Comprehensive Drive stopped working, super buzzy/fuzzy.

[alsiv23]

Hi, my Comprehensive Drive build worked well at first, but after a couple days it became a little buzzy/cheap sounding at even low drive and now is a complete fuzz-fest even when drive is at its lowest setting. The signal is clean entering the IC, and coming out super distorted. All components between IC input and IC output are outputting fuzz signal, including potentiometer in and out, as tested with an audio probe. I have attached photos, showing the pedal with an audio probe temporarily installed. Does anyone have any suggestions, advice or insight? This is my third pedal build, after building two Propolis Fuzz. I'm curious why it would start fine and then become messed up. Maybe there was a short that ruined the chip itself? I have a multimeter but I wouldn't know what exactly to test to see if this is the case. Thanks ahead of time!

 

[Brett]

What voltage do you measure on each pin of the opamp?


It's difficult to tell from the pictures, but which IC did you use for IC1and where did you source the part?

 

[DailyDovetails]

I might check the resistance between legs 1 and 2 on the IC. It should go up and down when you adjust the drive pot between maybe 10k and 500k as a guess.

 

[alsiv23]

What voltage do you measure on each pin of the opamp?
View attachment 84466

It's difficult to tell from the pictures, but which IC did you use for IC1and where did you source the part?

Thanks for responding! The IC is a Texas Instruments RC4558P from Mouser.

My readings are:
1 - 7.4
2 - 7.4
3 - started at 7.4 then gradually kept going down 0.02 at a time. I let it get down to 2.8 before I stopped testing. Pin 1 and 2 voltage always matched pin 3 after I started bleeding (if that's what was going on).
4 - 0.0
5 - 3.56
6 - 4.56
7 - 4.56
8 - 8.9

 

[alsiv23]

I might check the resistance between legs 1 and 2 on the IC. It should go up and down when you adjust the drive pot between maybe 10k and 500k as a guess.

Thanks for responding! My resistance is jumping around wildly, even when just testing a single resistor out of circuit. Something must be wrong with my multimeter. I'll see what I can do about getting my hands on a better one and will post my findings here. This one was free from Harbor Freight.

 

[alsiv23]

Thanks for responding! The IC is a Texas Instruments RC4558P from Mouser.

My readings are:
1 - 7.4
2 - 7.4
3 - started at 7.4 then gradually kept going down 0.02 at a time. I let it get down to 2.8 before I stopped testing. Pin 1 and 2 voltage always matched pin 3 after I started bleeding (if that's what was going on).
4 - 0.0
5 - 3.56
6 - 4.56
7 - 4.56
8 - 8.9

After taking these readings the pedal is operating correctly again. Any ideas what could have happened? The distortion was truly intense before. Could I have bled some electricity from the circuit by testing the IC?

 

[DailyDovetails]

After taking these readings the pedal is operating correctly again. Any ideas what could have happened? The distortion was truly intense before. Could I have bled some electricity from the circuit by testing the IC?

What I was wondering was if something was wrong with the drive pot or one of the adjacent resistors. That part of the circuit sets up how much gain the IC will have. The higher the resistance the more gain it will have. If there is a bad solder joint or pot in that area it can increase the resistance, setting the gain too high and cause clipping.

It’s possible you moved the component to where it was making good contact again. I would try giving things especially the drive pot a jiggle and see if you can recreate it.

 

[Brett]

My readings are:

Your voltages on pins 1-3 seem off, but it's early and it's possible I'm entirely wrong. I'll measure the voltages on my build to confirm.

 

[Brett]

I just confirmed, your voltages on pins 1-3 are wrong. You're looking for 1/2VCC on those pins. Something has gone awry with your 1/2 supply bias network (R15, R16, C12). I'd check closely around those parts to make certain you don't have a bridge somewhere.