[sdiy] Re: [AH] Prophet 5 Repair Questions

[klosmon]

Checking the tantalums is a good (& easy) place to start.

If that doesn't yield results, check back & we'll continue.

~GMM

At 09:29 PM 8/25/2004 -0400, elmacaco wrote:
>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
>
>
>