My thief hums in the 2nd speaker, if you are counting from the left facing the front of the machine. I swapped my I/O board with another I have and the hum got considerably louder. I believe the quieter I/O board has had the caps replaced because they are all the same color unlike the louder one. I have tried to unplug each harness from the I/O board one by one to rule out the hum coming from some where else in the cab. I also unplugged the monitor and the light fixture and still have the hum. Today I tried switching out the power supply with another one and still no change. I cannot imagine that both power supplies have the same issue. I would also think that if it was the power supply I would get humming in all 4 speakes not just one. I was looking at a restoration thread yesterday for a EDOT and his hum came from what I believe he said were caps. I have 2 of the same thing on the back of my I/O boards. They are little and red on one of the boards and green on the other. I believe they say +35v. I am wondering if I should try that next? Could it be the volume pots, even though I can adjust the volume on it? Any help would be appreciated. Thanks