Distortion/weirdness in older Shruthi - FIXED

My original Shruthi (v0.5 control board, v0.5 SMR4 v1 filter, v0.97 firmware) started acting up recently. I have been swapping filter boards in/out of it in recent times, but when I reassembled, I now get a different, somewhat “distorted” sound coming from it. Overall, the audio coming out of it is louder than before, and several patches don’t sound “right.”

For all I know I have some setting that I’ve forgotten about wrong. I did try to go in and muck about with the resonance trimmer, so that is likely off, but I don’t think it can account for what I’m hearing. I’ve made several audio demos comparing my correctly working Shruthi (in my Programmer). The filter you’re hearing in that is an IR3109, but again, I don’t think it can account for the differences I’m hearing. Here are some stock patches - the first note is played on the good Shruthi, the second is from the ailing one:

bacon
cavemen
flateric
flo bass

Any ideas for where to start looking?

I would say a gain problem in the mixer…

As usual, signal chain tracing…

So I used a DIY audio probe, and hear a big difference in sound character and the inclusion of distortion between points 3 and 4. Does that mean the TL074 needs replacing? Sorry for my noobishness.

You can try swapping it with one of the other TL074s on the board to see if this changes anything. Check all soldering joints on IC7 and IC8’s sockets - and on all neighboring parts.

Swapping TL074’s - no change.

Reflowing IC7 and IC8 and surrounding stuff - FIXED!

Thanks Olivier