Help! Vegas 400 problem.

richard burton · From: Britain

My Vegas 400 (220 volt) has developed an alarming hum when a guitar lead is plugged in, with nothing on the other end of the lead, and nothing touching the exposed jack plug. I checked on some of my other amps to see if this was normal, but they were silent. If I touch the earth on the jack plug, the hum diminishes slightly. Have I got a problem, or am I worrying unduly?
Thanks in advance, R B.

Ken Fox · From: Nashville GA USA

An unloaded guitar cable is a great antenna for hum (so is a single coil pickup). Even worse is a speaker cable, unshielded. Does it hum with the guitar plugged into the amp?
Amps have a shorting jack input that keeps the input shorted to ground until a guitar is plugged into it. With just a cord, you have opened the circuit to all sorts of noise pickup. I see a lot of older amps that have dirty or worn input jack shorting points. These amps will exhibit noise when the volume is turned up. Often I just cleaned and retension the jack and the noise is gone.

[This message was edited by Ken Fox on 05 October 2003 at 09:03 AM.]

richard burton · From: Britain

Thanks Ken for your input. All three input jacks exhibit the same problem, so I'm guessing it's deeper in the amp than the inputs. I double checked with another Vegas, set up alongside my Vegas, plugged in the same wall socket, but my amp is way noisier. Any suggestions, anybody?

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Adding a "polarity switch" (like most amps have), just a small capacitor which links the AC input to the chassis, might help. In the old days, we'd just reverse the plug to "polarize" the amp properly. Normally, this is the same way I test an amp to see if it's properly polarized. You plug in an open instrument cord (just like you're doing), and then throw the polarity switch to the position that gives the least hum.

If your line plug is polarized, simply reversing the line cord leads (inside the chassis) might also work, but always check with a voltmeter after you make these type of mods. You don't want 220v. on the chassis!

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