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.

[Jon Snell]

The most important piece of information is missing;
What frequency is the hum?

From that you can decide whether power supply or an earting/chassis fault, (or that the amplifiers just doesn't know the words).
Recheck your work!

[Pedro Vecino]

That amp (the entire F series) has a clean and spacious sound, although surprisingly that channel has more hiss than the lead channel at half gain and the same volume. The latter is quieter, although with contour mode activated it goes up slightly compared to normal mode.
It is not the quietest amp but it should not have any noticeable hum except when the clean channel gain is turned up above half (in this case it is noticeable).
It is a bit of a confusing amp to determine if the noise levels are normal.
As for the diodes, the blackened area sometimes becomes conductive and a few megaohms can be measured between some points on the board and the diodes. If so, it is a sure source of noise. It is not a clean and constant hum but an annoying electrical noise with some variable activity.

[bobloblaws]

The most important piece of information is missing;
What frequency is the hum?

From that you can decide whether power supply or an earting/chassis fault, (or that the amplifiers just doesn't know the words).
Recheck your work!

Hard to say the frequency. Very low in any case, so if it could only be 60Hz or 120Hz, I want to say it is 60Hz. It is more a low hum than a buzzy type hum.

When I was reassembling the amp, there was a point where I had pushed the PCB down on a few of the standoffs but not all of them and the board was curved. I'm wondering if I ended up cracking a solder joint when it was in that state.. I may need to disassemble again.

[bobloblaws]

That amp (the entire F series) has a clean and spacious sound, although surprisingly that channel has more hiss than the lead channel at half gain and the same volume. The latter is quieter, although with contour mode activated it goes up slightly compared to normal mode.
It is not the quietest amp but it should not have any noticeable hum except when the clean channel gain is turned up above half (in this case it is noticeable).
It is a bit of a confusing amp to determine if the noise levels are normal.
As for the diodes, the blackened area sometimes becomes conductive and a few megaohms can be measured between some points on the board and the diodes. If so, it is a sure source of noise. It is not a clean and constant hum but an annoying electrical noise with some variable activity.

As a matter of fact I am hearing something similar to what you describe in addition to the hum.

[bobloblaws]

This might be a clue. The photo below shows the area of the 560R resistor. There is a hole visible that is not populated. Despite the amount of obvious heat damage, the pad has continuity to the 560R (didode end), you can see the trace in the second photo of the bottom of the board. When I was replacing the resistor I wondered about it but I just went according to the schematic which shows a connection only to the 2 diodes. Just now I looked closer at the script on the board. In full it reads "560 ohm 7W (120 ohm 2wX)". Any thoughts on this phantom 120R Resistor that doesn't seem to be shown on the schematic? (Side note: I've found a few discrepancies on the schematic already, whether it is a revision situation I don't know.) Incidentally, the two green wires showing in the photo are the heater supply to the power tubes.

[bobloblaws]

I found another piece of weirdness. On the power Amp/Record Out page of the schematic (pg 8) on the left hand side there is a FET. I have no idea what it is for. Anyway, the actual wiring is different than shown in the drawing. The leg that is shown connected to the bottom of the 22K and 56K resistors and the tube grid is in fact not connected to those components but rather connected to the junction of the 33K resistor and the .005 cap. Might this discrepancy be significant?

[Tom Phillips]

I found another piece of weirdness. On the power Amp/Record Out page of the schematic (pg 8) on the left hand side there is a FET. I have no idea what it is for. Anyway, the actual wiring is different than shown in the drawing. The leg that is shown connected to the bottom of the 22K and 56K resistors and the tube grid is in fact not connected to those components but rather connected to the junction of the 33K resistor and the .005 cap. Might this discrepancy be significant?

Although I cannot comment on the significance at this time, I will say that differences between the Mesa schematics and the actual circuits found in the amps is not unusual. To be fair, such differences are not unique to the Mesa Boogie brand.