Hello, TLDR; is there an easy way to test to see if an OTA is broken?
my MS–20 filter cut out after working for a moment, and now the only way it produces any sound at all is touching anything between the two pictured resistors with my finger.
the filter isn’t processing any external signal, and the cv and cutoff flat out do not work. the resonance (with finger applied) does change by turning its pot, but the pitch is fixed since the filter appears to be totally closed.
The TL074 works, I tested it with the blinky light op amp tester. I tried with another LM13700 as well but it produced the same results leading me to believe it’s not broken and somehow something magically shifted causing the problem (that I haven’t found thru hours of testing) or my current configuration is chewing thru OTAs (I hope not).
Is there a circuit or a multimeter setting I can use to test to see if the OTA is broken? Thanks.
If it behaves similarly with another 13700, I’d assume they’re are fine.
I suggest to start with the signal path and go through the circuit. Measure voltages at each node and think about it, if it makes sense. Feel free to post the schematics with the measured voltages, it’s easier to help you out if you are stuck!
If a finger touch has auch a huge effect, I would guess it’s a broken solder joint or maybe copper leftovers from cut traces but it can also be related to the transistors which easily open with a bit of voltage dropped over your fingers which touch some source…
I didn’t have the components for the VCO but it led me to locating the example VCA from the datasheet, and it appears that my OTAs are in nice working order, and I know how to build a quick dirty VCA!
Will keep the VCO design in the pocket for future use though.
As for the main issue, it could be as simple as the IC popping out of the breadboard.
