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.
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.
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.
Quote from: Marshall_Lead12Guy on August 22, 2020, 10:16:44 PM
I'd love to get this figured out.
I know this post was some time back, but I have a Bass 12 with exactly the same problem. It is driving me nuts! Did you ever get it figured @Marshall_Lead12Guy?
I've posted here:
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/937816-1980-s-marshall-bass-12-hum-problem (https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/937816-1980-s-marshall-bass-12-hum-problem)
and here:
https://www.thefretboard.co.uk/discussion/210064/marshall-bass-12-repair-help/p1 (https://www.thefretboard.co.uk/discussion/210064/marshall-bass-12-repair-help/p1)
about my experiences, and I'm still none the wiser. I've managed to reduce the hum down by replacing R17, and as you can see on the fretboard posts, there is a Japanese chap who had the same issue. It's still not fixed though, so not sure what to do next. Any assistance/ advice would be much appreciated.
@Enzo as a Jedi master in all things of this nature ( in the past you helped me out with some advice on my Peavey Classic VT) do you have any thoughts on what could be the problem?
Well, I don't mean to be unkind, but one problem I see - and that is in many many threads - is people assuming that hum (the most common amp complaint there is) will be caused by the same thing.
DO you have that same "hum is less in the middle rather than the ends" on the volume control? Are you sure your hum is 60Hz and not 120Hz?
There are a zillion sources of hum in amps.