Obsidius preamp issue

[paulharvey]

I built this a while ago, worked fine for a long time. Now it seems to have very low output. The bypass signal is louder than with the pedal engaged and the level and gain cranked. Anyone else experience this?

 

[DailyDovetails]

My guess is somewhere there is a short going from positive to ground maybe a small portion of loose wire somewhere. I would also probably clean up the board with some isopropyl alcohol and a toothbrush as it looks like there is a lot of flux that might have some stuff entrapped in it.

Other than that my guess would be maybe an OP or transistor. I would check that you have voltages on OP amps and transistors. I don’t know what they should be but on the OP amps one leg should have nothing from ground 1 should have around 9V from ground and the others should have something in between. The transistors should have different voltages that are greater than 0 on all the legs from ground.

Someone else will probably pop in that has less general advice. Check the wires around the foot switch for shorts also.

 

[DailyDovetails]

My guess is somewhere there is a short going from positive to ground maybe a small portion of loose wire somewhere. I would also probably clean up the board with some isopropyl alcohol and a toothbrush as it looks like there is a lot of flux that might have some stuff entrapped in it.

Other than that my guess would be maybe an OP or transistor. I would check that you have voltages on OP amps and transistors. I don’t know what they should be but on the OP amps one leg should have nothing from ground 1 should have around 9V from ground and the others should have something in between. The transistors should have different voltages that are greater than 0 on all the legs from ground.

Someone else will probably pop in that has less general advice. Check the wires around the foot switch for shorts also.

Reflow any solder joints that look suspicious after cleaning.

 

[Erik S]

TL072 with the little divot at the top might be questionable according to this thread:

TL072 vs "TL072"

I did not want to hijack the "TL072 substitute? (They're expensive now)" thread, hence a separate topic. Short visual and audio comparison between fake and genuine TL072 chips. Simple setup, one inverting stage. Genuine vs fake and LM358 for a comparison. Here some oscilloscope images...

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