marshall TSL 100 scratchy gain pot

[xjbear]

I already cleaned the pot, took it apart and cleaned it properly

its the clean channel, it has scratchiness throughout its range except all full and all down of course, its the only pot in the whole amp doing it, I replaced c10 and c3 on the clean board problem persists and c8 on the tube board still no good, i'm getting 22ma? dc on the leg of the pot it looks like.
It could be coming from the switching fet's maybe, if someone has experience with this amp or a good idea please chime in, thanks

main tube board schematic
https://docs.google.com/open?id=1NWL...lriNscK8p3oI9P

clean board schematic
https://docs.google.com/open?id=1l_C..._q0nzOS9FSRvyG

[xjbear]

c16 on the tube board feeds signal to the clean channel gain pot through con6, ill try swapping out that cap tonite

[Enzo]

Is there any DC across the pot?

[xjbear]

Is there any DC across the pot?

Yes, I was getting some dc across the pot, I took the connector loose and put a .1uf cap in line with it and it was still scratchy, so my belief is the optos or fet 1 or 2 are back feeding something to the gain pot.

I did get good results cleaning the board with carb cleaner, reflowing all the joints and then cleaning the board again with alcohol and a fingernail brush, the pot is much quieter now but I still get odd noises once in awhile that sound just like 'silver mica disease' in old radios.

All leads me to believe maybe there is a bad spot somewhere in a trace that I can't find, I'm going to try and procure a new clean channel board, do you know how to get those parts? I have an email into marshall tech support

[Enzo]

You added a DC blocking cap, but after that did you measure to see if there was STILL any DC?


If DC is still present, then start disconnecting things until you find what is letting the DC in. If everything that is connected to that node is disconnected, THEN I'd look for surface leakage on the board. And depending upon where in the circuit, due to grid leak action, some tubes will generate a small negative voltage at their grid. You occasionally see this causing the volume pot IN THE GUITAR getting noisy when connected to the amp.

[xjbear]

C40 on the overdrive board has been removed, I wonder if that cap blocks dc from the switching system into the clean board, I don't have the amp at the moment so I can't do any testing or replace that cap but it will be back here next week

TSL100 overdrive channel front control PCB circuit diagram, TL10-61-02 Issue 4, (Marshall, 1998).
http://www.drtube.com/schematics/mar...98)%20iss4.pdf



Thx Enzo for giving this a shot for me, yes the pot still had some dc leaking in, it's an odd amp, since I own one now and the TSL in question is a good friends amp it's important for me to nail this one right

[Enzo]

Um... just what VR number control is the noisy one we are chasing? Where is C40 on which schematic? I can't see where you removed it from, so I can;t comment what it would do.

Don't focus laser-like on caps, you could have other sources of leakage current.

[xjbear]

vr1 on the clean board
http://www.drtube.com/schematics/mar...98)%20iss4.pdf


c40 is on the overdrive board, lower left corner
http://www.drtube.com/schematics/mar...98)%20iss4.pdf

[Enzo]

Oh. I was looking for things that might make a pot scratchy, I was looking in the signal path. Your C40 is just a cap across the transistor driving a relay coil. Or was, anyway. It serves as a timing tweak. If it were present, when TR1 releases, RL1 and RL4 would drop out instantly, except if that cap is there. The cap charge would keep the relay coils enegized a brief moment longer. WHy wold they want that? I don;t know. Perhaps during channel switching, some other relay didnl;t pull in fast enough and some noise happened until they slowed down these relays dropping. If it is not mounted, they may have decided it was not needed. Or it may be used on some other model that shares this board. Was it actually "removed," or was it simply not there?


VR1 is noisy? And you have some small DC on it? Then presumably removing the DC would remove the noise. That would be my operating premise until shown otherwise. So watch that small voltage. Unplug CON2, any change? Unplug CON6. ANy change? REmove FET2. ANy change? C3 can't put voltage on the pot that isn;t already there. Both ends of it are wired to the pot. Obviously we can;t run signal through the amp with those connectors off, and we cannot hear the pot being noisy, but if we consider the DC voltage as an avatar for the noise, we watch it instead of listening.

[xjbear]

Your comment about c3 is funny I guess I shoulda saw that, at this rate I'll be pro in about 50 more years haha

C40 was removed by me at one point to speed up the switching according to a mod list I found on the web, just wasn't sure it could make things noisy or not, I don't have much transistor/op amp experience, thanks for the direction Enzo