Xpantastic!

> at this point its a process of elimination,
> working backwards from a known operational
> point and ruling out all factors along the way.

Yes, this is what I am trying to do.
There is sound from both voice cards. All 12 voices sound well, and  
after tuning one card is perfect, while the other one is almost in  
tune, but not quite. Some VCOs are off by a few notes, and some VCFs  
are audible different, but not much.
With the scope I can follow all the sound paths, and I can follow the  
sound path of the tuning. All good.
I have also followed all the digital control lines to see what they  
are doing, and they all seem to come through.
I have swapped a lot of ICs between the voice cards, one by one,  
checking to see if there is any change. Still no change.

It seems to me to be a systematic error somewhere that eludes me.
There is a network of 6 sets of DACs around UX06 and UX07, leading  
through a resistor network to UX08 before going to the VCOs, VCF and  
VCA, but there is one set of these for each individual voice, and I  
find it hard to believe the the CV control for all voices would fail  
at the same time. But it is interesting, because the PW and Resonance  
passes tuning each time, and they uses another DAC (U816) that is  
common for all the 6 voices on the voice card.

But as I now have swapped all the ICs in the critical positions  
between the card to no avail, I am not sure what to do next. I could  
shift the position of the ID strap. How many possible position are  
there for that thing? (It is really just a jumper).

BTW: some of the ICs gets fairly hot. I have noticed that in other  
Oberheim synths as well (almost burned myself on a OB-8 once). Is that  
normal? I have checked power many times, so all power supply voltages,  
frequencies and reference internal voltages are correct, and so far I  
have found no short circuits. But still: they are somewhat hot.





Terje Winther
terje.winther@...
http://www.wintherstormer.no/