[sdiy] OB-SX auto tune problem

[Magnus Danielson]

Matti,

On 09/24/2010 05:32 PM, Matti Kari wrote:
> Hi,
>
> I recently acquired an Oberheim OB-SX for a good price. It is the last
> model built with 56 presets and in perfect condition externally &
> internally, except that the autotune sometimes (say: usually) fails on
> some voices, except that voices #1 and #2 never fail and the voice #6 is
> the most unreliable.
>
> I have already verified that the problem is not on the voice cards
> themselves by swapping them around - no change. Also I've changed the
> 4051 and TL082 chips that generate the (I assume) tuning voltages for
> the oscs - no change.
>
> What is interesting and may have something to do with the problem is
> that now that I calibrated all the voice cards (auto tune switched off,
> of course), the auto tune tunes them almost exactly a whole step higher
> (i.e. playing a C key plays D), and I need to compensate this using the
> master tune pot.
>
> Any ideas? I've already calibrated the synth according to the service
> manual and all the voltages are what they should.̈́

Check the signal level from the various voices as they appear to the 
counter path. The counter needs to get sensible signal back from the 
VCOs in order to count them properly and tune. Poor signal quality can 
cause extra transitions or dropped transitions... and autotuning goes 
haywire...

I know more or less how the OB-8 does it, so I imagine the OB-SX does 
something quite similar. If somebody can give me the service manual for 
the OB-SX I may be more specific.

Cheers,
Magnus