Error in OB-8 service manual

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Tue Apr 23 03:10:00 CEST 1996


Hi!

Someone has asked me about this, and I didn't know an answer back then,
though I had encountered the same problem as well: It's about the envelope
time calibration in the OB-8.
In the manual it says that you have to adjust some voltages at some pins
to 0.000 volt (in calibration mode). This is wrong. In my OB-8 (as well as 
in
at lest one other one) you cannot even reach 0 volts, so what I did so far
was matching the voltages for the whole synth at least.
Now I found out that I had ended up with far too slow envelopes (I often
asked myself: Why is the attack so tame in spite of hardware envelopes?)

You get the fastest envelopes with 0 Volt at the control pins of the 
CEM3310's
(higher voltages are not allowed). With 0 Volt at the control pins you get
a *slightly* slower time than in a Prophet for example, but the difference 
is
minimal (the Prophet uses a smaller capacitor).
The point is, that 0V in calibration mode do *NOT* corespond to 0V in
normal running mode !!!

I did some iterations: Setting some value in calibration mode,
storing it (important!), then switch to nornal mode, turn attack pot
fully ccw, and measure the actual value.
After some iterations I found that at -250mV in calibration mode I had
the desired 0V in normal mode. This was the right value for *all* time
constants in my machine, so probably it's just a software error and
applies to all OB-8's. At least, the -250mV would be a good value
to start with.

Yes, my OB-8 can say "JUMP" now. (;->)

JH.



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