[sdiy] despicable delta destroys detective's day (VCF insanity)

[Bob Weigel]

This tops 'em all.  Ok... I'm using a Delta I repaired months ago the 
other night to give it a test and avoid taking the one out of my 
studio.  It crackle-pop-fizzles within a few minutes of use.  I opened 
it and wiggled/tapped and it seemed to have some mechanical correlation 
but I couldn't isolate it.  One of *those*....

So back at the ranch..er...bench...I look and find that the signal is ok 
up past the first ota stage.  Noises happen in the signal at the inv. in 
of second stage..which..means nothing since there's feedback there so we 
can't tell where it's originating from that of course w/out some deeper 
evaluation.  I *assumed* it's probably just the 13600's and changed them 
both as I've done on several units with success.  This time..uhuh.

I re-soldered everything in the area.  Still no result.

Now when it's failing...it sound kinda like a resistor going open type 
effect.  And indeed when i finally get it to cut in and out regularly 
enough to see what is happening... the offset voltage at pin 5/7 I think 
it is...before the buffer...is zigging from 1.1V or so offset to near 
zero when the signal gets attenuated to a noisy low level.  NOW what 
resistors could affect that?  Well the 10K feedback, the 10K input or 
maybe the 15K or the 220ohm ones.  Replaced them all....no result!  
Still exactly the same. Usually plays a few seconds when you turn it on 
as you hear the noise gradually coming and then BONK....or it would go 
into a rapid 'switching' between working and not working ... anyway....

I'm like..well..there's only one component (after changing the OTA a 
second time...) that we havent' done ..the .001 uF mylar cap which 
showed NO particular corrleation while squeezing or twisting it.

I pulled it..put the diode checker on thinking it would pick up a leak 
if it was leaking since mine goes up to 3. some volts.  Showed no 
issue.  Anyway I put a new cap in anyway... and it is working now.  I 
haven't heard it crack up once in hours now.   Has anyone EVER seen a 
failure mode of this type with what appears to be a mylar cap?  -Bob

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