[sdiy] CA3080 VCF and mysteries

[TheMysticSatin]

Hello all, you might remember a post a while ago...
working on a CA3080 based VCF and the buffer op amps seem to lock up...
switched them from followers to integrators on Mike's suggestion and this
improved the sound of the one that was already working (I built two, one
works one doesn't) but didn't fix the one that wasn't working.  Voltages
were checked from on top of the IC's and all pins are correctly connected. 
First I observed that... the filter works okay at first but over the first
two minutes its been powered up the voltages climb and eventually pin at
supply.  Audio output (which is quieter and less sharp than on the good
filter) disappears.  While I was measuring voltages I noticed that when I
metered from GND to the CV pins of the OTA's the audio would return, so I
threw 1M resistors from GND to the CV pins of the OTA's and sure enough the
audio returned, but the Cutoff reference was way off...   

SO... here's the next update...
1. I've noticed that when the CV for the non working filter is positive the
output of the first 3080 goes way low, like down towards -15v.  And it does
it in a slow curve just like when the op amps do it.  It doesn't go high
when the CV is negative and none of the other OTA's do this.  I've replaced
this OTA with no change of results.  Although the OTA doesn't go high like
this when the op amps are pulled out and it's not getting an input.


2.  While I was measuring voltages I think I crossed the probes while
touching the CV pin of the above mentioned OTA and ground and I think I
shorted out the OTA at this point.  Suddenly no voltage measured at pin 5 or
on the opposit side of the resistor b/w pin 5 and the 3906.  I wondered is
this because the 3080 isn't drawing current any more because it's shorted
out?  Is that normal?  Anyway I replaced the 3080 again.



If any of this gives anybody any ideas please pass it on... this things
killin' me!