[Boatanchors] An interesting meter repair

[Arthur Shulman]

I recently acquired a 'junk' HP 651B Test Oscillator - basically an
audio-10mhz generator, which I hoped to put in working order. Upon
powering up the unit, the first thing noticed was that the output meter
pointer jumped to 20% of full scale and locked. After a few seconds, it
was possible to return the pointer to zero by tapping the meter face.
The test was repeated several times with the same results, indicating
there was an internal meter mechanical problem, not an electrical
failure.

Well HP really put these things together! It took about an hour to
remove the front panel, and then to remove the meter from it, mainly
because of my unfamiliarity with the assembly method

I inspected the casing and found no external evidence of damage, so I
tested the movement with power supply, series resistor, and a VOM on the
ua scal. Same result. There was no possibility of replacing the meter,
so, with nothing to lose, I decided to open the case to inspect the
movement.

My metod of handling these delicate devices is to avoid touching the
internals. Blowing gently on the meter pointer is sufficient to deflect
it to full scale without damaging delicate parts.

After another hour of careful investigation, an oddity was noticed.
First, repeated deflection of the pointer yielded further movement in
the 30% range. Second, it was possible to get full-scale deflection by
blowing harder, at which point the needle would lock at 60% when trying
to return to zero. This seemed to suggest some mechanical binding at a
reproducible part of the range.

With the pointer locked, a careful inspection under a strong light of
the internal aspect of the left magnet pole ajacent to the armature
bobbin showed what appeared to be a bright orange irregular patch
corresponding to the position of the interference. I thought that if I
could clean this unknown staining without causing damage, I might be
able to restore the meter.

I cut a length of 4 thou mylar film about 1/32 wide an inch long, and
very carefully inserted it between the bobbin and the magnet, while
blowing on the pointer to position the armature effectively. Sure
enough, my efforts resulted in a noticeable improvement. I tried moving
the mylar around to abrade off the deposit, and then it occurred to me
that I had a better solution. I had on hand a quantity of mylar abrasive
strips in various grits, the finest of which were invisible to the naked
eye. I tried these, starting with the finest, thinnest grade, and
working up to a somewhat coarser grit. I was careful to gently blow out
any debris, but none was ever seen.

It took about 1/2 hout to completely free up the stuck movement, with a
little extra sanding and polishing for good measure. The result is a
perfectly working meter, earlier deemed unsalvageable!

I don't recommend this procedure unless there is no other option, you
have a very steady hand, and are respectful of the delicacy of the
mechanics involved. Believe me, doing root canal treatment is much
easier and more relaxing than salvaging meter movements! But there is a
great deal of satisfaction if you succeed!

I would like to hear if anyone else has encountered this kind of a
problem, and what the cause might be. There was no evidence of moisture
anywhere, and I am at a loss to explain the rust/corrosion/fungus that I
observed.

Arthur Shulman VE3ZV