Ammeter movement adjustment ( or repair ).

[golddart0]

Dear Sirs

I have a problem with an ammeter movement ( moving coil, permanent magnet ) that I need to repair.

Electrically the movement is working fine.
I adjust the mechanical zero, the movement is connected to the calibrator and the needle deflects to full scale deflection according to specs.

The problem is, when the energy is removed, the needle doesn’t return to zero every time.
Sometimes it rests above the zero mark ( most of the time ), others below.

I have checked if the coil frame rubs against the core, or if there is friction between the frame pivots, but everything seems to be all right, moving freely.

I am not sure if the movement was “ serviced “ by someone before…

Can you give me some insight about what should I look for in order to put the movement working properly ?

Thank you very much in advance for any help you could provide.
Hope to hear from you soon.

P.S.- The movement is part of an analogue ammeter made by Chauvin Arnoux, model CA 401

[JAlberts]

First, check that your test reference amperage is correct so that you are not trying to over set the meter to compensate for a low test amperage. Also test the meter at different scale selector range maximums to determine that there is a consistent meter maximum reading accuracy or error across all selector settings. I would also recommend checking the meter's fuse contacts for any oxidation.

If there are issues found there and there is no erratic indicator motion; then, the moving of the spring end connection to the adjusting screw may have affected the spring's action (these very low rate clock type meter springs are very delicate). I had a similar problem with a VOM, when trying to adjust it, the needle action was free but became very erratic for no apparent reason; but, after a few repetitions of gently rotating the adjustment screw back and forth and resetting it to the correct position the problem disappeared. So you might try that.

If that does not correct the problem then wearing of the indicator shaft ends creating friction in the jewel bearings may be the problem; or, the needle return spring may be weakened or damaged; and, as a result, when you lower the spring tension with the adjusting screw enough to allow the indicator to reach the required scale maximum value, then there is not sufficient spring tension left to return the indicator to the zero point. To test for that, I would reset the indicator to a lower scale value and see if the indicator then moves closer to the scale zero point.

If so, then the spring has probably lost some of its tension or the indicator shaft is worn and replacement of the meter is going to be the best bet; but, depending upon the meters replacement cost, you may want to contact the meter mfr or try to locate a qualified meter repair facility before buying a replacement meter.

[golddart0]

Dear Mr JAlberts

Thank you very much for your contribution.

By the time I received your reply the problem was solved.

After some time trying to adjust the galvanometer and not being able to return the needle to zero consistently, I decided to disassemble the PMMC movement ( The d’Arsonaval type PMMC galvanometer calibrated to Amperes ) to see what was happening.

I turned out, that, there was little debris particles between the core and the moving coil frame, that made the frame stop near the end of the travel to “ zero “.

After the movement have been clean and assembled again, everything was fine.

Once again thank you very much for your time and attention.