bitman
Well-known member
So my little solid state Marshall lead 20 developed a hum.
Replaced all the electrolytic. - no change.
Spinning the bias pot from one side to the other changes the hum from real loud on one side to less hum than the center spot on the pot, but not completely gone.
Today I went to check for DC on the speaker outs to see if one of the 2 output transistors was bad. I set the meter to 20 vdc and put the negative lead to the chassis and when I touched the speaker terminal at the speaker the hum completely went away! I thought oh crap I shorted it out and pulled the lead away. I thought that weird so I disconnected the negative lead from the chassis and again, touched the positive meter lead to the speaker and the hum went away. When I lifted the lead away, after a second or so the hum came back. So I grabbed some needle nose pliers and touch the same spot on the speaker and the hum went away again!
WtH?
Replaced all the electrolytic. - no change.
Spinning the bias pot from one side to the other changes the hum from real loud on one side to less hum than the center spot on the pot, but not completely gone.
Today I went to check for DC on the speaker outs to see if one of the 2 output transistors was bad. I set the meter to 20 vdc and put the negative lead to the chassis and when I touched the speaker terminal at the speaker the hum completely went away! I thought oh crap I shorted it out and pulled the lead away. I thought that weird so I disconnected the negative lead from the chassis and again, touched the positive meter lead to the speaker and the hum went away. When I lifted the lead away, after a second or so the hum came back. So I grabbed some needle nose pliers and touch the same spot on the speaker and the hum went away again!
WtH?
