Dirty Knobs?

[skweeks]

my marshall jcm 800 lead series head is causing major problems... the master volume and i think the preamp knobs are the problems, the volume knob is "unstable" i guess, like i can turn it up to 2 and it'll be cranking then i go a little higher and it cuts in volume and i can goto ten and it won't get any louder... and drops in volume occasionally once i do get it set.. someone, who is somewhat experienced, although not in the repairing of amps said that it was "dirty knobs" (that just sounds odd...) if it helps... does this sound right? and if it is how do i fix it, or how much will it cost for parts and service at a reasonable store/maintnence shop?

 

[gaffa]

Hmmm....Interesting.

Sometimes the knob turny bits (properly known as potentiometers) get crap in them (dirt, dust etc.) This can be cleaned out reasonably easily by getting some electrical component cleaning fluid from an electrical parts store (I don't know, maybe RadioShack would have some, although the Australian equivalent, Tandy, doesn't carry it).

Spraying that into the pot (turny bit) will clean it. This approach usually works best if the knob is crackling as you turn it.

If it's changing volume randomly, it sounds like there something wrong in the amp circuitry itself - it's probably one for a repair technician. You could open the amp up and see if anything is fried, but fixing it still requires someone who knows electronics

- gaffa

PS. If you do get the cleaning fluid, make sure that it is 'non-conductive' fluid - there is a whole range of electrical cleaning fluids on the market, so its easy to get confused

 

[Daddy-O]

This may be a problem with the volume pots(defective solder joints)and cleaning may help, but I would take the amp in for service. My best bet would be that there is a flakey power resistor and or bad connection on the circuit board. Worst case would be an output transformer startind to go bad. Just keep in mind that most electronic problems are simple to fix. You can most likely get this repaired for less than $100. I would guess 1 to 1.5 hours bench time and a few dollars in parts

 

[Even]

Its also not extremely hard to replace the pots. If you are mildly handy with a soldering iron its a quick and easy job. And a 250-500ohm pot only costs between $3-$8