Vintage Synth Repair

Hi
 
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.
 
Regards
Brian
  _____  

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Malte Rogacki
Sent: Saturday, 10 May 2008 14:43
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] The ARP Omni and the infamous tantalum caps
 
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@gacki. <mailto:gacki%40gacki.sax.de> sax.de
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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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