Mechanical damping of panel meters...

[Andrew Smallshaw]

Hi all

I\'ve recently picked up an old Weir bench PSU and recalibrated it
to within quite sharp tolerances. However, the front panel meter
(switchable between voltage and current, although the schematic
shows it is working as voltmeter in both modes) reads accurately,
but the settling time must be a good 10-15 seconds before it stops
oscillating back and forth.

I haven\'t got as far as pulling the meter out yet but I expect
obtaining a compatible replacement will be difficult. Looking at
online it seems most practical sources sources address electrical
damping for brief transients in the signal, this is rather the
mechanical side of things. Basic research suggests pneumatic
damping is the usual order of the day but I haven\'t found anything
relating to actual maintenance and adjustment.

Does anyone have practical experience or is a replacement meter
the usual approach?

--
Andrew Smallshaw
andrews@sdf.org

 

[legg]

On Mon, 11 Sep 2023 11:27:28 -0000 (UTC), Andrew Smallshaw
<andrews@sdf.org> wrote:

Hi all

I\'ve recently picked up an old Weir bench PSU and recalibrated it
to within quite sharp tolerances. However, the front panel meter
(switchable between voltage and current, although the schematic
shows it is working as voltmeter in both modes) reads accurately,
but the settling time must be a good 10-15 seconds before it stops
oscillating back and forth.

I haven\'t got as far as pulling the meter out yet but I expect
obtaining a compatible replacement will be difficult. Looking at
online it seems most practical sources sources address electrical
damping for brief transients in the signal, this is rather the
mechanical side of things. Basic research suggests pneumatic
damping is the usual order of the day but I haven\'t found anything
relating to actual maintenance and adjustment.

Does anyone have practical experience or is a replacement meter
the usual approach?

.. . . . unless the meter is reading a real disturbance . . .

There\'s probably a failed capacitor inside that needs replacing.
Could even be an electrolytic across the meter terminals.
Most sensitive moving coil meters (<1mA) are pretty much self-
damping and won\'t require much filtering to average a measurement.

RL