Guitar amp problem

[joe-90]

Hi folks, don't know if anyone can help me but my problem is this:

I've got a Sessionette 75 guitar amp that is about 25 years old. It was working fine until the other day when it started to develop a mains hum from the speaker. Not dead loud but far too loud to ignore. Then it started to get distortion and lose power. I took a guess that it might be a dried up electrolytic in the power supply so change to two big ones. However, it made no difference.

I had a look at the circuit board closely and noticed that the 1 watt resistor leading to the external speaker socket was burned -and so I suspect it might be one of the MOS-FET power transistors that has burned out. I can get a whole now power amp board from the guy that still services them for about 30 quid.

However, (and this is the bit I don't understand). I read somewhere that if you plug your guitar into the faulty amp and then take a lead out of the 'send' socket (the output from the pre-amp) and send it to a different good amp rather than back in the 'return' socket it will give you an indication of whether the preamp is OK as it will sound OK through the good amp, if you see what I mean.

I did this and although it seemed to lose most of the distortion - the mains hum was still there as loud (or louder) than ever.

So I switched the faulty amp off at its on-board on/off switch - but the mains hum was still coming out of the good amp. Yet when I unplugged the faulty amp altogether the hum stopped.

What's going on? Why does it still hum when the faulty amp is plugged into the good amp but switched off at the switch - yet doesn't when I unplug it from the socket? I don't want to order a new module if I've made the wrong diagnosis. Can anyone shed any light on the matter? Cheers.

Oh and I forgot to mention I only got the problem after I connected it to a couple of 2x12 speakers. Don't know if that is relevant or not.

[vtech]

Keep in mind that in general, any guitar amplifier input must see some sort of a resistance between Tip to ground at all times or else it will translate in to a hum at it's speaker. So, if you look at the schematic HERE on page 4, the "effect send" J3 will see a 10k resistance (through J4 and monitor out). So short of a broken trace or bad connection at J4 there should not be any hum?

Of course there still may be a problem in the main amp section; lets say one of the shared voltage rails is pulled down due to a defect in the Amp module. It can easily upset the TLO71/72 Opamp's operation, throw a DC offset at the output and create a hum. --although this would only be valid when the amp is ON. Can you do a basic voltage measurement on the amp?