Boxer 30 Bass Amp hum. (Trace Elliot)

[Devs]

Hi.

I got the above amp with a Peavey bass guitar. It's in very clean, little-used condition and works well, but it does have hum...

The hum is noticeable whether an input lead is plugged in or not. With nothing plugged in, the hum is quite audible from half-vol upwards. In this state, it's more of a low-toned background 'internal' hum. At full vol with the bass control on max and the mid-shift button pressed, I'd call the hum 'quite loud'! Very intrusive.

With a guitar plugged in, the hum increases and is now a higher-toned, 'crisper' hum, clearly now being 'picked up' by the guitar itself. With a different amp - a 100w Torque keyboard amp - the same guitar & cable is virtually quiet even at full vol, and this amp has no perceptible hum at all with no inputs plugged in.

I've checked the earthing points and find no pd between the chassis and input socket 'sleeve', potentiometer bodies, etc. The main power supply earth is also connected to the chassis and is making good contact (this is in the UK, just in case things are different elsewhere!)

Any suggestions where the 'internal' hum could be coming from? Could the power supply smoothing capacitors be worth replacing as a first test - easy and low cost?!

Thanks for any advice.