Mesa Boogie F-30 Hum Problem After Repair

[Chuck H]

Freeze spray might help to localize a noisy component.

Nothing to be lost trying for sure. +1 upvote for this.

[bobloblaws]

Nothing to be lost trying for sure. +1 upvote for this.

I'll buy some. I'm gonna put this amp on the back burner for a week or so and try and get a few others out of the way.

[Tom Phillips]

... I'm gonna put this amp on the back burner for a week or so...

Sounds like a reasonable plan. I have had to do that too. One time an idea came to mind in the middle of the night and it turned out to be the correct diagnosis. I can't really remember if it was an actual dream or just one of those times between fully awake and truly asleep. Never-the-less, it was a cool experience.

[Chuck H]

Sounds like a reasonable plan. I have had to do that too. One time an idea came to mind in the middle of the night and it turned out to be the correct diagnosis. I can't really remember if it was an actual dream or just one of those times between fully awake and truly asleep. Never-the-less, it was a cool experience.

This happens to me also. It's easy to get horse blindered itno some specifics and the best way to see out of that is to remove yourself for a little while. Sometimes the answer comes to you out of the blue and sometimes returning to a project with a fresh perspective does it. Either way it's often the solution.

[bobloblaws]

I'll buy some. I'm gonna put this amp on the back burner for a week or so and try and get a few others out of the way.

Thanks for all the help so far, people, I truly appreciate it!

Anyway, I got back to it this weekend. I pulled the board out and double checked the work I did on reflowing the solder joints, etc. I then went back to trouble-shooting, but this time I didn't entirely re-assemble it, so I can access the bottom of the board relatively easily if I need to. I still have a lot of noise in the form of RF, Rice Krispies (snap, crackle, and pop), and 60Hz hum. It could very well be more than one problem at play.

When I audio probe the plates of V2B and V1B (with my monitor turned up fairly high) I'm detecting the noise I mentioned, minus the hum element. However there is nothing equivalent on the corresponding grids. Strangely though, I'm measuring 350mV DC on V2:7. If I plug a dummy plug into the input jack that voltage rises to 800mV. I don't know if it's related to any of the noise issues, but it seems like a good place to start. Any thoughts on what could be going on?

[Helmholtz]

Is the grid voltage at V2 pin 7 positive or negative?
Is your dummy plug shorting?
Did you try a different V2?
Sometimes a bad plate resistor causes noises as described.

[bobloblaws]

Is the grid voltage at V2 pin 7 positive or negative?
Is your dummy plug shorting?
Did you try a different V2?
Sometimes a bad plate resistor causes noises as described.

it
Helmholtz, the voltage I mentioned is positive. Dummy plug is not shorting, just a regular 1/4" plug. I did try one other tube in V2, also an older tube, it happens to test better on my tube tester, but who knows, I'll try something newer.
Barring that, I'll look into the plate resistors. Thanks for the tips.

[Helmholtz]

Helmholtz, the voltage I mentioned is positive.

In that case the 0.001µ coupling cap at V2a pin 1 might be leaky.

[bobloblaws]

In that case the 0.001µ coupling cap at V2a pin 1 might be leaky.

That is interesting. If that were the case would the position of the lead channel gain pot affect the amount of voltage at V2:7? In this case the lead gain pot has no effect. I should also mention that V2:7 goes to 0V if I change from the rhythm channel to the lead channel.

[Helmholtz]

Yes, coupling cap leakage will only show if V2a is engaged (RY 1 A and B in right hand NC position acc. to schematic).

[bobloblaws]

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

X-Pro, based on some previous input it seems like you have quite a bit of familiarity with this particular amp. I've suddenly seemed to have lost the chassis ground connection to the PCB and I can't figure out where the connection is supposed to be made. Can you help?

[x-pro]

I've never seen this device in person.
I rely on your actions and schemes, analyze and give advice.
I'm very busy right now. Later, I will re-read your topic again, and if someone doesn’t give you a solution earlier, maybe I’ll recommend something else.
But at the moment I would like to see detailed photos of your device. Photo of the entire board, printed circuit board, layout in the amplifier case.

[bobloblaws]

I've never seen this device in person.
I rely on your actions and schemes, analyze and give advice.
I'm very busy right now. Later, I will re-read your topic again, and if someone doesn’t give you a solution earlier, maybe I’ll recommend something else.
But at the moment I would like to see detailed photos of your device. Photo of the entire board, printed circuit board, layout in the amplifier case.

Thanks. It could be that the ground connection is made exclusively via the input jack connected to an area of bare metal on the back of the front panel. But I'm pretty sure I was able to troubleshoot previously without the jack in place, I just don't remember for sure.

[Tom Phillips]

. I've suddenly seemed to have lost the chassis ground connection to the PCB and I can't figure out where the connection is supposed to be made.

The input jack being used as the ground point is a possibility.
You should also look at the underside of the board where the standoffs are located. One, but not all, may be a ground point. If so, there will be a tell tail ring of bare PCB trace contacting the metal standoff.

Another procedure you could do with the amp off and the filter caps discharges is to make resistance measurements from a known ground point such as the negative of the reservoir cap to various points (such as the input jack) looking for continuity.

When doing R&R of circuit board assemblies, there is always the chance of breaking a wire loose that is providing the ground.

[bobloblaws]

The input jack being used as the ground point is a possibility.
You should also look at the underside of the board where the standoffs are located. One, but not all, may be a ground point. If so, there will be a tell tail ring of bare PCB trace contacting the metal standoff.

Another procedure you could do with the amp off and the filter caps discharges is to make resistance measurements from a known ground point such as the negative of the reservoir cap to various points (such as the input jack) looking for continuity.

When doing R&R of circuit board assemblies, there is always the chance of breaking a wire loose that is providing the ground.

Yeah, it could very well be the input jack alone. It's the only possibility I could find. Anyway, I made that connection and soldiered on. As far as the issues I'm still dealing with, I think the preamp tube sockets might be a factor. At one point I had tried tightening up the clips, I may have done more harm than good.