[sdiy] Re: Oberheim OB-XA autotune procedure questions (UPDATE)

[doof]

Funny that you mention the pulsewidth processing during the autotuning
procedure.  Yesterday evening I fired up the scope and calibrated the
pulsewidth of each board to a 50% duty cycle as suggested in the service
manual, and surprise of surprises autotune was working once again.

I spent a few hours following the exact tuning procedure outlined in the s
manual, all of which convinced me that the OB-XA's Osc tracking is really
pretty crummy,  I've come close but it seems impossible to eliminate audible
beats between voices over a say, 3 octave range.  My OB-8 is far more stable
in terms of tuning.  Likely this is what accounts for the "thickness"
generally attributed to the XA.

One glaring omission in the service manual is that it neglects to mention
after adjusting Osc1 on each individual board, you also need to turn on
adjust the Osc2 section to match the tuned first voice.  This *minor* detail
was in the back of my mind, but I figured that since the service manual
doesn't address this, Osc2 must somehow be slaved to Osc1 on each board and
it would magically correct itself.  After lugging my XA up two flights of
stairs (why oh why didn't I test this on the bench) I heard miserable out of
wack tuning when patching with the two Oscillators per voice.

Another thing I noticed is that calibration of of the Pitch/Mod section is
vital to the proper tuning of the voices.  If the pitch flapper isn't
calibrated correctly the slightest movement can throw the basic pitch off
(including breathing on it too heavily).

Oh and the molex connectors (used extensively in this board) are a breeding
ground for bad connections, I was offered a few suggestions to clean and
reseat (ideally replace) those.  The dip switch that turns voices on and off
(next to the two test switches) also seems prone to oxidation.  So if one of
your voice cards fails to light up, try switching the dip key for that card
on and off a couple of times... should clear it right up.

Thanks for all your responses.  Hope these observations can be useful to
others.



>
> 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