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Cannot zero offset error on Unigor A43 analog multimeter

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pbs74:
I have just purchased a Unigor A43 analog multimeter. It is in really good condition, but I cannot adjust it to zero with the offset screw below the meter scale. The lowest possible adjustment is about 5 on the 100 scale or 1.5 on the 30 scale, see picture (note, same result in off position). In the photos from the seller, however, the meter was adjusted to zero, and seller confirms it was fine prior to shipping. Needle does not look bent.

I suspect damage during transport due to tossing/bad handling. There were some further evidence of that: part of the outer box has been torn and taped by shipment company, the meter case has some cracks in two corners which seller claims was not there, and the strap around the meter inside its carry case is partly cracked as if it has been stretched.

I have tried measuring with the meter, and it works quite accurately across the ranges, except for the offset error. Also, the needle is not physically constrained, i.e., it can go towards zero/beyond when getting a small and safe negative signal.

When lying on its sides, exposed to gravity, the needle will move to close to zero on one side, and 10 on the 100 scale when lying on the other side - i.e., symmetrical movement around the "offset" of 5 on 100 scale.

Anyone with experience in analog meters with an opinion on what could be the cause? Is it likely that the meter has been damaged due to excessive force during transportation? Anything that can be done, or checked to verify the cause? Could it be balancing weights?

When taking apart, there is another adjustment screw on the backside of the meter casing, but I have not tried adjusting that yet - could it be a further range for offset adjustment on the front?

Thanks,
Peter

wasedadoc:
Meter movements typically have spiral springs. Imagine a vinyl LP of extremely short playing time. Try to see if excessive mechanical shock has resulted in adjacent turns getting caught together. If they have it may be relatively easy to separate them.

bob91343:
If there is no evidence of spring damage, then bend the pointer slightly at its base.  Or adjust the spring.  While these are delicate things, they are not very precise so you won't cause any trouble.

Another way is to bend the two ears that engage the zero corrector screw.

pbs74:
Some more details after "taking it apart" :)

I've managed to zero it in (image 1) by moving the blue wire a bit (image 2), so it measures correctly across ranges. However, the new zero position means the zero adjustment screw/tap does not fit anymore, which is kind of annoying when assembled (image 3).

I wonder if the zeroing plate has somehow been skewed vs the metering base (close up images 4 and 5). I've tried to put light pressure on the zeroed plate while holding the base metal parts, but nothing moves, and I do not want to turn too hard, these are delicate things (notice the very, very thin string from the needly mount to the top place, through the hole the zeroing plate is centered around).

Anyone with some further suggestions based on the close-ups? Worst case I would be able to assemble without the zero adjustment screw and poke with a small screwdriver (or maybe non magnetic plastic piece), but would be a lot nicer making it work like it should, so help much appreciated.


--- Quote from: wasedadoc on June 26, 2022, 09:41:50 pm ---Meter movements typically have spiral springs [...] Try to see if excessive mechanical shock has resulted in adjacent turns getting caught together.
--- End quote ---

I do not see any spiral springs, maybe covered under the "zeroing plate" at its center/mount?


--- Quote from: bob91343 on June 26, 2022, 10:57:42 pm ---If there is no evidence of spring damage, then bend the pointer slightly at its base.  Or adjust the spring.  While these are delicate things, they are not very precise so you won't cause any trouble.
--- End quote ---

Might work, but with above zeroing progress, I am not too keen on bending needle, as it will introduce some non-linearity across the different scales (although probably negligible). I will say it is pretty precise though, see accuracy examples in images 6-8 vs Brymen 829 (and keeping in mind that my zeroing is not dead on, but half a needle thickness off, see first image).



--- Quote from: bob91343 on June 26, 2022, 10:57:42 pm --- Another way is to bend the two ears that engage the zero corrector screw.
--- End quote ---
I think this would have been ideal, had these "ears" been of metal and not something resembling thin pcb.

wasedadoc:

--- Quote from: pbs74 on June 27, 2022, 08:45:00 pm ---I do not see any spiral springs, maybe covered under the "zeroing plate" at its center/mount?

--- End quote ---
There must be something that acts against the clockwise rotation of the pointer.  Apart from making the pointer move back to zero when no current is flowing it is that anticlockwise force which should be directly proportional to the degree of clockwise rotation of the pointer which determines the linearity of the device.  The meter seems to need only 30 microAmps for full scale deflection which means that the clockwise force from the coil and magnet are quite weak and thus the anticlockwise force will be equally weak, meaning a very weak spring.  Often the forked piece that is turned by the front zeroing screw is attached to one end of a spiral spring. In this case there appears to be a wire going vertically downwards from a solder joint that rotates with the forked arm. Possibly that is a spring which is operating in torsion mode. Can you rotate the brown forked piece to have its slot at the 6 o'clock 3 o'clock (see my next post below) position while holding the metal piece with that solder joint to remain in the position shown in the photo?  I mean is it just friction that makes the two move together when the front adjusting screw is turned?

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