Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

Hello everyone, I'm working on a Mesa Boogie F-30 that had a couple of areas overheating on the PCB. In particular, the 560R resistor feeding the +15/-15 supplies had made the board pretty crispy. I pulled the board out and replaced that resistor with a 560/10W and raised it a half inch or so up off the board. I also soldered the leg ends directly to connected components because the traces in that area had been heated so much I did not trust them to hold up.

The four 4007 diodes in the heater supply circuit were placed very tightly together and right on the board and the board was discolored in this area as well, so I replaced them and lifted them up away from the board as shown in the photo below. Then I reassembled everything and fired it up. All the voltages seem fine but now there is a significant amount of hum being generated that wasn't there originally. I did some audio probing and the problem seems to be in the V3B area. The hum is audible when probing V3 pin 6, but not when probing V3 pins 1 or 7. I'm not sure of the best way to continue troubleshooting this problem. Could the change I made to the diodes be problematic? I appreciate any suggestions, thanks. (Schematic attached)



f30.pdf

[bobloblaws]

The four 4007 diodes in the heater supply circuit were placed very tightly together and right on the board and the board was discolored in this area as well, so I replaced them and lifted them up away from the board as shown in the photo below. Then I reassembled everything and fired it up. All the voltages seem fine but now there is a significant amount of hum being generated that wasn't there originally. I did some audio probing and the problem seems to be in the V3B area. The hum is audible when probing V3 pin 6, but not when probing V3 pins 1 or 7. I'm not sure of the best way to continue troubleshooting this problem. Could the change I made to the diodes be problematic?

I may have just answered my own question. I tried a little troubleshooting trick someone suggested back in another thread where I had a hum issue with an old Gibson amp. I grounded the grid, V3:7, and the hum at pin 6 disappears. Am I therefore correct in concluding that the hum is therefore not heater related but is in fact coming in through the grid?

[Tom Phillips]

Attached is a copy of the F-30 service information. It will be a good reference to help clarify the circuit references you are using.
Mesa Boogie F-30.pdf