"Phil Allison" wrote in message
...
Arfa Daily wrote:
** The schem of the 2.3 should be very close:
http://www.mediafire.com/view/bmotmj..._manual+8.jpg#
Thanks for that, Phil. Much appreciated. Odd how it works for a short
while.
Before having the schematic, I gave everything in the area a good dousing
with freezer when it failed, but nothing really that I would say was very
positive. Maybe - and it *was* a maybe - the 5532 was a bit sensitive,
but
it was inconclusive. I did notice that when it was wrong, the waveform on
pin 7 I think it was, was much bigger than 'normal' and the output TRs
on
that side got quite hot quite quickly.
** Is there a dummy load connected ?
The clip indicator is probably just showing the channel is being
overloaded, as is the rapid heating. Could there be an intermittent short
in the output wiring ?
Also, I see there are two PCBS, small signal and power stuff,
interconnected by some kind of multi-pin. You must know how reliable they
all are...
I'll try and find time to get another look today or tomorrow with the
benefit of your schematic. Thanks again.
** I found the Alto schem on a net forum, it looked terribly familiar.
Have you not seen the QSC's USA models or the MX700 ?
... Phil
Yes, and embarrassingly, I was even working on one last week ... :-\
Funny how just a different name on the front blinkers you to the obvious. I
was even discussing that odd output topology with a colleague.
Back to the problem in hand. I only had a few minutes to look further today.
In answer to your questions, yes, there was a dummy load on the end that has
a built in power meter. Yes, there are two main boards, each of which has a
power amp and power supply, and one of which also has the driver / clip
sense circuitry for both channels. Initially today, it was giving its
problem. Couple or three volts RMS going in, transformer balanced into the
channel 2 XLR input. As soon as the front panel level control is advanced
just a gnat's cock, on comes the clip light, very solid. The amp still has
output, but it's pretty strangled, as you would expect if the clip limiter
is genuinely coming in. When looking at the output of the opamp on pin 7,
when it's wrong, it's massive, and asymmetric, so I guess that's the reason
for the clip circuit to come in and the light to come on. It's as though the
opamp has suddenly attained 10 times its normal gain. I thought at one point
that maybe the level control was going intermittently open at its groundy
end, but when it's working, with that level of input signal, you can wind
the level pot full up, and not make it clip. It's as though a feedback
resistor is going open intermittently. Trouble is, all of the resistors
around the opamp are those tiny tiny sm devices that are so small you can
barely see them.
Actually, a thought now occurs to me. I wonder if the level control is
actually nothing of the (conventional) sort, but rather a variable feedback
resistor for the opamp, as they sometimes are in some amps. If that went
open, it might allow the opamp to go to full open loop gain. I don't
remember the level control being shown on the section of schematic that you
linked for me. I'll maybe have a look tomorrow if I get time, but today, a
****load of work has poured in the door, including four Technics SL1210s
that are needed by the weekend, an Evans tape echo, two Voxs and a Marshall
....
As I finished checking around the opamp today, it came back right, and
wouldn't go bad again, with the control as smooth as you like from zero to
flat out, so at that point, it got flung off the bench again to make room
for something else.
Oh yes, and the ribbon. There is one of about ten or twelve ways that links
the two boards, and I have had problems in the past I seem to remember, but
in this case, it seems quite solid, as do all the other connectors. In fact
I haven't found a way to provoke the problem into either coming or going. It
just does it, as and when it feels like it.
Thanks for your input. Any and all suggestions and insights are welcome with
problems like this
Arfa