Marshall 1987x making noise

[leewinn]

I have a 90's 1987x with an fx loop that makes noise similar to having the instrument cable plugged into the amp but not the guitar (sort of buzzing). The amp makes this noise regardless of whether a cable is plugged into the input or not. With vol 2 turned up to around 3/4 the noise nearly goes away. Turning vol 1 up at all makes it much worse. Also, the guitar can still be heard with vol 1 all the way down. I was leaning toward replacing the vol 1 pot but the resistance is pretty much the same as the vol 2 pot (measured from the center leg to each of the outside legs; turned up and down). I measured resistance to ground from the ground leg of both vol pots and came up with .2 ohms. Any thoughts?

 

[mickeydg5]

Clean the all cable plugs, amplifier jacks, tube socket prongs and tube pins with an electronics contact cleaner.
See if that helps at all first.

 

[neikeel]

Clean the all cable plugs, amplifier jacks, tube socket prongs and tube pins with an electronics contact cleaner.
See if that helps at all first.

Yes probably one of the unused switching jacks not grounding properly.

 

[leewinn]

Thanks for the recommendations. I cleaned the jacks and sockets with very little improvement. Then, just for kicks, checked the adjustment of the hum balance pot. Turns out someone had turned it all the way in one direction. I used a meter to adjust it to the center and then my ear to get rid of the rest of the noise. There was still a slight power supply hum in the background, so I replaced the filter caps which pretty much did the trick. There is still a VERY slight hum but I think that it's manageable. The only thing left is some hiss on the bright channel (probably normal). I was wondering if replacing the plate load and grid resistors with metal film would have any benefit.

 

[mickeydg5]

That is good to hear.