Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

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

OK, do you think it might be the -15V supply? If that is the case it brings me back to what I mentioned in post #13. Or maybe conductivity on the PCB because of the the area that was overheated. Since I jumpered the ends of the 560 ohm resistor to the respective diodes, maybe I can scratch out the traces that are no longer needed and see if that makes a difference.

[Tom Phillips]

It would still be good to determine if the hum is 60Hz or 120 Hz.

[bobloblaws]

It would still be good to determine if the hum is 60Hz or 120 Hz.

I am sure it is 60Hz, actually. That's a good point. AC in the -15V supply would be 120Hz, yes?

Could it be a bad resistor? Or a cold solder joint? I haven't been able to find anything by chopsticking. I can't think of any other possibilities

[Tom Phillips]

If you are sure that it's 60 Hz hum then, as you said, it won't be from bad filtering on any of the DC power rails.
60Hz could come from pickup of radiation from AC line input wiring or any of the power transformer secondaries feeding circuitry. It could also be a bad tube that is putting the AC heater hum into the signal path.

Note that the FX circuitry is very close to the AC line wiring and the power transformer. Perhaps the wiring dress was disturbed/moved when you did your initial work described at the beginning of this thread. You did say that you noticed the hum after you completed that work. Attached is a gut shot of the F-30 showing the FX board proximity to the AC wiring. How does your project look in that area?

If you wanted to do an experiment, it looks like the FX board could be dismounted from the rear panel and then moved away from the back corner of the chassis while still functionally connected to the main PCB. This is just an idea I thought of. I have not seen other discussions about excessive 60 Hz hum in the F-30 so it's strange that your amp has that problem.

Another thought: Is the hum level affected if you shield the top of the chassis as it would be when the amp is in the cabinet?

[Chuck H]

Is it possible that some of the "toasty" board left behind in the repairs is semiconducting? It would be very hard to diagnose easily but I wanted to put it on the table.

[bobloblaws]

If you are sure that it's 60 Hz hum then, as you said, it won't be from bad filtering on any of the DC power rails.
60Hz could come from pickup of radiation from AC line input wiring or any of the power transformer secondaries feeding circuitry. It could also be a bad tube that is putting the AC heater hum into the signal path.

Note that the FX circuitry is very close to the AC line wiring and the power transformer. Perhaps the wiring dress was disturbed/moved when you did your initial work described at the beginning of this thread. You did say that you noticed the hum after you completed that work. Attached is a gut shot of the F-30 showing the FX board proximity to the AC wiring. How does your project look in that area?

If you wanted to do an experiment, it looks like the FX board could be dismounted from the rear panel and then moved away from the back corner of the chassis while still functionally connected to the main PCB. This is just an idea I thought of. I have not seen other discussions about excessive 60 Hz hum in the F-30 so it's strange that your amp has that problem.

Another thought: Is the hum level affected if you shield the top of the chassis as it would be when the amp is in the cabinet?

Thanks, Tom. I have had the loop board out and it did not change anything. Also, I believe I've ruled out the Fx loop based on the troubleshooting I've done with the help all y'all on here.

As for the shield idea, it's hard to tell at this point because I have a couple components clipped and jumper wires making connections that are sticking up above the PCB. When I tried to cover the chassis with a piece of steel it actually increased the hum as it came near the jumper wires.

[bobloblaws]

Is it possible that some of the "toasty" board left behind in the repairs is semiconducting? It would be very hard to diagnose easily but I wanted to put it on the table.

Yes, I wondered about that myself in post #31 where I considered removing some of the trace material.

[Tom Phillips]

...As for the shield idea, it's hard to tell at this point because I have a couple components clipped and jumper wires making connections that are sticking up above the PCB. When I tried to cover the chassis with a piece of steel it actually increased the hum as it came near the jumper wires.

I expect that the hum would only increase if the steel shield was not yet grounded to the chassis. Without the ground connection the steel acts as an antenna. When the amp is installed in the cab, it is grounded by contact with the four screw heads protruding above the chassis lip.

Another thing to check on that amp. With the amp off and the line cord unplugged, verify that the chassis is at system ground potential. If it were me I'd do that by checking continuity between the screw heads I just mentioned and the negative side of the filter caps.

[bobloblaws]

I expect that the hum would only increase if the steel shield was not yet grounded to the chassis. Without the ground connection the steel acts as an antenna. When the amp is installed in the cab, it is grounded by contact with the four screw heads protruding above the chassis lip.

Ahhh, OK. I don't have the amp's cover here with me. But what I did is I clipped a wire to the cover I used from another amp and grounded it. That resolved the antennae behavior and as it happens it made no difference to my hum issue.

Another thing to check on that amp. With the amp off and the line cord unplugged, verify that the chassis is at system ground potential. If it were me I'd do that by checking continuity between the screw heads I just mentioned and the negative side of the filter caps.

I measure virtually no resistance between the screws heads and the reservoir cap, at most 0.05 ohm.

[Tom Phillips]

Thanks for the quick report.
I'll re-read the thread and see if anything else comes to mind.

[Chuck H]

Since you've reported that you can not confirm the hum wasn't there before the repair and you report that there was some discoloration to the board at the filament DC circuit that prompted you to replace and raise the diodes I would at least check and/or replace C1 and C2. They may have been damaged or derated by the same heat that that discolored the board?

And even though you're fairly confident it's a 60Hz hum I think it would be good to confirm this.

And, at V4b the schematic shows the cathode bypass as .22uf. But since this is an effects loop reamp stage that then feeds the phase inverter I would think that maximum gain would be needed so check that cap value just to see if it isn't actually 22uf. And if it is then I would try jumpering another 22uf cap over it to see if the original part might not be fully bypassing the cathode.

[bobloblaws]

Since you've reported that you can not confirm the hum wasn't there before the repair and you report that there was some discoloration to the board at the filament DC circuit that prompted you to replace and raise the diodes I would at least check and/or replace C1 and C2. They may have been damaged or derated by the same heat that that discolored the board?

OK, I'll have to pull the board out again in that case.

And even though you're fairly confident it's a 60Hz hum I think it would be good to confirm this.

I confirmed with my signal generator, it is definitely 60HZ. Would you still recommend pulling the board to remove C1 and C2?

And, at V4b the schematic shows the cathode bypass as .22uf. But since this is an effects loop reamp stage that then feeds the phase inverter I would think that maximum gain would be needed so check that cap value just to see if it isn't actually 22uf. And if it is then I would try jumpering another 22uf cap over it to see if the original part might not be fully bypassing the cathode.

It appears to be .22uF rather than 22uF. The yellow one in this photo.

[Tom Phillips]

...Would you still recommend pulling the board to remove C1 and C2?

If you do decide to pull the board for any reason, let's talk about the ground points before you re-install it. If the objectionable hum appeared after you did your initial work, I'm wondering if something happened that created a ground loop.

[bobloblaws]

If you do decide to pull the board for any reason, let's talk about the ground points before you re-install it. If the objectionable hum appeared after you did your initial work, I'm wondering if something happened that created a ground loop.

OK, I'm planning on pulling it.

I'm still wondering about what I mentioned in post #13. I'm certain that when I replaced the 560R resistor I followed the schematic exactly. But I'm still curious why there is an unpopulated hole and the marking on the top of the board refers to a 120 ohm resistor. Any idea what that might be about? I certainly did not find such a resistor when I began working on this.

[Tom Phillips]

OK, I'm planning on pulling it.
I'm still wondering about what I mentioned in post #13. I'm certain that when I replaced the 560R resistor I followed the schematic exactly. But I'm still curious why there is an unpopulated hole and the marking on the top of the board refers to a 120 ohm resistor. Any idea what that might be about? I certainly did not find such a resistor when I began working on this.

I don't have a good answer to offer about the parts values nor the unpopulated holes. From experience, I can say that both occurrences are fairly common so they do not necessarily stand out as a red flag. In addition, I have seen add on parts, sometimes tacked onto solder pads under the PC board.

My thoughts about the grounding are that I'd expect you to find only one metal standoff that connects a pad on underside of the PCB to the chassis where the paint is removed under the standoff. I can not say for sure that you won't find more than one. However, the goal is to be sure that the ground points are in the stock configuration. That's all I can contribute for now. Difficult to do when we both are not looking at the amp first hand.

Remember that, since the PCB is a two sided with plated through holes, you should watch for damage that could disturb a through hole connection.