[time-nuts] Solartron 7150plus DMM Failure
GandalfG8 at aol.com
GandalfG8 at aol.com
Mon Sep 21 13:57:51 UTC 2009
Apologies to any who may receive multiple copies of this but I thought it
worth sending to those lists where I've previously seen mention of this
I have now had two identical failures of Schaffner filtered mains sockets
on ex-mil 7150plus DMMs, although I'm not suggesting the ex-mil source is
In both cases there has been an internal component failure within the mains
socket that caused it to emit smoke around the edges of the metal filter
cover, accompanied by a brown goo and a very unpleasant smell.
The brown goo is deposited inside the meter in the area of the filter, and
very messy it is too, with a small amount seemingly contained within the
smoke and splattering on adjacent external surfaces.
In neither case did the meter stop working, or even pause to cough:-), so
I'm assuming it's most likely to be the parallel capacitor directly across
the line input that's failed. According to the Schaffner schematic there's
no varistor in there, which might have been a preferred suspect, but there
are two series inductors followed by two more capcitors, one between each
power line and ground.
After the failure the meter can continue to be used as it stands but
really needs a strip down and good internal clean in the area of the mains
socket, and even then it's still likely to smell unpleasant.
I don't have time at the moment but will open up the filters eventually
and see what might be done to rebuild or just bypass them.
Just cleaning the filter internally too would probably help reduce the
As a general precaution I would suggest, at the very least, standing these
meters on a sheet of plastic and, if there's anything reasonably close
behind it, wall etc, folding the plastic up behind it too as extra protection.
The unit that failed here today was sitting on the top shelf of a test
bench with its back fairly close to the wall, the wall is looking better after
a good clean but is still not very pretty!
A longer term fix might be to remove the filter components entirely and use
an external inline filter if required, there isn't much spare room inside
the unit so I don't think an internal stand alone would be an option.
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