Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

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.

Point taken.

Anyway, I'm measuring 0.09DCV on pin 2 there. Would that be enough to introduce noise?

[x-pro]

J175FB changes the frequency response of the output stage when RHY mode is enabled.
Your hum disappears when you ground pin 7 of V3b. So you should carefully check the moving contact in the FX LOOP SEND connector, as well as the quality of resistors 22k near this connector and 47k at pin 7 of the lamp (according to the diagram).
In post 13, second photo. Ring cracks on diode contacts should be eliminated.

[bobloblaws]

J175FB changes the frequency response of the output stage when RHY mode is enabled.
Your hum disappears when you ground pin 7 of V3b. So you should carefully check the moving contact in the FX LOOP SEND connector, as well as the quality of resistors 22k near this connector and 47k at pin 7 of the lamp (according to the diagram).
In post 13, second photo. Ring cracks on diode contacts should be eliminated.

Thanks for the info and tips, x-pro. It's hard to say for sure, but the level of the background hum may be a bit lower than when I first posted about it a few days ago. If that's the case it may be because I reflowed the solder joints on the power tube board in the interim.
Anyway I looked into the things you mentioned. The resistors appear to be OK but I did not measure them out of circuit. For the moving contact in the Send connector, what should I be looking for, just good contact? I've already sprayed it with deoxit but I haven't yet removed the Send/Return jack board to look at the outside of the jack.

[bobloblaws]

Thanks for the info and tips, x-pro. It's hard to say for sure, but the level of the background hum may be a bit lower than when I first posted about it a few days ago. If that's the case it may be because I reflowed the solder joints on the power tube board in the interim.
Anyway I looked into the things you mentioned. The resistors appear to be OK but I did not measure them out of circuit. For the moving contact in the Send connector, what should I be looking for, just good contact? I've already sprayed it with deoxit but I haven't yet removed the Send/Return jack board to look at the outside of the jack.

I removed the Send/Return board and inspected the jacks, everything looks OK. But I did discover something interesting. If I insert a dummy plug into the send jack and turn the FX loop level pot wide open (labeled 90% on the chassis), the hum is much lower. It lowers the amp output as well, to a smaller degree. I tried spraying the pot, no change. What does this tell us? I can follow the signal path and the function of the jacks and pot but I don't understand why the scenario I described results in the hum being decreased or whether this area is where that hum is originating to begin with.

[x-pro]

pot but I don't understand why the scenario I described results in the hum being decreased or whether this area is where that hum is originating to begin with.

Temporarily, to check the presence of background, connect the left lead of the 470 ohm resistor from the lamp grid to the right lead of the 1µf50V capacitor from the collector of transistor Q4.

[bobloblaws]

Temporarily, to check the presence of background, connect the left lead of the 470 ohm resistor from the lamp grid to the right lead of the 1µf50V capacitor from the collector of transistor Q4.

I'm confused about exactly where you want me to connect a jumper. In any case, I tried the 4 different combinations. When the jumper is connected to the negative side of the cap there is no hum (but no signal getting through either) with the other end connected to either side of the 470R. When the jumper is connected to the positive end of the cap the hum is still there, again with the other end connected to either side of the resistor. In this case there is signal coming through as well.

[g1]

I'm confused about exactly where you want me to connect a jumper.

See pic below. The cap is necessary to block the -10VDC from Q4. If the -10V gets to the V3b grid, it cuts the tube off, resulting in no sound (also no hum). This is what you found when you attached the jumper to the negative end of the cap.

So with the jumper xpro suggested, the hum was still there.

[bobloblaws]

See pic below. The cap is necessary to block the -10VDC from Q4. If the -10V gets to the V3b grid, it cuts the tube off, resulting in no sound (also no hum). This is what you found when you attached the jumper to the negative end of the cap.

So with the jumper xpro suggested, the hum was still there.

Right, so can we rule out the loop and the pot?

[g1]

Right, so can we rule out the loop and the pot?

I would say yes, but I will defer to x-pro .

[Chuck H]

How is that jumper any different from simply confirming the FX loop switching jack is functional and then simply NOT plugging anything into the loop???

That said... Mesa had some trouble with hummy effects loops of this type something like twenty five years ago. They have long since solved the problem and I haven't heard about it since. But I'm not in the repair game either other than here on the forum so my perception may not be the real picture.

I had a Mesa amp with the hummy effects loop problem and when I asked Mesa if there was an approved solution they said if I sent them the amp they would fix it for free. This amp was a couple of years old and I had bought it used. There was no warranty. When I got the amp back they had simply taken out the old board and put in a whole new fully loaded board. I couldn't say about the difference in the loop circuit because the original was encased in black epoxy. The new board was not. This would have been about twenty four years ago. They had so much trouble with those hummy effects loops around that time that I would have to think they took measures to make sure it never happened again. And they were certainly very accomodating in my own circumstance.

[bobloblaws]

How is that jumper any different from simply confirming the FX loop switching jack is functional and then simply NOT plugging anything into the loop???

That's a good point. Note that I snipped the positive leg of the cap, FWIW. But it still seems to me that the fact that my hum problem remains suggests that the problem is not with the fx loop. Until now I had been trying to avoid any other snipping or God forbid taking the board out again, but I'm inclined to start checking components around V3B.

[bobloblaws]

That's a good point. Note that I snipped the positive leg of the cap, FWIW. But it still seems to me that the fact that my hum problem remains suggests that the problem is not with the fx loop. Until now I had been trying to avoid any other snipping or God forbid taking the board out again, but I'm inclined to start checking components around V3B.

I might have to take that back. I just snipped the loop side of the 470R resistor and that got rid of the hum. Then I connected a jumper straight from the positive leg of the cap to the loop side of the 470R resistor (and I have the 22K resistor snipped and another 22K jumpered from the cap to ground). So the circuit is identical except for the removal of the fx loop in its entirety. Again, breaking the circuit at the loop side of the 470 resistor eliminates the hum. So where is the hum coming from? The cap should block any DC, right? I tried a replacement cap, no change. Could it be AC in the transistor section from the -15V supply? I don't measure anything significant with my DMM.

[Pedro Vecino]

You can try bypassing the equalizer completely to see what happens. Just lift the appropriate pin on the capacitor and resistor and connect them.

[bobloblaws]

You can try bypassing the equalizer completely to see what happens. Just lift the appropriate pin on the capacitor and resistor and connect them.

Yes! I had the same thought. I did this and it eliminates the hum. I guess I have my work cut out for me now figuring out where in that section the problem is. Best bet rule out transistors first?

[Helmholtz]

A transistor can amplify hum but it's never the source of hum.