OBERHEIM SYNTH

Clearly something in the autotune circuitry is not working.   There is a comparator 
that gets fed one voice at a  time.   The comparator output is used to gate a higher
frequency clock into a counter, for one or sometimes more cycles of the comparator
waveform.   By counting high frequency pulses that fit into a period, or several, of the 
oscillator output, they can measure its frequency, and figure out what the control
voltage needs to be to get it exactly in tune.   Usually 4-6 different notes up and down 
the keyboard are measured, and CV's for notes in-between them are interpolated when 
you are playing.

I would use a 'scope to see what the waveforms look like during tuning in this circuit.
Was helping a friend with his OBsx that had a tuning problem.   We ended up replacing 
a bad data bus buffer that was used to read the counter after tuning had completed. 

Good luck,

       Bob


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On Tue, 7/21/15, richtrix@... [oberheim] <oberheim@yahoogroups.com> wrote:

 Subject: [oberheim] Autotune on OBXa suddenly shutting down all voices
 To: oberheim@yahoogroups.com
 Date: Tuesday, July 21, 2015, 12:28 AM
 
 
 I have an OBXa that I finally got to 100% percent
 with a lot of help from here and some dumb luck a few months
 ago. I have the Encore MIDI kit. I haven't touched it
 for about 2 months or so since I've spent any free music
 typed time fiddling with some Linn 9000s I'm trying to
 get working. The Obie worked just swimmingly the last time I
 used it. Now though... everything works find and sounds good
 but when I hit autotune instead of zipping through the
 voices like it does usually it spend about 2 seconds on each
 voice and then disables all of them. Any ideas what could
 have happened in these two months of inactivity? Doesn't
 matter how long the board is on before hitting autotune...
 get same result. 
 Sincerely,Rich
 Clarke