Mesa Boogie F-30 Hum Problem After Repair

[Helmholtz]

The EQ inductors like to pick up hum fields. If e.g. you routed a power supply wire close to one of the inductors, this could be your problem.

[bobloblaws]

The EQ inductors like to pick up hum fields. If e.g. you routed a power supply wire close to one of the inductors, this could be your problem.

OK, I don't think either of my jumpers came into that vicinity, but I will look into that possibility.

[bobloblaws]

***Sigh*** The amp from hell. I pulled the board, got rid of the jumpers. While I was at it I took a soldering iron to the bottom of the board and reflowed pretty much the whole thing. Put it all back together again. The hum I was trying to get rid of appears to be taken take care of. But now there is a louder hum, seems to start around V4. Also periodic crackling sound is back that I thought I had been resolved before I started this thread. Tried swapping out the 100K and 82K plate resistors at V4 and the 0.047 coupling caps, no change. I'm getting close to giving up on it and moving on to the other amps I have in the queue..

[Chuck H]

But now there is a louder hum, seems to start around V4.

That the hum is louder is telling. Because Helmholtz called it out. There may have been multiple hum sources and I suspect they were out of phase so somewhat cancelling. You removed one and now the hum is louder. I'll bet if you scope it again that dip in the hum signal will be gone. You wouldn't have detected the V4 hum because the one you corrected was earlier in the circuit.

Not sure if this helps.

EDIT: Since the crackle was gone and is now back tells me it's more likely a physical contact problem than a component problem. Have you cleaned all the contacts? When you resoldered did you touch up the tube pin joints? These are common failure areas because players sometimes plug and unplug tubes repeatedly while "tube rolling" and Mesa's PCB mounted sockets can make that a durability issue.

[bobloblaws]

That the hum is louder is telling. Because Helmholtz called it out. There may have been multiple hum sources and I suspect they were out of phase so somewhat cancelling. You removed one and now the hum is louder. I'll bet is you scope it again that dip in the hum signal will be gone. You wouldn't have detected the V4 hum because the one you corrected was earlier in the circuit.

Not sure if this helps.

EDIT: Since the crackle was gone and is now back tells me it's more likely a physical contact problem than a component problem. Have you cleaned all the contacts" When you resoldered did you touch up the tube pin joints? These are common failure areas because players sometimes plug and unplug tubes repeatedly while "tube rolling" and Mesa's PCB mounted sockets can make that a durability issue.

Thanks, Chuck. Yes, reflowed the tube pin terminals and cleaned the tube sockets. Wiggling the tubes does not make difference so I don't think the problem lies there.

Come to think of it, the term cracking might not be the best description. A little more subtle, like random blasts of white noise with a crispy component at times?

[Helmholtz]

Use your scope.

[Tom Phillips]

...Come to think of it, the term cracking might not be the best description. A little more subtle, like random blasts of white noise with a crispy component at times?

Is the noise intermittent or does it occur fairly often when the amp is just sitting on the bench undisturbed?

[x-pro]

Is the noise intermittent or does it occur fairly often when the amp is just sitting on the bench undisturbed?

And what is the input to the amplifier? Is there any cable connected?

[bobloblaws]

And what is the input to the amplifier? Is there any cable connected?

No cable connected.

[bobloblaws]

Is the noise intermittent or does it occur fairly often when the amp is just sitting on the bench undisturbed?

It is sporadic/random, but always happening. Kind of hard to describe, a little bit of everything, white noise, pops, clicks. It is in addition to the loud hum (will post scope traces shortly).

[bobloblaws]

Use your scope.

Here's some traces. 1st two are from the speaker output. Next two are the PI plates. Last one is V3B plate.

[Tom Phillips]

Bobloblaws,
The scope traces are interesting but hard to relate to the noises you are describing. Your first post in this thread seems to indicate that you have audio signal tracing capability. Is that true? That's what I often use in these circumstances. I was recently successful in tracing some really annoying noise to an individual capacitor. The signal tracer indicated a nice quiet spot at the input side of the coupling cap and, on the other side of the cap, I could hear the exact noise as was present at the output of the amp. There was actually no "signal tracing" because I could just hunt for the objectionable background noise. Changing one cap fixed the noisy amp.

[bobloblaws]

Bobloblaws,
The scope traces are interesting but hard to relate to the noises you are describing. Your first post in this thread seems to indicate that you have audio signal tracing capability. Is that true? That's what I often use in these circumstances. I was recently successful in tracing some really annoying noise to an individual capacitor. The signal tracer indicated a nice quiet spot at the input side of the coupling cap and, on the other side of the cap, I could hear the exact noise as was present at the output of the amp. There was actually no "signal tracing" because I could just hunt for the objectionable background noise. Changing one cap fixed the noisy amp.

Yes, I've been using audio tracing as well to try to solve this.

Bear in mind the traces I just posted are of the loud hum, not the sporadic noises.

[Helmholtz]

Freeze spray might help to localize a noisy component.

[g1]

This may be coincidental, but glitchy clicky popping noises can be caused by oscillation, as can hum. However, that hum should be 120Hz, which I don't think you have.