Xpantastic!

Hi Terje,
  
>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.

These are analog switches which multiplex the master DAC voltages to provide 
individual sample and hold circuits for each parameter in a given voice. The 
resistor and capacitor networks provide a means of holding the CV voltage while 
the multiplexer moves to the next parameter.
Analog switches are notorious for becoming leaky and failing, but if you have 
switched these out and the problem still exsists, then the problem is somewhare 
else.

>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).

I see the Straps on the voice board schematic below the 6809 CPU 
It appears you will have to move these in order to " switch voice boards"
That looks like the best aproach to isolate the problem to at least 
" which board is failing"

>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.

These are older technology digital and anlog integrated circuits and they do 
tend to run on the warmer side.
The CPU and memory chips are running at maximum speed so they will run hot. One 
thing to remember is if you place your finger on an IC you are adding heat to 
the circuit and if you hold it there long enough ( more than say 4 secs) the 
chip will heat up.

Unless we are talking about power supply components (ie Voltage regulators), If 
they are burning your finger then yes, it's probably sourcing to much current 
to the next stage it is connected to.

One common trouble shooting technique is to spray "Freeze spray" onto the chips 
one by one until the circuit works.It's an easy way to isolate a heat problem
We can still get this in the US. But not sure about Europe. Usually obtained 
from an electronics supply house.   
Note: Don't spray this on the VCO/VCF chips because thses are temperature 
sensitive, and obviously it would change the tuning.