[sdiy] Prophet 5 Repair Questions

[elmacaco]

Hello everyone,

An old friend of mine has a Prophet 5 that was giving him some trouble and
so remained unused in a case for years.

We switched it on, and it was working for a while but engaging osc B made
patch 11 stop making sound, but a patch change fixed it.  One of the
transformer screws was missing, (that had a ground wire and nut supposed to
be attached to it in some rubber cement) so we fitted a replacement and the
weird drop outs stopped happening.  This also stopped the transformer buzz
vibration from happening.

We happily played it for about 30 minutes, then all audio ceased.  When we
cranked the amp, we could hear weird noise like sound and the patches were
weakly audible underneath it.  The filter did not affect the weird noise.

I clearly suspect the audio output vca, not the Voice VCA's, since the A440
is also quiet and I have come across this a few times.

The output circuit seems pretty simple, it has a TL072 as a voice summer and
the other half taking in the Volume pot's CV (we swapped the pot to see if
it was causing trouble, it wasn't the problem, although when you moved it
you got weird reactions in the noise.  When it was working it wasn't crackly
at all, but interesting to notice the synth stopped working while I was
adjusting the volume knob...)

(you can see the schematic in question here, it's on the bottom of page 48
in the PDF)

http://www.hylander.us/images/schematics/sequential/Profit5v3TechManual.pdf

After the 072 it goes to a 3280 (only half of it used in this circuit) which
does I don't know what, then it goes to a NE5534 which is at the final
output and seems to be a buffer before the audio out jack.

Now assuming one part can cause all this chaos, I am suspecting a capacitor
of some sort (lot's or Tant's in there and one elco in this circuit) that is
causing an output amp stage to oscillate (TL072 style?)which is the crazy
racket we heard, assuming that this oscillation behavior would prevent the
amp from amplifying, it would point to some associated cap around the 72 or
the 3280 perhaps.

Where the volume CV comes in is a bit weird, looks like a buffer with a
filter network before it of some sort, and it has a transistor at it's
output 2N4250 which I am not sure what purpose it serves or if it could be a
cause of the failure.

My chief suspects right now are capacitors, but I am unsure as to what a bad
cap would look like on a scope.  I guess I could use continuity testing on
the tants, since they conduct when they fail (right?)

I didn't have my scope with me, but next time I am over there I will bring
it, and any guidance from the list would be most helpful.

Humbly yours,

Ed