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Old 24th May 2023, 8:51 am   #21
pmmunro
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Angry Re: MoD Voltmeter - wiring arrangement?

Herald1360,

Thank you for pointing out and correcting my serious mistake - a bad slip of the brain. (Shame emoticon x 1000)

6SN7GT,

There's a good possibility that the fault with your meter is an open-circuit rectifier, especially if it's copper oxide or selenium. This can fairly easily be replaced, ideally with four germanium diodes, but silicon will probably give sufficient accuracy.

You can test the basic meter (without the rectifier) with a variable power supply and a 12k or 15k resistor in series with the movement. Set the supply to around 9V and increase it until you get full scale deflection.

PMM
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Old 24th May 2023, 11:19 am   #22
6SN7WGTB
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Default Re: MoD Voltmeter - wiring arrangement?

Quote:
Originally Posted by pmmunro View Post
You can test the basic meter (without the rectifier) with a variable power supply and a 12k or 15k resistor in series with the movement. Set the supply to around 9V and increase it until you get full scale deflection.

PMM
I often use a DMM set to ohms and manual range where you can therefore 'drive' a movement with a precise and limited current.
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Old 24th May 2023, 1:35 pm   #23
karesz*
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Default Re: MoD Voltmeter - wiring arrangement?

Hi Richard,
I do it often too, but these testing currents are, usually ~1mA (not exact=1mA!), so can be too much for some sensitive movement--so: be careful please, begin in higher range!
K.
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Old 27th May 2023, 2:12 pm   #24
6SN7WGTB
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Default Re: MoD Voltmeter - wiring arrangement?

Quote:
Originally Posted by karesz* View Post
Hi Richard,
I do it often too, but these testing currents are, usually ~1mA (not exact=1mA!), so can be too much for some sensitive movement--so: be careful please, begin in higher range!
K.
Fortunately my DMM goes down to 500nA at high ranges, and then steps through 5µA, 10µA, 100µA, then max of 1mA.

But, yes, you do need to be careful - a bit like some older ohm meters and testing 1.4V filament battery valves...
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