SOLVED UniCab not working

[Crowbar]

Hello all,

I just finished building a UniCab pedal, but it's not working. I get a high pitch noise and turning the knob changes the pitch of the noise. It sounds like when you are trying to tune an old AM radio

I'm a complete amateur at this, any suggestions where to start looking for the fault?

Thanks in advance

 

[Crowbar]

Are you sure none of your IC legs are folded under now? That could give you intermittent contact.

I think I have them correctly in the sockets, but Im also thinking one of them might have gotten damaged from being removed and inserted several times.

I have ordered 3 new ones, just to rule out a broken IC

 

[Crowbar]

I’m not very experienced with troubleshooting so take this FWIW…

Have you tried testing the PCB outside of the enclosure? I’ve had a few cases where a PCB worked prior to boxing, but had weird issues after. You might get some clues at least if it exhibits different behavior outside the box.

It’s curious to me that you see a lot of issues when turning one of the pots. It looks like you’ve got covers which is good but is there any chance contact is getting made somehow to the legs or case?

I was having trouble with a project one time and got weird behavior when turning a pot and finally figure out the pot was the issue. I think maybe I damaged it when bolting it into the enclosure.

Also, sometimes it’s good to take a break and come back to it later.

I have tried it both inside the enclosure and outside of it, there is no real difference except its a bit more noisy outside of the box.

Its one of the pots that has a strange behaviour, but looking at the schematic I don't understand how, it's just connected in series between two stages of the circuit

 

[Feral Feline]

I have tried it both inside the enclosure and outside of it, there is no real difference except its a bit more noisy outside of the box.

Its one of the pots that has a strange behaviour, but looking at the schematic I don't understand how, it's just connected in series between two stages of the circuit

So it's the B100k "TOP"?

Try running a test-signal (signal-gen/MP3/Looper/etc) through the circuit while you turn the pot slowly... If the pot is the weak-point and faulty, try a trimmer/other pot (whatever taper — anything), or even a fixed resistor-value to eliminate the potential problem-pot from the equation.


I tend to over-tighten things — I know I've overtightened a pot to the enclosure (possibly damaging it in the process) when it's difficult to turn the durned knob.

 

[Crowbar]

So it's the B100k "TOP"?

Try running a test-signal (signal-gen/MP3/Looper/etc) through the circuit while you turn the pot slowly... If the pot is the weak-point and faulty, try a trimmer/other pot (whatever taper — anything), or even a fixed resistor-value to eliminate the potential problem-pot from the equation.


I tend to over-tighten things — I know I've overtightened a pot to the enclosure (possibly damaging it in the process) when it's difficult to turn the durned knob.

Yes it's the "top" pot. I have only tried with my guitar as input, but by turning that pot you can sometimes find a "sweet spot" where sound is transmitted to the output, and you can also find positions where it just crackles, and other positions where it goes "beep"

My knowledge of circuits is not enough to even try to understand why this happens, given this pots location in the schematic

Another observation is that if I connect leg 1 of the pot to the output (discovered this by mistake) you get a loud constant "beep" sound. But not from any of the other two legs. This seems odd since leg 1 and 2 are connected according to the schematic.

So maybe this pot is broken. Or the PCB connections are damaged.

EDIT:
I was wrong, it's pin 3 on the pot that provides a constant beep if connected to output, so it would seem it's the first part of the circuit that is faulty.

I'm sorry for the quite confusing info here, it seems my testing is not ver structured

 

[Feral Feline]

Dinna fash!

This is how you become knowledgeable about circuits and begin to understand how circuits work — by fixing things that aren't working.

I'd definitely try removing the pot, and chuck in a fixed resistor value anywhere between 50k and the full pot value of 100k. If the problem is gone then you had a faulty pot. If the problem persists, then ... you keep hunting until you find the Gremlin and CRUSH it.

Real Crushed Gremlin

 

[Crowbar]

I am happy to announce that the problem is solved and the pedal is now working flawlessly.

It turns out one of the IC:s (TL072) must have been faulty, most likely broken by me fumbling them into the sockets.

I replaced all 3 IC:s and now all is good.

I want to thank everyone who offered help and encouragement, this forum is really great.

 

[Crowbar]

I will finish this thread with a photo of the final result, with slightly neater wiring