TSL 60 Channel volume problem

[chungaleta]

Hi mates, this is my first post here, I hope I can get help and help others too.
I have a JCM2000 TSL-60 on the bench, the individual channel volumes for both od1 and od2 channels don´t work at all, the master volume and the clean channel volumen do work perfectly. There are bo noises, or lack of distortion or gain on the od1 and od2 stages. Just the Volume knobs not working at all.
I cleaned the whole amp which showed terrible signs of humidity, checked both volume pots and they meassure correctly, however, they don´t do a thing.
They´re stuck at full volumen. General Amp volume being controlled only by the Master Volume knob.

At first I checked if they were chained in some sort of series link where the failure of one would hinder the other one from functioning, but that is not the case. They´re mounted on two separate little PC boards and linked by a wire chain that was checked and passed the test. So I dismounted both pots and tested them both to make sure they´re ok, they are.

There are no resistors blown, no shorted caps and no apparent malfunction in any of the adjacent circuits. Both channels sound perfect and when changing from od1 to od2 and vice-versa there are no noises, popping sounds or lack of gain in any of the two.

There are a couple ICs in the circuitry that I thought would´ve shorted due to voltage arching produced by moisture, but both of them are linked to the master volume line. So, If the master volume works fine and the clean channel clean and crunch independent volume controls work fine, I guess those ICs are not shorted.

I performed other maintenance work on it, cleaned all boards and removed the moisture tracks, re-valved it, tightened pre-amp socket pin-recievers, changed one octal socket and re-biased.
The volume problem existed when I first got the amp. I thought i would disapear right after the other jobs were done, but it didn´t

I´m puzzled

Cheers!

[paddy]

Hi there,

I had a quick look at the scematic and it seems as if some of the volume pots are grounded via FET's in the switching circuitry.
You might like to look in this area.

Hope this helps.

Cheers,
Paddy

[rfgordon]

If the amp was damp, then corrosion under the copper traces could cause them to break--thus opening the circuit. You might have to check each PCB path for continuity.

[selloutrr]

these are tricky amps to work on, everything can look and test good but still not work as a whole. truely a gremlin! try the two suggestions above if that doesn't solve the problem turn the lights off and see if you can visually see arching on the circuit board. I've had 3 on the bench that had traces that were to close and needed to be hand wired to bypass the poor quality construction of the PCB. (assuming you discharge all caps and the amp is not plugged in) I've noticed a lot of variation in the production on the TSL and DSL models, and found they are built with under spec'd parts that lead to failure. While you are in the amp and have it tore apart if you haven't already done this go threw with the schematic and make sure each part is to spec and the correct voltage. If you replace the IC's keep in mind they don't take heat at all!!! Some have stand off's installed from the factory so you can pull out the IC and swap. If the IC's on the amp are directly soldered to the board, solder an IC holder in first or the IC will burn from the heat while you solder. (general caution tip, not saying you didn't know that)
best of luck!

[chungaleta]

Problem solved mates.
Bloody FET was the culprit!

Thanks a lot mates!!!
This one got me thinking for hours.

Cheers