"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Anybody got a copy of the schematic that's in the back of the user guide
that they can scan for me, please ? I've searched all my files and
drawers high and low, and can't find my book anywhere ... I thought
that the 'net would be awash with copies of the schematic, but it seems
only for the Mk 2, which is totally different to the MK 4.

Have you tried AVO direct? They still make the Model 8, IIRC.

--
*Dance like nobody's watching.

Dave Plowman London SW
To e-mail, change noise into sound.

Thanks all. Some useful suggestions there. You are of course all quite
correct in that it is very difficult to damage an AVO. This one is now
around 40 years old, and it has been in use on a daily basis for all of
those years, and has withstood much normal workshop abuse - yes, including
measuring the resistance of the mains ! - many times over. It was a very
high pulse voltage that caused the problem that I now have. I have recently
been working on some pro lighting equipment that makes use of short-arc
discharge lamps, and it is important that the running voltage, once they
have struck, is correct. They are fed with a 50% duty factor square wave at
about 200v p-p, resulting in an average 'AC' voltage of 100V across the
lamp. The AVO reads this nicely on either its 100V or 250V AC ranges. When
the lamp is being started, however, it is subjected to a pulsed igniter
voltage of several kV. When the lamp strikes, its impedance drops quite low,
and this swamps the igniter circuit and stops it. Stupidly, I had the meter
connected during the ignition sequence, and now, 100V of 'real' sinusoidal
AC reads about 35V ...

I am at this point thinking that it is going to be one of the two rectifier
diodes, which seem to be "OA" germanium types, but I guess it could also be
a fried metal film precision resistor. Needless to say, there is no visible
evidence of any problem :-\

Arfa