Fender Bassman 100 Silverface weird hum problem.

Psygoat

Hi.

I have a Fender Bassman 100 with this odd problem.
It has a really loud mains hum when switched on, I initially removed the preamp tubes and this brought the noise down a lot, so initally suspected some kind of grounding problem,

If it's been on for a while, there is a pop and the hum disappears

I've left it running for hours and there is the occasional pop.

Other than that it sounds fine, clean signal despite the hum.

I've done quite a lot to it already - new filter caps, one of the power supply dropper resistors was way out so replaced that, new electrolytics in bias circuit and on preamp cathodes.
New control and screen grid resistors and a couple of plate resistors on the normal channel (didn't do anything)

The story behind it is that it has been standing for a very long time and the mains transformer doesn't look that great - the point where the wires come out of the mains transformer - the transformer case has a crack and it looks like kinda melted or something has leaked out of it.

Apparently it worked before then started red plating, after which it was put aside for 15 years.

I'm baffled by the fact that it is 'fixing itself'

Any ideas gratefully appreciated.

ICBM

Check the connections to the brass grounding strip under the pots. It does sound like there's a bad connection in the preamp ground path somewhere, which probably as the amp warms up the metal flexes a bit and makes a good contact.

These things can be an absolute nightmare to find...

Psygoat

Thanks I'll give that a go.
I've never seen an amp with so many messy grounding points -  and that waxed eyelet board - yuck.

ICBM

Psygoat said:

I've never seen an amp with so many messy grounding points -  and that waxed eyelet board - yuck.

I just had to fix a friend's '66 Vibro Champ with all sorts of hum, crackling and tremolo not working properly issues - it had both a cracked ground wire connection to the brass sheet *and* eyelet board conductivity issues... as usual, the underlayer rather than the eyelet board itself, so I did the Kelloggs Mod on it

Psygoat

I'm really hoping it's the brass plate. I can see a hairline crack at one point, I bridged across it with a crocodile clip which made no difference, but it might have multiple issues. I need a more powerful iron to go over the various grounding spots, so I can hide it some place where I don't have to look at it for a week, and get on with something else.