De-buzzing MusicMan HD130 - any suggestions?

[Leo_Gnardo]

I've tried all the usual, got brand new F+T filter caps etc. Buzz level depends on setting of master volume - I've moved the PT wires away from it, no dice. Also copper shielding around MV control & wiring has no effect. Buzz diminishes by about 3/4 when in low power setting but still there and annoying . I'm suspecting rectifier switching noise from the voltage doubler power supply coupled thru PT magnetic field, I'm considering putting in a couple of 1100PIV FRED rectifiers. Buzz doesn't change with settings of channel volumes. Augmenting low voltage filters or bypassing with film caps has no effect. I'm at my wits end, more than usual, and would appreciate any suggestions especially if you've successfully tackled the same problem.

The amp's in practically brand new condition, no funky mods or service work has been done.

[drewl]

It's coming from the preamp, most likely the first stage.
You can try the low voltage filter caps and the 16volt zener diodes.
Try and find a nice quiet opamp for the first stage.

[loudthud]

Just to be clear on the Master Volume, does turning the Master all the way down kill the hum, or make it worse? When you flip the Standby switch off, does the buzz stop immediately? These things are known to toast output transformers. Everything good there?

[Leo_Gnardo]

Just to be clear on the Master Volume, does turning the Master all the way down kill the hum, or make it worse? When you flip the Standby switch off, does the buzz stop immediately? These things are known to toast output transformers. Everything good there?

Master vol down does kill the hum. Standby does stop it immediately. Output section just fine, a little shy on bias current at the moment. Has a set of EL34 that are working just fine, turns out a nice clean 110W at clip. It's the tube driver type, no pesky sandristors here.

drewl, if the first IC was a problem, buzz would vanish when channel volume turned down. The only thing that modulates the buzz is the MV. In all the time I've tackled MM's, from late 70's to now, total of two bad IC's found, ever. I'm thinkin' maybe, maybe the last IC that sums the channels and drives the MV may be the culprit, but it does do its job amplifying. Typically bad IC's produce either a lot of distortion, whoosh noise, or simply lock output to one of the power supply rails. In Peaveys, there are usually a lot of little diodes that clamp signal levels, sometimes these fail shorting to a supply line, no signal, just buzz, but there are none of those here.

One more coffee, then I'm going back in. Thanks for your input loudthud & drewl! Onward thru the fog.

[Leo_Gnardo]

Tried all sorts of fixes: de-power the neon pilot, nope. Thinking "Hiwatt", de-bundled the neat harness that runs near the PT, connecting signals to MV & main board, out drive board, 360V, 340V & IC power supplies, nope. Pull filament wires away from signal wires, nope.

Did confirm buzz develops in the final op amp stage that sends signal to the drive tube. I'm going to fix the symptom since I can't locate the problem. Clipped in a 100K in parallel with the final op amp's 100K feedback R - that drops the signal a bit and the buzz a lot. There's plenty of gain in the pre stages so the player may need to dial a volume control up a number or 2 to get to the same loudness. And with power switch set at low, noise is negligible. Gonna draw a line under it, call it done.

Thanks again for reading & sending your ideas!

[Randall]

Wow, that's an odd one.

[Leo_Gnardo]

<Randall - Wow, that's an odd one.> Make it two. Same customer has the head version too, same symptoms. I won't be re inventing the wheel. A two-penny resistor is going to save the day in as much time as it takes to solder it in. Put this one in your crime-stopper's notebook friends.

[drewl]

I've seen this alot in these.

The plus and minus 16volt diodes usually get very hot and need to be replaced or reflowed.
Some use older 1458 opamps which are noisier than newer ones.