Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

I'm still playing whack-a-mole. Every time I think I made some progress, something else pops up.

Anyway, I hope someone can help me with the particular issue I'm trying to solve at the moment. It's a 60Hz hum in the V4B area. With all of the preamp tubes pulled and the power tubes installed there is a hum at V4:7. If V4:7 is grounded (e.g. via the mute switch) there is no hum, including no hum at the amp output. If I ground the other side of the .02 cap that is connected to V4:7, the hum is present. How best to further troubleshoot? I feel like I'm spinning my wheels here big time.

[Chuck H]

Is the hum also at pins 3 and 8 (connected together on the schematic)? Check the 1000uf cap for the PItail and bias supply. If the hum is not on pins 3 and 8 then what is the signal like coming out of the headphone output?

[bobloblaws]

Is the hum also at pins 3 and 8 (connected together on the schematic)? Check the 1000uf cap for the PItail and bias supply. If the hum is not on pins 3 and 8 then what is the signal like coming out of the headphone output?

Yes, the hum is also at pins 3 and 8, but not quite as loud. If there is any hum on pin 2 it is quite a bit lower. A couple of things to note. I have the .02 cap lifted on the V3 side. Also, I'm hearing the hum on pins 7, 3, and 8 immediately when turning on the amp, even before the tubes have a chance to warm up.

[Chuck H]

I would just replace the the 1000uf cap for the PI/power tube bias supply as a matter of course. Even with the circuit values the time constant that cap instigates would mean hum on initial turn on anyway so that's not a surprise. But if it's not doing it's job once charged the problem continues. Other than that...

The only other thing connected to the PI grid is the headphone/recording circuit. If there are failed transistors there it might cause a problem at the PI grid. (the PI input grid would be V4 pin 7 you indicated above). So HOW IS THE OUTPUT FROM THE HEADPHONE JACK?!?

[bobloblaws]

I would just replace the the 1000uf cap for the PI/power tube bias supply as a matter of course. Even with the circuit values the time constant that cap instigates would mean hum on initial turn on anyway so that's not a surprise. But if it's not doing it's job once charged the problem continues. Other than that...

The only other thing connected to the PI grid is the headphone/recording circuit. If there are failed transistors there it might cause a problem at the PI grid. (the PI input grid would be V4 pin 7 you indicated above). So HOW IS THE OUTPUT FROM THE HEADPHONE JACK?!?

Right, sorry, also hum at the record out jack.

[Chuck H]

Start by replacing the 1000uf cap for the PI cathode/power tube grid supply.

[bobloblaws]

Start by replacing the 1000uf cap for the PI cathode/power tube grid supply.

Will do, I have it in stock.

[Chuck H]

Will do, I have it in stock.

How old is your stock? I never keep electrolytics past about three years. They go bad even faster with disuse and have a shelf life. Sprague Atom caps used to have a five year expiration date on their packaging when I bought them at brick and mortar stores.

[bobloblaws]

How old is your stock? I never keep electrolytics past about three years. They go bad even faster with disuse and have a shelf life. Sprague Atom caps used to have a five year expiration date on their packaging when I bought them at brick and mortar stores.

They were shipped to me Dec 2023.

[Chuck H]

They were shipped to me Dec 2023.

[bobloblaws]

OK, I changed out the bias supply cap, no change. I guess I want to be investigating the recording circuit now. Lifting the .002 and .047 caps seems like a good place to start.

[Helmholtz]

Is the +/-15V supply clean (scope)?

[bobloblaws]

Is the +/-15V supply clean (scope)?

I think it is. I recall that was corrected when I removed the jumpers I had attached to the 560 ohm resistor for reinforcement. I can certainly double check it, but in any case I went ahead and lifted the .002 and .047 caps to isolate the recording circuit from the V4B grid and the hum still persists at pins 7, 3, and 8.

[bobloblaws]

Is the +/-15V supply clean (scope)?

To answer your question more directly, I just checked and when audio probing I'm am hearing 60 Hz hum at the +15V supply ("H" on the drawing). Awhile back I was trying to troubleshoot hum in the graphic EQ/FX loop section and had identified hum on the -15V side. I'm not hearing that any longer, presumably because I removed the jumpers that I was advised might be causing it.

I'm also detecting hum at both bias supplies, E and F, as well as the point that I think is shown as "I" on the drawing, although the actual circuit does not seem to be exactly as shown.

[bobloblaws]

...I'm am hearing 60 Hz hum at the +15V supply ("H" on the drawing). Awhile back I was trying to troubleshoot hum in the graphic EQ/FX loop section and had identified hum on the -15V side. I'm not hearing that any longer, presumably because I removed the jumpers that I was advised might be causing it.

...I changed out the bias supply cap, no change. I guess I want to be investigating the recording circuit now. Lifting the .002 and .047 caps seems like a good place to start.

....I went ahead and lifted the .002 and .047 caps to isolate the recording circuit from the V4B grid and the hum still persists at pins 7...

I'm also detecting hum at both bias supplies, E and F, as well as the point that I think is shown as "I" on the drawing, although the actual circuit does not seem to be exactly as shown.

Given the above, I'm wondering what my best course of action is. Divide and conquer and try to find a bad component in the Bias Supply area other than the cap I already changed? Could it possibly be due to a grounding issue with the PT?