
of the thing with screwy values.  If there's a way to re-initialize or reset

it somehow,  try that,  and see if that doesn't fix this...