Marshall JCM 900 100w DR 4102

ToneWitch

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Got this guy on the bench.

Owner told me he swapped out the pots to cure a hum issue only to find it didn’t cure the hum and was messed up more from there.

All the traces to the pots were lifted from over heating with a soldering iron. Also, the pots were placed randomly into the board. They didn’t follow the schematic and the values were not put in the correct locations.

This left channel B completely useless as a 10k pot was placed on its master volume (1M being the correct value).

I replaced the filter caps because they are over 15 years old. Pulled all the pots and put them in their correct locations via the Schem. I also had to solder leads from the pots to their correlating components as the traces were gone.

Replaced all the 5201 IC’s and waiting on the TL071/TL072 and 1458 IC’s to show up.

Now the amp works as it should however, it still hums and the hum increases with the volume controls on both channels.

I believe the Hum issue is coming from the reverb fet IC (TL071) or a transformer but I am replacing all IC’s since I am in there. This is speculation and need to test further.

Troubleshooting the issue I swapped tubes (pre and power) to see if a tube may have been the cause.

Bias is set to -49.87v running 6L6 tubes. Adjusting to more negative bias doesn’t adjust/stop the hum.

I am getting correct voltages at the +\- 15v circuit

Tried adjusting lead dress as the hum sounds almost like a transformer hum however adjusting the lead dress did nothing to cure it.
Will update further here shortly.
 

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