Usually panel meters in this range starts life as 5 mA FS movements, and then
an internal shunt resistor
provides the final FS value.
It has been reported that the permanent magnets in d'Arsonval meter movements
their flux with age, so old panel meters often can read somewhat low.
An internal adjustable magnetic shunt is often provided in moving coil
movements, and by decreasing
the shunting action, the calibration can sometimes be restored.
If the magnetic shunt range should prove to be insufficient, the remaining
remedy is to remove the internal shunt resistor,
and provide an external shunt with the proper value to get the advertised full
----- Original Message -----
From: "Bill Turner" <firstname.lastname@example.org>
Sent: Wednesday, December 28, 2005 7:20 PM
Subject: [Amps] Calibrate a panel meter?
> I have a Simpson 923A panel meter, 1 amp full scale. It is reading
> low by about 6%. When it has 800 ma through it, it indicates about
> 750 ma. I'll be using it in my 8877 amp at 800 ma and I'd like it to
> be more accurate.
> If it were reading high, I could just shunt it, but reading low...???
> Is there a way to calibrate it by adjusting anything inside? Moving
> the magnets around, etc? I'm sure the factory calibrates them somehow
> but I've never read anything in the ham literature about how to do it.
> Suggestions appreciated.
> 73, Bill W6WRT
> Amps mailing list
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