Vintage Synth Repair

After a somewhat closer inspection and some measurements I believe to have
gotten a better grasp on the infamous tantalum cap problem for the gating
circuits.

Observation 1: The caps are not operated generally under a +15/-15V voltage
difference. There are some errors in the service manual for the original
Omni (not Omni-2). As correctly described on the string control board
schematic the voltages are +8 Volts for shortest release and -15V for
longest release.
This also explains why comparatively (!) few caps fail - I guess in many
cases people will use very short release times; and if this actually would
equal 15V the caps would be permanently under a 30 Volts difference.
The +15V are indeed the sustain reference that goes to the string control
board; what comes back is between +8V and -15V; this is the voltage applied
to the sustain buss. So nominally there's only a 23V max voltage difference
on the sustain buss.

Observation 2: But that is not all that happens on the sustain buss,
though. There's also the suppression trigger. And - big surprise - this is
a (very short) +15 spike that is sent for each newly pressed key. Which I
guess is what in the end kills the caps.

Also interesting: The tantalum caps in the bass gating circuit are rated 35V...

There's something that I haven't completely figured out so far: The release
time depends on the number of keys held down. A single key will have a
shorter release time than a five-note chord. Or maybe this is another
problem...


-- 
Malte Rogacki gacki@...
----------------------------------------------------------------------------
"Don't forget to TURN ON THE SYNTHESIZER. Often this is the reason why you
 get no sound out of it." (ARP 2600 Owner's Manual)
----------------------------------------------------------------------------