Marshall G80R cd

[b757bob]

I need some help here. My Marshall G80 has a loud hum. I think its from the power supply as non of the volume or gain pots have any effect on the volume of it. I've got hold of the ancient manuscripts - revision 4 Schematic diagrams. I just need some guidance.
Thanks in advance

[pontiacpete]

I would check the solder joints, e.g. around the input jacks, filter caps , pots and then anywhere else.

[Enzo]

Look at your speaker cone. When you turn the amp on, does it move one direction and stay there? If so, you have DC on your output. Turn the amp off immediately and disconnect teh speaker wires until it is fixed. Connect your voltmeter to those loose speaker wires and see if the amp puts DC voltage there.

That is usually the result of a shorted output transistor or IC.

Alternatively, if one of your filter caps has failed or - more likely - cracked free of its solder, then that leaves very rough DC power in the amp. Your output powr amp stage uses both polarities of voltage. I don;t know what without looking, something in the 30-40v range I'd assume. Are BOTH present at the power amp, and are both about the same voltage. If one DC voltage is a lot lower thaqn the other, flip your meter to AC volts and see if there is a substantial amount of AC where that DC ought to be. If that is the case, then the filter cap for that power supply is either bad or not connected. Not connected meaning a crack in the solder to it or the pc board traces to it.


There are3 of course other poossibilities, but those two should cover 99% of the LOUD HUM complaints.

[b757bob]

Thanks to both of you for taking the time to help. When I took the board from the chassis I checked all joints to the pots and the jacks and then started looking around the capacitors in the power supply section. Physically tapping around the two large capacitors C69 & C71 seemed to activate the problem and on closer inspection, revealed a dry looking joint. A dab of solder on all the joints and it's working fine again. Keep up the good work and thank you once again.
Bob