Excessive Hum for Marshall JTM100

[cluster]

Hello all, I have a hum issue which I am trying to track down. Recently, at my studio someone accidentally knocked over my JTM100 head on to the carpet. It fell off it's half stack. It took out a couple of power tubes which I replaced but now I get excessive hum at idle (nothing plugged into the jacks). Here are some observations:

1) Hum is affected by all pots but especially the bass pot. Turning up the bass pot really accentuates the hum and makes it louder.

however 2) if i turn both volume pots up full, the hum disappears (even if i turn the tone pots and presence). that is, with all pots on full (amp cranked on 11), the amp vanishes and it just sounds like a normal cracked plexi.

I replaced all tubes and poked around with a chop-stick but no luck. Banged around on the transformers and knocked on the tubes as well with no luck. Can anyone give me a clue as to where I can look to so as to find where the hum might be coming from?? Input jacks? Leaky bias cap/diode? Any help is welcome.

Thanks in advance...

[52 Bill]

My first thought would be a mechanical problem like a broken ground connection. That doesn't mean a big broken off wire, even though it could be that. More likely a oxidized jack ground or a loosened pc grounding screw that was made high resistance by the fall.

[cluster]

My first thought would be a mechanical problem like a broken ground connection. That doesn't mean a big broken off wire, even though it could be that. More likely a oxidized jack ground or a loosened pc grounding screw that was made high resistance by the fall.

thanks bill. i'll go over the amp again tonight.

[cluster]

went over the amp and finally found the problem. the power board stand-off had come loose which grounded the power caps. tightened it up and and the hum went away. Thanks again bill.

joseph