sketchy volume pot - marshall mg30cfx

[metalmania408]

just picked up a used mg30cfx.   great sounding for a mess around amp.......when the volume stays put, that is.   it seems that the volume on this thing wants to gradually fade out.   

I notice when the volume fades, I can touch the vol pot (there's also a master vol pot - not that one) and the volume jumps back to it's current level based on the position of the pot wiper.   the volume issue happens regardless of playing the amp through the speaker or the headphones.

I did a visual inspection of the inside, the pcb connections/joints seem to be solid; nothing loose.    for certainty, I  re-wet the solder on the vol pot jugs.  still no go.   I then went ahead and sucked up the old solder on the pot and applied some new solder, making new connections.  continutity test with my meter are good, but still a no go a properly functioning vol pot.

I tried sourcing a new oem vol pot (linear 10k ohm), but it seems like they're hard to come by (as far as oem).     the pot has three lugs up front and two at the side/rear.  the rear lugs look like they might just be there for physical support of the pot when mounted to the pcb.

I may have to find a "good enough" replacement for the pot, or see if i can salvage the existing pot first.   actually, i may do this first.   if i can fix it, great.   if i wreck the pot, well....it's already malfunctioning.  again, something in the pot is touchy.

any other/better recommendations on my approach to fixing this is greatly appreciated.

by any chance, does anyone have the schematic for the mg30cfx?   i sent an email request to marshall this morning for a copy, but i'll have to wait and see if they can help.

anyways, thanks in advance for any suggestions.


EDIT:  no go on the schematic from Marshall.   Apparently, schematics are only given to authorized service centers.    This is why i like Peavey and Fender.   Great support model.   I guess I'll try to rig something up in order to salvage this little mg.   No more Marshall for me though.  Support is lacking in my opinion.

[Roly]

Chances are its got a dirty track.

Sealed pots are less likely to get dirt in them, but when they do they are harder to clean.

What I would do is remove the pot form the board, closely inspect it to see how it is held together (most likely by that U-saddle) then very gently try to ease it apart.  Once open give it a good wash with metho or Deoxit (being careful to avoid mobilising any grease on the shaft), work the wiper on the track if possible, then reassemble and restore.

HTH

[Enzo]

In the photo, the pot is vertical.  So sitting like that, spray some Deoxit down along the plastic shaft.  That should cause some to ooze on inside along the shaft through the bushing.   Work the pot shaft back and forth to spread.


Schematic?  Well, if it is a bad pot, the schematic won;t help you anyway.

[metalmania408]

thanks Roly and Enzo.   I failed to mention in my original post that i sprayed the pot with some deoxit.  that didn't seem to do the trick.

i went ahead and removed the pot from the pcb.   the pot is disassembled, sprayed with deoxit.   i'll put things back together the next day and see how things are. 

thanks for the help.

[Roly]

Good one.   :tu:

This stuff is just tedious ain't it?  But I know no other way.  Here's hoping, fingers crossed.