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Author Topic: Marshall 5005/Lead 12 Hum Source?  (Read 498 times)

Marshall_Lead12Guy

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Marshall 5005/Lead 12 Hum Source?
« on: August 22, 2020, 10:16:44 PM »
Trying to track down the source of 60Hz hum in a Lead 12 I picked up last week. So far, I've not fixed it. So, I could use your help, guys.

Main clue: volume pot all down or all up yields loudest hum; hum nearly disappears when pot is at 2 o'clock.

Here's what I've done so far:
(1) Replaced all electrolytic caps (they were probably over 30 yrs. old.)
(2) Removed volume pot for out-of-circuit check; 1.14M, smooth from 0 to 1.14 through rotation.
(3) Hit the whole board with the soldering iron (in case of a non-visible bad joint).
(4) Re-soldered the ground wire connections (the one from transformer to chassis, and the one from transformer to one of the filter caps).

[Probably not related to the hum, but worth mentioning: the transformer needs a varnish dip, since there's a medium-loud mechanical hum.]

I'd attach an image, but I'm new here and so don't yet have the ability to do so (as far as I can figure, anyway). I can assure you, though, that the internals are clean. All looks as you would expect.

I look forward to hearing from you. I'd love to get this figured out.
« Last Edit: August 22, 2020, 11:02:25 PM by Marshall_Lead12Guy »

Enzo

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Re: Marshall 5005/Lead 12 Hum Source?
« Reply #1 on: August 23, 2020, 01:24:00 AM »
You knew the volume pot worked because you could hear it change things throughout its range.

Determine whether your hum is 60Hz or 120Hz.   Or if not in USA, then 50Hz and 100Hz.

Turn it all the way down and solve that hum first.  I suspect you have two hums going on.  You have some sort of hum coming in from the early stages, and it is out of phase with whatever hum is after the volume control.  SO at some middle setting of that control the two cancel.  That explains your 2 o'clock thing.

A schematic would be nice.

Marshall_Lead12Guy

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Re: Marshall 5005/Lead 12 Hum Source?
« Reply #2 on: August 23, 2020, 01:37:58 AM »
Many thanks for the reply, Enzo.

Here's an SSGuitar thread with the schematic for the head version (the 3rd schematic from top; marked 3005 (rather than 5005, which is my combo version)): https://www.ssguitar.com/index.php?topic=3787.15

Ah, right, I shouldn't have questioned the volume pot, since it was doing its job successfully regulating volume.

It's 60Hz.

Again, thanks for replying. This is great.