Crackle Not Okay - Fulldrive 2 Clone

[r080]

I built a clone of a Fulldrive 2 with a GGG TS board over 5 years ago, and it has started to continuously crackle like a bad pot. I am hoping to get some ideas on what the cause might be.

-The crackle is not caused when turning any pots
-I put a new power jack and new 1/4 jacks in, no change
-There is no change when tapping on any components or pulling wires. I could not get any change when wiggling the socketed components pretty hard.
-I inspected it under a microscope but found nothing that looked cold. Just to be sure, I reflowed about 90% of the board.
-It crackles powered by battery, 1Spot, and Cioks DC7

The build is pretty much like this without the diode switching:
http://1.bp.blogspot.com/-6Jl-1PWqSDU/VYplag3p2qI/AAAAAAAAAZc/78LbbwXLxLQ/s1600/fulldrive.BMP

Right now, I don't have any way to post images, unfortunately.

Also, what is the purpose of the big cap across the boost switch? As far as I can tell, it could only possibly do anything while the switch is in the middle of its throw. It looks like it would not make a pop because of the resistors, but why is it there?

[ThermionicScott]

How about different guitars and cables?  Amps?

[anotherjim]

Have you used tantalum caps for C8 & C9? Are you sure you have them the right way around? Polarity can be unclear on some tantalum packages. I don't know why polarized tantalum are specified since 220nF is easily had in non-polar film types. Anyway, I don't trust tantalums in audio paths.

C6 shouldn't be a problem. If the build is correct, it will have "VEE" at both ends so shouldn't pop or suffer leakage noise.
Have you used the 1uF NP types for C3 and C10? If R4 increased to 100k, C3 could be 100nF film. C10 could be a 220nF film.

[r080]

How about different guitars and cables?  Amps?

It happens with different guitars and amps, as well as with the input shorted.

[r080]

Have you used tantalum caps for C8 & C9? Are you sure you have them the right way around? Polarity can be unclear on some tantalum packages. I don't know why polarized tantalum are specified since 220nF is easily had in non-polar film types. Anyway, I don't trust tantalums in audio paths.

C6 shouldn't be a problem. If the build is correct, it will have "VEE" at both ends so shouldn't pop or suffer leakage noise.
Have you used the 1uF NP types for C3 and C10? If R4 increased to 100k, C3 could be 100nF film. C10 could be a 220nF film.

No tantalum. Film 1uF caps for C3 and C10.

I realize C6 isn't causing a problem, I am just curious what the point of it is.

[r080]

I think probably my next step will have to be an audio probe starting from the beginning to see if there is a specific point where it starts.

I have been using this pedal frequently since 2013 and it only started having problems this winter.

[r080]

Using an audio analyzer on a tablet with some creative adapters, I looked at the transistors and opamp pins. I figure an active device failure could be the cause, given I can't make the noise change with physical manipulation of anything.

There is definitely noise at the output of the first opamp. Because there is no gain at the opamp's first input, I couldn't tell whether anything was there. I then swapped the input and output buffer transistors, and the noise seemed to be unchanged. I will try to replace the opamp next.

[r080]

After some more troubleshooting, I think I was able to eliminate just a few components as the cause. I swapped the opamp, still crackles. I removed the input buffer transistor (so opamp input pulled to ground), still crackles.

[antonis]

I removed the input buffer transistor (so opamp input pulled to ground), still crackles.

It needs to be connected at VEE..

P.S.
Try to delete R10..

[r080]

I removed the input buffer transistor (so opamp input pulled to ground), still crackles.

It needs to be connected at VEE..

P.S.
Try to delete R10..

Removing R10 was no change. In the process of verifying I had the opamp input at VEE, I noticed that is at 2 volts, rather than ~4.5V. Supply was 9.5V.

I clearly have more work to do. Since it has been working great all these years, I don't remember ever checking voltages, so I have no idea whether this is new due to something I screwed up during my troubleshooting.