Mesa Boogie F-30 Hum Problem After Repair

[x-pro]

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.

The hole in the board is an option to install a different form factor 560 ohm 7 watt resistor

[bobloblaws]

The hole in the board is an option to install a different form factor 560 ohm 7 watt resistor

Ok, I'm glad that got cleared up, thanks!!

[Tom Phillips]

I found a thread at https://music-electronics-forum.com/...-tremoverb-hum that discussed a Mesa Hum Fix that turned out, according ti the OP, to be a design flaw. It may not help in your case but can't hurt to read about what other's have experienced.

[x-pro]

.... AC in the -15V supply would be 120Hz, yes?

No. If you look at the formation of the +15 and -15 voltages, you can see that they come from one winding through a single half-period rectifier. The smoothing circuits are two RC circuits - 100 ohms+1000uF. The capacitors are of course large, but they may have lost their rating. Then there will be a background from 60Hz.

[x-pro]

In general, the rules for routing conductors with alternating voltages (power or sound) for background suppression involve shielding or twisting the conductors and placing them close to the surface of the metal chassis.

[bobloblaws]

No. If you look at the formation of the +15 and -15 voltages, you can see that they come from one winding through a single half-period rectifier. The smoothing circuits are two RC circuits - 100 ohms+1000uF. The capacitors are of course large, but they may have lost their rating. Then there will be a background from 60Hz.

Ok, good to know, thanks for that info.
I have a a dual rail DC power supply I built as a Covid lockdown project, it's +12/-12. It would be a bit tricky, but I was thinking of disconnecting the -15V supply from the contour EQ section and injecting -12V from my gadget. Do you think that's worth a try?

[Helmholtz]

Do you have a scope?
Seeing the waveshape of the hum would help to identify the hum source.

[bobloblaws]

I found a thread at https://music-electronics-forum.com/...-tremoverb-hum that discussed a Mesa Hum Fix that turned out, according ti the OP, to be a design flaw. It may not help in your case but can't hurt to read about what other's have experienced.

It looks like his solution was this:
It was a ground loop in the reverb recovery circuit; the stock setup has each channel's master volume, presence, and reverb pots sharing a ground bus. I removed the reverb pots from the bus and grounded them at the grid reference resistor for the post-master triode.

[bobloblaws]

Do you have a scope?
Seeing the waveshape of the hum would help to identify the hum source.

Yes, I have a digital scope, but I'm not very good with it. I've only ever used it to check output power. I don't know how to set up the scope to isolate specific frequencies. I could give it a shot though.

[Helmholtz]

Just post a pic of the hum at the speaker output. Should be mVs.

[bobloblaws]

Just post a pic of the hum at the speaker output. Should be mVs.

I think I managed it. First I "tuned" the scope by putting 64HZ through the amp from my signal generator and then using the "auto" button on the scope. Turned off the test signal and was left with this:

[Tom Phillips]

That's a nice scope capture.
Another scope application tip when looking at a signal related to the line frequency is to set the trigger mode to "Line."

[Chuck H]

Interestingly there looks to be a 120Hz artifact? I hope Helmholtz or someone else has insight on this and you can source this problem.

[Helmholtz]

First I "tuned" the scope by putting 64HZ through the amp from my signal generator and then using the "auto" button on the scope. Turned off the test signal and was left with this:

No need for "tuning" as your scope has autoset and automatically displays period time and frequency.
(I would change the settings to 50mVdiv vertical and 5ms/div for a larger display.)
The hum pattern shows single sided (negative going) peaks, typical for power supply ripple.

Now I would scope the +/- 15V supplies with the scope input in AC coupling.

[Helmholtz]

Interestingly there looks to be a 120Hz artifact? I hope Helmholtz or someone else has insight on this and you can source this problem.

The small dips on the positive side are spaced at 60Hz.
Not sure what it is but the output hum might be composed of 2 different hum signals.
I would ignore the small dips for now.