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Hi, I am trying to find the cause of loud popping in Marshall super lead 100 re issue , it's in the preamp and I have removed 1st two tubes . There is DC appearing on tone stack caps which can only be leaking through the board from other tracks . I have seem terrible cases of this in JCM 2000 amps , has anyone seem it in a Super Lead 100 ? thanks
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Can you post the schematic? -
Originally posted by Enzo
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Where are you measuring the Vdc on the tone stack?Originally posted by Luthin You View Post
You should be measuring on the potentiometer side of the tone capacitors .
Otherwise you will just be measuring V2A pin 3 cathode voltage.Comment
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HI Jazz P, yes I am reading up to 10Vdc on the pot side of caps , and as the 1st 2 tubes are not fitted , there is no DC on V2a cathode. I can see sharp static noises on CRO so I think its the board breaking down and leaking DC into tone stack .With V1 &2 fitted , there is also DC getting into the bright channel vol pot, and if you turn up the bright vol it biases the input to v2a , then if you turn it down the DC is shunted to ground and channel 1 can still work , it is not leaky caps as I disconnected them . I am surprised no one has seen this , surely I cant be the 1st ?Comment
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It is a leaky pc board , Marshall UK say they haven't seen it , but thats what it is , I removed the pate resistors and supply from the board and coupled the signal back in with caps , and the noise is gone , and so is the biasing problem with the bright vol pot . I see this as potentially worse than the TSL thermal runaway , because this amp isn't even warm and its unusable with popping .Comment
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