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

[bobloblaws]

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

Errr...., thanks, but that's the same doc that I attached.

[Tom Phillips]

Errr...., thanks, but that's the same doc that I attached.

Oops & apologies. I see it now. I looked at your photos but totally missed the PDF link and the "Schematic Attached" comment.

[bobloblaws]

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?

On the other hand, there is a normal signal to noise ratio when I probe V3:7. In other words I don't hear hum on pin 7, but I do on pin 6. My brain is going in circles. Is it the heater circuit causing it? Or something else? I tried a different tube, but no change..

[g1]

When you grounded the V3 pin7, what happened to the hum at the amp's speaker?

[bobloblaws]

When you grounded the V3 pin7, what happened to the hum at the amp's speaker?

The hum at the output also disappears, and of course the guitar/test signal as well.

I'm now second guessing whether the hum was present from the start or not. It was generally noisy, and I may have overlooked it. It is a constant volume, in other words, the hum level does not change when adjusting the gain or master pots. these amps are reputed to be somewhat noisy but I hope the background hum is something I can eliminate. And the fact that it seems to originate specifically at the V3B plate and is quite prominent makes me think it is not just something to be expected.

One of the reasons it was brought to me was lack of clean tone on the clean channel even at low levels. I noticed this the first time I tried it but it corrected itself and seemed fine for a good while. It went into that state again and I thought it might have been the power tube swap I did as an experiment. However it seems OK again now, with either set of power tubes. I may need to reflow solder joints on the power tube board and possibly the main board also. I've been hoping to avoid doing that though, getting the amp disassembled to get access to the bottom of the board is a royal pain.