Fulltone GT-500 Issue

[drummer4gc]

Hey all, I've got a malfunctioning GT-500 on the bench that is giving me a hard time. I'm hoping if I describe some of what I've observed, a brighter mind than mine might have an idea or two for me to try.

The issue is with the Boost side - the volume knob crackles like hell and doesn't give consistent/expected volume levels through it's rotation. Replacing the pot did nothing.

There is no available schematic for the circuit on the net, but it sounds like there are similarities between it and the Fat Bastard/Boostered:



Unfortunately, the circuit board is multi-layered and has internal traces that are nearly impossible to follow, so it's been a lot of guess and check with a DMM. A difference I have noticed is that the volume pot is 1M Audio and appears to connect the output to the gate of a fourth JFET - I'm guessing a gain recovery stage, but that's approaching the edge of my knowledge around boost circuits. The signal traces fine before the pot and sounds fine coming out of the pot if not connected to the JFET, so I'm assuming the trouble is here.

If I'm correct that I'm basically working with a Fat Bastard-type circuit with a JFET recovery stage after the volume pot, are there any ideas for what to check out next? Faulty/leaky cap putting DC on the pot maybe?


Thanks for reading!

[PRR]

DC voltages on pot. (Better all be exactly the same.)

[drummer4gc]

The pot is disconnected right now, but I'm measuring almost full battery (~8.5) on the wiper's pad on the board. Pads for the pot input and ground measure 0.00.  Wish I could make an inference from here but I'm not sure...faulty jfet?

[drummer4gc]

Must've been a bad jfet - works like a charm with a new one.