Marshall MG100DFX making weird noises

[lee]

Hi Guys need a little advice please I have a MG 100DFX Amp that recently started making a terrible noise like a loop back, the noise is intermittent and not always comes when you play. I have had the main board out and cleaned it up also take the FX card off which was quite dirty, Changes a dodgy Volume pot out. Also re soldered a few dry joints (suspect ) on the board but the noise is still there, Its like interference that is in a loop, I think it has to do with the FX card as when I turn all dials down on the FX side I haven't heard it as of yet. Please advise if anyone has seen this before please. Also  Its my sons Amp and I am trying to fix it for him.Appreciate any help you have. 

[thetragichero]

does he use the onboard effects? if not i *believe* you can just remove it and play without
if it is that board you're hoping it's few through hole components on it because otherwise it's some smd digital chip not working
they're not bad sounding amps save for the current onboard effects