buzzing amplifier

[Space cat]

Fenderman, since you're a musician you might get some information out of the sound. An earth loop/missing earth problem will give you a relatively clean 50Hz hum (60Hz in US) with a few odd harmonics. If the third harmonic is particularly strong it's probably coming from a transformer core.

To me the word 'buzz' suggests a sawtooth waveform with loads of harmonics. Can you identify the fundamental? Is it 50Hz or 100Hz (US, 60 or 120)? A 100 Hz sawtooth is highly indicative of power supply trouble. Maybe a smoothing capacitor has failed. If you can get inside, look for visible signs of damage. Electrolytic capacitors go bad over time. This is sometimes visible as bulging or fluid leakage. In extreme cases they explode!

While you're in there you can tell us whether this thing uses valve (tubes) or not.

 

[tim west]

When I used to have to test a lot of musicians equipment for shows I used to find loads of guitarists had removed the earth from the terminal inside the plugtop to stop the hum. I used to replace it before testing knowing full well that they would remove the earth again once i had gone.

 

[plugwash]

I don't buy this one being an earth loop problem. Usually those kinds of things happen when you have multiple peices of earthed equipment connected together. I wouldn't expect it to affect a simple setup with just amplifier and instrument. Nor would I expect it to be the only sound audible and be unaffected by the volume controls.

 

[tim west]

An earth loop can be an effective pickup for noise especially when the earth supply is a dirty one as expected in residential properties.

Although not condoning it, I was pointing out what a lot of musicians especially professionals do by removing the earth from the plugtop earth terminal. I have seen so many guitar amps like this that the problem must be quite widespread.

 

[bernardgreen]

The original poster mentioned two LEDs on the PCB were not lit.

Could it be that part of the power supply has failed. many power amps have a dual supply (positive ---0v --- negative) and one of these failing could be the cause of buzzing and non lit LEDs.

Or one of more of the smoothing capacitors has become disconnected leaving the supply with mains ripple. A broken track on the PCB where the capacitor is soldered in is a common fault.

 

[plugwash]

Although not condoning it, I was pointing out what a lot of musicians especially professionals do by removing the earth from the plugtop earth terminal. I have seen so many guitar amps like this that the problem must be quite widespread.

I'm sure some annoying noise is picked up due to earthing related issues, especially when the gain is high. I'm just saying it doesn't seem to fit the reported symptoms in this case.

I would expect a problem caused by earthing to mostly affect the earlier stages of amplification and so the noise volume be changed by the volume control. I would also expect it to mostly be a problem when there is more than one peice of earthed equipment involved (e.g. multiple instrument amplifiers connected to a mixing desk).