excessive feedback?

elmegil · Posted: Tue May 15, 2018 10:48 pm Post subject: excessive feedback?

I'm troubleshooting someone else's build of one of the Benjolin PCBs I did a run of a little over a year ago.

I've built and troubleshot a fair number of these over the course of the last year, including troubleshooting several from a workshop at Knobcon in September. But I haven't run into this particular problem....

When the resonance is all the way up on a "normal" build, it's still just shy or just at self-oscillation.

When the resonance is all the way up on this particular build, it's just mad oscillation swamping out everything else.

There's a ton of rosin flux all over, but it's an "all in one" build (pots an jacks on one side, chips resistors caps on the other). So I can't see my way to doing a thorough cleaning of the PCB without risking damage to the pots in particular. I have done some scraping with a small wire brush in a few places, but that hasn't seemed to make any difference.

I thought I saw some high frequency oscillation on the op amp feeding the resonance pot, so I tried a small cap across the feedback resistor, but that doesn't seem to have any impact.

I've done some a/b comparisons of various component values, and the only one that really sticks out is the 100K resistor feeding back the bandpass to the CV inputs. On my "good" build, measuring in circuit, I see about 85K. On the "bad" build I see 99.7K. Everything else I've measured, resistances and capacitances, the in circuit values between my board and the one I'm working on are within 10% of each other, usually less.

Wondering if anyone has run across this particular symptom before.....

This is the last of several problems with this build and I'd like to be able to declare success, but the behavior of the resonance is really bugging me.

Grumble · Posted: Tue May 15, 2018 11:01 pm Post subject:

- Are the used opamps the same as the good build? Because I have noticed that some opamps are more susceptible to oscillation as others and need a decoupling capacitor close to the power pins.
- When placing a capacitor over the feedback resistor, the feedback signal increases so the opamp will oscillate sooner.

elmegil · Posted: Wed May 16, 2018 5:29 am Post subject:

elmegil · Posted: Wed May 16, 2018 5:31 am Post subject:

Well, C/I shouldn't be the issue:

Grumble · Posted: Wed May 16, 2018 6:46 am Post subject:

elmegil · Posted: Wed May 16, 2018 6:50 am Post subject:

Replaced the op amp and the bypass caps, just in case, and it seems a tiny tiny bit better (a little more range on the resonance pot) but still basically the same problem.

Considering just going ahead and replacing the other op amp used in the filter (A TL074) for good measure just to rule it out, given the power problems this one has had in the past.