"Gareth Magennis" wrote in message
...
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.


Whilst it's all very clever sticking a micro in there, realistically, what
is the point ? When has a shorted input, as is the case with most amps with
a switched input socket, ever caused any kind of problem ? It serves to keep
the amp pretty silent, and is simple, and reliable. And if you really want
to shut the output stage off along with the rest of the amp, what was wrong
with a standby switch ? When I see some of this stuff, I can't help but
think that it is design for the sake of the designer's ego.

I have a friend that is an ex commercial designer, and his answer is to put
a PIC in everything from a toilet seat to a kitchen blender. If ever I
happen to mention that I've knocked up some little service aid out of bits
in the junk box, his first comment is ALWAYS "If I was designing that, I'd
have done it with a PIC ... "

Yes, sometimes what he says is valid, but often, when I've achieved the
desired result with a couple of transistors and a few other bits, without
having to sit down and write and debug code to make it work, I really can't
see what the point of the needless complexity would be ...

Arfa