[Boatanchors] my old VOM - anything I can do ?

[Barry Hauser]

Hi Emanuele & list:

There are a few possibilities I can thing of.

Does the meter have a mechanical zero set screw in the front?  Most do,
although some may have a trim cover over it.  Try adjusting this with the
meter vertical -- very gently.  Then lay the meter down flat and see how
far below zero it goes, if at all.  The spiral springs have progressive
tension so it might not be that far off.  If you mostly use the meter in
the vertical position, it might be OK even if it reads below zero when
flat.  Check it against another meter known to be working.  If the
adjusting screw has no effect, the little loop on the mechanism may have
bent so that the tiny nub on the inside surface of the meter is not
connecting with it.

Another possibility is that the needle bearing ends have worn so that the
pointer is falling out of position, though I doubt this. Many meter
mechanisms have adjustable bearings -- they screw in and out to apply
just the right hold on the needle bearing ends into what is usually a
"jewel" -- often two, front and back.  These could have backed out or the
needle ends worn down so the thing is not centered.  This will result in
wobble and sticking.

 I have also noticed electrostatic problems with some equipment due to
contact with modern plastics.  If you pass your fingers over the face of
the meter and can make the pointer move as if by magic, the meter glass
is electrostatically charged.  But this is a temporary problem I've found
only when a meter or has been wrapped in bubblewrap or poly and it goes
away by itself after a while.

Still another problem -- an extreme form misadjustment -- can occur if
the adjustment screw was turned too far, full around, etc. or the
mechanism has loosened up enough so the spiral spring is completely out
of range.

Hopefully just the screw adjustment will put things right. If not, you
might as well carefully disassemble the meter and very carefully look it
over.  I understand meter mechanics are few and costly.  I recently fixed
a meter where the pointer was way out of position.  The problem was a
combination of what I described -- adjustment far out of place, adjusting
screw not engaging the loop, and the bearings were loose.  However, that
meter -- also old -- had just been shipped to me and probably was bounced
around.

Hope this helps.

Barry


On Mon, 4 Feb 2002 23:21:24 +0100 "Emanuele Girlando (IW1DHI)"
<egirland@tiscalinet.it> writes:
> I am trying to revive my first 25 years old VOM (CHINAGLIA model 
> Cortina
> Major/made in Italy almost unknown in USA - very nice and with 
> great
> performance).
> It seems to work properly but the needle doesn't stay at zero when I 
> put the
> instrument in vertical position.
> Rather it moves to the middle scale position under the effect of 
> weight of
> the lower (heavier) part of the needle. I am sure, it didn't behaved 
> like
> that before.
> Does it mean it has lost magnetic field? or spring strength?
> Anything I can do?
> Tks in advance.
> ciao
> 
> Emanuele Girlando (IW1DHI)
> http://space.tin.it/clubnet/egirland/
> mirror at
> http://www.qsl.net/iw1dhi/
> 
> _______________________________________________
> Boatanchors mailing list
> Boatanchors@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/boatanchors
> 
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