Replacing all the pots on a Marshall 80V 8080

[rockhorst]

A friend of mine has a Marshall amp that he thought was near dead. Turns out it's just extremely dirty pots with dead spots. I managed to get out the PCB in a decent way and thought about contact cleaner, but we decided it'd be cool to just replace all of them. I've got a couple of questions/sanity checks (it's been a while):

• Pots have three terminals connected to the PCB. What would be the fastest/best procedure of desoldering them?
• How can I cleanly get rid of the extra solder?
• What types of pots should I use? I have the schematic with all the resistance ratings, but should I take a powerrating or something in consideration too?
• Are there alternatives to standard pots? Can they be replaced with a sort of closed pot less prone to collect dust?

Slightly off topic:

• I know with that with a tube poweramp you need to drain the caps for save tweeking. The valvestate has a few (relatively small) caps as well, should those be drained? (Valvestate = tube preamp, solid state power amp). What do you use to drain them safely?

[thereverend]

• Pots have three terminals connected to the PCB. What would be the fastest/best procedure of desoldering them?
• How can I cleanly get rid of the extra solder?

2 questions with 1 answer. heat up the solder and then use a solder sucker to remove the solder around the pots you wish to remove from the board.

• What types of pots should I use? I have the schematic with all the resistance ratings, but should I take a powerrating or something in consideration too?
• Are there alternatives to standard pots? Can they be replaced with a sort of closed pot less prone to collect dust?

i dont know about closed or open pots. what i do know is that if i was replacing the pots on an amp i'd replace them with pots of the same rating, resistance and tolerance.

[rockhorst]

I've been out of this game for a while but just dug up my soldering iron and found the desoldering braid as well. Totally forgot about that.

About tolerances and ratings: how do you determine those? For instance, one of those pots has B22K and 2JA written on it. I understand the 22K bit. Do the other letters mean anything? The schematic is here http://www.drtube.com/schematics/marshall/pc0689.pdf by the way

[thereverend]

B would be the taper, linear in this case. i've havent got the foggiest what 2JA means. i'm sure someone more knowledgeable around here might know.

[anchovie]

I once replaced all of the pots on the head version of this (model 8100). You don't need to worry about power ratings or anything like that, the pots only have audio signal passing through them. With regards to alternative pots, I'd say the best alternative would be decent quality ones! The head that I fixed had the same problems, lots of dead spots and one pot that didn't even change value.

There are a lot of cheap components in those amps, all hyped up with the "it's got a valve in it!" sales pitch. They never advertised that there are clipping LEDs on the distortion channel, though!

[drewl]

It's a common repair on these, I probably do at least one Valvestate a month.
Get the schematic if you want to make sure of the pot values, or you can measure them.
They desolder and come out pretty easily with solder wick.

[rockhorst]

Did it, works like a charm. Although the pots were a little longer than Marshall ones so they had to be sawn down to size. And some knobs turn overly smoothly. And it still emits the smell of a bit of contact cleaner. But, no crackling, yaaaay.

[petemoore]

  Good job, about time it sounds like.
  That should boost the performance/reliability to like new.
  I haven't had any trouble with jack / plug contacts cleaning, they seem to stay clean enough .