Blimey, this had me stumped for a while.

Blackstar valve amp.
Unless you insert a plug into the input jack, the Power Amp section is on
mute, but I already knew that.
So you can't check the Power Amp section by injecting a signal into the FX
return or Power Amp In socket, thereby bypassing the pre-amp section, as I
normally do, unless you insert a dummy jack.

Now this amp has a PIC onboard that (as well as controlling the footswitch
functions, also) senses the inserted jack in the input socket.
But, in addition, it also disables the Bias control circuitry, so as well as
muting the signal to the Power Amp, it also puts -100v bias onto the output
valves, thus cutting them off completely, unless a plug is in the input
socket.

But what I didn't know was that the PIC also gets a signal from the Speaker
jack sockets, and will again not take off the -100v bias unless THIS also
has a plug in it.


So after repairing the faults, I follow my normal procedure of checking HT
and bias voltages with no output valves or speaker jacks in place, but I
remember to put a dummy jack in the input socket to unmute the amp.
But having no plug in any speaker socket means the amp will not come out of
the -100v bias position and I think for a long long time there is another
problem to fix.




Hope that makes sense.




Gareth.