Electrical help needed. Get yer thinking caps on for me please experts!
 

So.

I bought a second hand ABM EVO 300w. It hums like a bitch.

I rigged a plug without the earth connected. It still hums, but when I touch the screws on the VU meter, the fan on the back or the D.I. socket, it goes quiet. I then tried it with a standard plug lead but with the earth wire inside the amp disconnected from the power lead input socket. Same thing, hums to start with but if I touch any screw or the D.I. socket the hum disappears.

Any suggestions as to where to go from here to fix it?

Thanks all.

You really need to get it to a shop before you electrocute yourself. Circumventing / defeating a ground is never a good idea.

Riis

Hi.

NOT very bright idea unless You used an isolation transformer.

+1 for taking it to a tech.
Based on Your actions so far, you'll either injure yourself or run the eventual repair bill higher. Or both.

Regards
Sam

Stop screwing around with the ground - that's not how your trouble shoot. Sounds like there is a bad internal ground connection somewhere. Take it to someone who knows what they are doing.

Are you sure the outlet you're plugging it into is correctly wired?

Take the tech route.

IF it has an electrical problem WRT the building's service, you would first need to tell us where you are- your use of 'earth' makes me think you're not in the US, or at least, not from hte US. If you aren't in the US, we need to know the line voltage and the wiring scheme- 120VAC with one Hot/one Neutral/one Ground (Earth), or 220/240VAC with two hot and one ground, etc.

Have you tried a different cable? Do this without any effects in the mix- troubleshooting requires starting with the minimum number of devices and connections before expanding once you determine any problems.

I would find out any changes that have been made to this bass. All it takes is one bad connection for this kind of noise to result.

Any grounding issues need to be checked, all the way from the instrument to the wall. If that's electrically sound, it's a problem with the building and you would want to start by moving to a different receptacle.

As others have posted, don't lift grounds unless you want to be the load, i.e., the light bulb.

If you 'know' enough to be disconnecting a ground, you shouldn't be opening an amp.

+1

A simple tester will help you eliminate a poor ground (just because there is a wire on the outlet does not insure a proper ground) or a reversed L1 / neutral connection.

Another easy test is to try a different outlet that is on a different circuit (could be noise on that line due to a dimmer, electric motor, etc.).

In this day and age, you want a proper 3-pin (grounded) outlet on the amp and in the wall. Not like the old days where you might have to flip polarity to lessen power noise.

Good points. In addition, the polarity flip switch in some early amps actually involved placing a capacitor between one of the conductors and the chassis of the amp. That capacitor is now fondly known as the "suicide cap."

Hah....... this is more typical of grounding problems on the INPUT...... amp input jack, instrument cable, or bass (jack or shielding).

Does it hum with NOTHING PLUGGED IN? (with all the proper grounds re-connected)

Thanks for all your responses.
I'm in the UK. 230v. The hum/buzz is without anything plugged into the amp. I've tested it on 5 mains leads, 4 locations around my house and as of today 5 buildings! I am beginning to suspect the mains input socket on the amp. How can I test that?

Cheers,

Kap

Oh, and I had two different "techs" suggest the non-earth lead as a quick test. I guess I have picked the wrong techs! One in a music store and the other a valve amp specialist. LOL. The amp and ext 1x15 cab sound great apart from the buzzing. (Sigh)

Cold be a bad power cord.

So, just to get it all in one place.....

Nothing plugged-in

Amp buzzes whether ground lead is connected or not.

if you go over to it and touch some metal on it, the buzzing stops.


OK........

If that is correct.....

I don't know this unit.... is it a head ?

Is it all metal case if it is a head?

If it is a combo, is it an all-metal drop-in chassis, or something with maybe an open-top or open bottom chassis like a Fender or Marshall, etc?

Reason I ask is because when you touch metal on it, you become "part of the shielding"..... if something is amiss inside, and the top is open (covered by wood cabinet material) then you leaning over it can shield the innards from lights etc that might make it buzz.