I've got a Trace Elliot BLX-130 SMH combo and it's got a hum around G# (420 Hz?) through the speaker.

The hum occurs immediately it is switched on, without anything plugged in, and is at a constant level regardless of the volume, eq, switches, gain settings.The amp also has a fan which kicks in on power-on; I can hear it spinning too (but the hum in question is from the speaker).

It'd be great to get some advice on what to check out for this (I'm thinking output diode/regulators?).

I don't have a circuit diagram and I haven't opened it up yet.


(P.S. The SMH is the model with a HF horn on the front, but the Fostex FH47 horn broke - ie. its plastic mounting became brittle and split - so at the moment there's a hole where it goes and its wires have insulation tape on them 'til I can get a replacement.)