Vintage Synth Repair

At 16:07 Uhr +0100 10.05.2008, Brian Davies wrote:
> I've been watching this thread with great interest.  One thing no one has
> mentioned is to what is the negative end of the tant cap returned?  If it is
> the 0V rail then it would appear as though at times the positive end of the
> cap sees a negative voltage, this can't be so can it?  If however it is the
> -15V rail then at times the positive end of the cap will see +30V - not
> conducive to long life for a 25V component.

The negative end of the cap sits at -15V.

I hate to admit that I somehow messed up my measurements (or actually: I
did them at the wrong place; I measured them at the sustain buss but should
have done them directly at the caps). The voltages seen by the caps are
actually a bit lower.

Everything I wrote before is basically correct, with one exception: The
sustain buss and supression trigger have to pass a 3.3k resistor, a diode
and another 470R resistor before the cap.
This results directly at the caps in 7.x volts for short release and 0
volts for long release (the sustain buss is indeed at +8/-15 instead). This
also means that the spike from the supression trigger is not at +15V but at
around +8 or +9V (definitely under 10V) for the shortest release time.
Which in turn means that the max voltage difference is indeed around 23 to
24V at the cap. Which is uncomfortably close to the rating of 25V but
within it.
Is this good engineering? Probably not.

-- 
Malte Rogacki gacki@...
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"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)
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