New poster...electrical wizards help needed

[Kapucheen]

Hi all,

I've just bought a second hand ABM 300w 1x15 combo. Mint condition. Sounds beautiful. But it has an intrusive hum with nothing plugged into it. I have undertaken all the checks possible and the only thing that stops the hum is when I used a mains lead without the earth wired up. It still hums, but disappears if I touch a screw on the VU meter, or the fan at the back, or the D.I. input.

Anyone any suggestions as to what could be causing it? and the exam question...how to fix it? LOL

Thanks in advance,

Kap

Edited March 16, 2013 by Kapucheen

[icastle]

When you touch the casing you're providing an alternative earth route.

If the existing route to earth is broken then the most basic fault finding exercise needs to happen first.

1) Is the power point you're using properly earthed?
2) Is the mains lead you are using damaged?
3) Have you tried an alternative mains lead?
4) Using a multimeter, can you get a connection between the earth pin of the mains plug (unplugged from the wall socket of course) to a bare metal part of the amp chassis (the screw holding the fan in would do).

I'd strongly advise you that you shouldn't be disconnecting earth wires from mains powered equipment - that wee scrap of wire is all that stands between you and meeting your maker should something catastrophic go wrong.

[flyfisher]

[quote name='icastle' timestamp='1363478910' post='2013300']
I'd strongly advise you that you shouldn't be disconnecting earth wires from mains powered equipment
[/quote]

Indeed!

[Kapucheen]

Understood gents. Thanks. I've tried the amp in 3 locations around the house, with and without plug boards, away from all things electrical, 5 different mains leads, 4 different buildings etc

The earth wire inside the head looks fine. I disconnected the earth wire inside the head and put a fully earthed mains lead on. It hummed then touching the screw then had the same affect ie the hum disappeared.

I'm stumped.

[icastle]

Well you've got an earth problem there somewhere matey.

It's very hard to diagnose without actually seeing the amp - do you have an amp tech somewhere near where you live?

[Kapucheen]

Hi icastle, there is a company within 5 miles that advertises on the web. £35 to open up and £25 an hour after that :-( Hence the attempt to solve first. Looks like I'll have to grt used to my tinnitus of the amp !

[Phil Starr]

You have an earth problem which makes your amp dangerous, it sounds to me as if an internal earth connection is broken or possibly something inside the amp is touching something it shouldn't.

Not getting it fixed and putting up with the noise isn't really an option I'm afraid. It is dangerous and time off work won't come cheap if the worst happens. Actually scrap that the worst thing that could happen is death, an unlikely but possible outcome.

[icastle]

[quote name='Kapucheen' timestamp='1363594771' post='2014368']
Hi icastle, there is a company within 5 miles that advertises on the web. £35 to open up and £25 an hour after that :-( Hence the attempt to solve first. Looks like I'll have to grt used to my tinnitus of the amp !
[/quote]

Which part of the country do you live in?

[MoonBassAlpha]

It sounds to me like it could be the input jack socket.
They are designed to ground the input when no plug is inserted. The connection which "makes" when the plug is removed can get tarnished (usually silver plated). This could also be the case in the effects loop, especially if rarely used, though this results in loss of output.