Mesa Boogie F-30 Hum Problem After Repair

[Tom Phillips]

I wonder why the scope thinks the signal is 95.24MHz.

[Helmholtz]

Oops, missed that. Good point. Also the signal looks ampltude-modulated
So the power amp might be oscillating at that frequency, though that would be an unusually high frequency oscillation.
Or some outside RF source is picked up by the amp.
Of course totally inaudible- except maybe for the modulation frequency.
What happens if the feedback is disconnected?

[Tom Phillips]

...So the power amp might be oscillating at that frequency.
What happens if the feedback is disconnected?

I would also double check by setting the scope sweep speed manually to rule out an issue with the auto mode operation.

[Helmholtz]

I would also double check by setting the scope sweep speed manually to rule out an issue with the auto mode operation.

The frequency correlates with the signal period of ~10ns.

[bobloblaws]

I wonder why the scope thinks the signal is 95.24MHz.

Here's a video version, it's a better representation of what the scope is showing. I set the sweep manually.

https://drive.google.com/file/d/1cHh...usp=drive_link

[bobloblaws]

What happens if the feedback is disconnected?

I'll check that out tomorrow.

[Chuck H]

FWIW Mesa had a few amp models that used an unterminated lead from the filament circuit to mitigate hum (via proximity NFB induced). Usually orange. Just a lead wire sticking up out of to board off some filament node and bent this way or that. Probably adjusted for hum mitigation individually for each amplifier? Mesa designs tend to emulate breadboarding for circuit topology and values. Rather than any refinement of the effect a circuit has and evaluation to simplify design. So it's possible this amp is an early model of a design that might have later used the unterminated orange lead and it wasn't implemented yet for this particular amp???

This is real info above and I'm just trying to consider what I know.

[bobloblaws]

I went ahead and lifted the .005 and 56k legs to remove the NFB. It had no effect on the hum issue.

Regarding the scope trace I posted in #180, I'm actually seeing the same thing at various places, e.g. V3:7, V4:7, speaker output, (basically anywhere in the circuit) with the amp unplugged. filter caps drained, standby and power switches OFF. Beats me what it means.

Here's V3:7 again with the amp OFF/unplugged.

[Tom Phillips]

...Regarding the scope trace I posted in #180, I'm actually seeing the same thing at various places, e.g. V3:7, V4:7, speaker output, (basically anywhere in the circuit) with the amp unplugged. filter caps drained, standby and power switches OFF. Beats me what it means.
Here's V3:7 again with the amp OFF/unplugged.

So now we know that the signal displayed is not being generated by active circuits within the amp. I'd say that it's being picked up by parts of the amp that are acting as an antenna or originating in the scope/probe setup.

The scope capture is still displaying "Freq(1)=93.46MHz" (What's up with that?) and "-400µV" (Does that mean 400µV per division?)
It appears that the scope sweep sped is the same as it was when you were using Auto mode.
I'm also curious where your scope probe ground is connected and what the scope would display if you disconnect it from the amp and just connect it to:
1. Nothing
2. A short bit of wire to act as an antenna or just touch the probe tip with your fingers.

[g1]

Sorry to jump in late here if it's already been asked, but a question based on: 1) the fault was not there before, and 2) supply node 'E' is related.
Are you 100% sure the PI tube is the exact one that was in the PI position originally (when fault was not there) ?

[Helmholtz]

I'd say -400µV is the trigger level and input sensitivity is 5mV/div.
I doubt the RF EMI is related to the 60Hz hum issue

[Tom Phillips]

I'd say -400µV is the trigger level and input sensitivity is 5mV/div.

I now see that the screen text includes "Vmax(1)=5.80mV". Therefore, I think the scope is listing what it thinks is the peak to peak value of the displayed waveform.

I doubt the RF EMI is related to the 60Hz hum issue

I concur. This is when I would be turning off other equipment nearby to try to determine where the signal is originating. For practical purposes,it seems that this high frequency waveform is just a distraction.

[bobloblaws]

Are you 100% sure the PI tube is the exact one that was in the PI position originally (when fault was not there) ?

No, the tubes have moved around some. Had them marked originally but the markings smudged. In any case the issue is the same no matter which of the 4 preamp tubes is in V4.

[bobloblaws]

For practical purposes,it seems that this high frequency waveform is just a distraction.

I think so. When I posted that trace I was thinking the frequency was 95Hz, I missed the "M".
The only idea I have left is there is maybe something connected to V:7 that is not shown on the drawing. I figure it's not outside of the realm of possibility since I've found other differences previously.

[Tom Phillips]

...The only idea I have left is there is maybe something connected to V:7 that is not shown on the drawing. I figure it's not outside of the realm of possibility since I've found other differences previously.

I agree. Mesa schematics have a reputation of deviating from the actual circuits. To be fair, other companies have the same issue, but Mesa seems to keep messing more with things after initial release in an attempt to fix issues with the originally released circuit design and/or layout.