Modifying meters?

If it really is several amps then most probably all you have to do is remove a shunt resistor that is placed parallell with the meter coil. It may not be 1mA but perhaps a 10mA coil.

But steer away from "moving iron" types. Often they are high-current movements - and with poor linearity.

Jakob E.

diito re gyraf regarding moving iron meters. You can usually tell from the outside about those: the vane/needle is just emerging from a semicircular slot in the faceplate. I think one brand of this type was called Readrite, which of course prmpted people to call them Readwrongs. The field generated by the current is pushing the magnet on the vane against a spring.

There is no modification to this type of meter, to speak of, that will give it milliampere sensitivity.

Decent high-sensitivity meters will have a more complex motor usually visible to some extent from the front. The D'Arsnoval movements have little rectangular coils with very fine wire and some mechanical bearings, sometimes jeweled. Another type is the taut-band variety which looks a bit different.

If you rock/rotate the meter and the needle doesn't move much, it's probably not a high-sensitivity movement inside. However, if there is a low Z current shunt inside this will tend to damp the needle motion quite a bit, but still it will be more responsive than a moving-iron one.

Brad

The mechanics in moving-coil meters are extremely delicate devices. Think classic watch-technology and -precision. Also the coils and springs are not easy to work on (to say the least)

One important rule, when working on open moving-coil movements is to keep your workspace very, very clean - as even the smallest piece of magnetic scrap will surely find it's way into the magnetic gap (blocking movement at some point) - and will be close to impossible to get out.

Jakob E.

I love finding old meters for applying to audio gear!

One tip- keep a 1.5V AA battery and a 100k lin variable resistor (connected in series as a "variable resistor" rather than "potential divider") with a 330R series resistor in your pocket with croc clips when you visit surplus places. Meters which have a high current rating on the dial can quickly be checked to see if they have a lower sensitivity. This also tests to see if they're working. Many meters at rallies and junk places have had a bit of a hard life!

Obviously start with the variable resistor at its highest resistance setting and work down. The 330R resistor limits the current at max (when variable resistor is at minimum resistance) to about 5mA, which is the maximum useful range of basic meter sensitivity. Obviously decrease this series resistor if you want to test shunted meters of lower sensitivity.

As Jakob mentioned, a meter can be ruined by small flakes of metal on the bench- e.g steel filings. Scan the surface of the bench with a permanent magnet before opening the meter case.

:thumb:

Mark

thanks Mark what a great tip, I've seen a lot of surplus stores/junk sales with strange meters that I never would consider buying because I couldn't test them, now I can. Excellent thank you!