[Amps] Calibrate a panel meter?

[Karl-Arne Markström]

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 can lose
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 scaled current.

73/

Karl-Arne
SM0AOM



----- Original Message ----- 
From: "Bill Turner" <dezrat1242 at ispwest.com>
To: <amps at contesting.com>
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
> 
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