MID 70S Marshall 50w JMP Service issue?

[bluze81]

hey guys I am working on a mid 70s JMP For a friend,I am having trouble with a Loud ground problem, When tesing amp guitar plugged in when I take hands off the guitar strings the amp just hums and hisses very loud, I have tested all grounds from the amp inputs all seems in order, I know the guitar and cable are fine, this is a very loud hiss,when I touch the amp chassis it stops, Have tried all i can think of to find a broken ground wire or cold solder joint ETC, have never ran into a problem as this on any builds or repairs, I have even tried a complete new set of tubes a problem persists, any help or suggestion would be welcomed at this point. thanks, Steve

[axeman]

The ground of the AC socket of your house is not grounded correctly.

[SDM]

The ground of the AC socket of your house is not grounded correctly.

Do check the above, if wall socket is correct though, check the power cable itself next. Aside from an obvious missing prong or such, wires can/do break inside it. Check its ground connection inside the chassis, check continuity of ground plug/prong to the chassis with a meter (can do same for neutral and hot, though to power switch rather than chassis). Amp must of course be unplugged from the wall to do this, be sure to move/bend the power cord around a bit while testing so to see if an intermittent loss of continuity occurs (if nothing shows up immediately).