On 15/03/2015 3:23 AM, Arfa Daily wrote:
"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
**So true. I serviced a (brand and model deleted) power amp awhile back.
Simple design, but with high(er) bias switching. It, too, used a micro
for muting and switching the bias switching. Needlessly complicated.
Particularly in light of the fact that when the amp landing on my bench,
it was the micro that had crapped itself. It was one of those mask
programmable things, which I had to order from the manufacturer
(overseas - New Zealand). Silly buggers sent me the wrong micro, which
meant my client had wait even longer for his amp.
The amp has three switches. Power (just a big honkin' switch for the
mains), a muting toggle and a bias toggle. It could have been far
simpler and more reliable to just use simple stuff. If they had wanted
to get fancy, then a few gates is all they had to do.
For ****'s sake the damned thing was complementary symmetry to boot!
--
Trevor Wilson
www.rageaudio.com.au
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