Hum in Marshall artiste 100 (2068)

[rbotn]

Hum in Marshall Artiste 100 (2068)

Hi.
I am a service engineer who try to help a friend with a Marshall 2068 from 1972.
The problem is that there is a HUM in the loudspeaker. The position of the volume controls has no influence. I am not shure if it is 50Hz or 100Hz hum.
My friend told me that the hum has been there from the beginning.
Any ideas how I can get ride of it?

 

[Procter2812]

Could be lead dress, filter caps, a valve?

Id start by cleaning all sockets/jacks... swap out pre amp valves.. Try pulling each stage one at a time e.g. V1 first to see if the hum stops at any stage.

Any modifications been done to the amp? e.g. transformer changes? Could be lead dress

Check filter caps.

Check bias, see if power valves are matched/biased

 

[rbotn]

I have change all filter caps, bias is correct the valves are matched. And if I pull out V1 to 3 no differences. But if I pull out V4 the amp is silence. Maybe the hum is coming from the feedback into the grid on V4. No trafo changes as long as I can see.

 

[neikeel]

V4 on one of these is the PI. So we can guess the problem is in the latter part of the preamp, probably a marked imbalance in the PI.

Have you run it through a 'scope to see where the hum comes from? This would tell you if you are you getting ac leaking back through the PI coupling caps (are they mustards or WIMAs, never seen a leaky mustard but WIMAs yes).

Presume heater voltages are correct and symmetrical throughout? Are the heater wires clear and none of the grid wires coupling with the heater wire?

So no one has modded it or messed with the output stage grounds (a ground loop will give you 50Hz hum too)

I'd be very interested to hear what you find, if problems might be worth a picture to see if it has been modded.