Cannot zero offset error on Unigor A43 analog multimeter - Page 1
Forum section: electronics / testgear

author: pbs74
author_rank: Contributor
timestamp: June 26, 2022, 08:18:53 pm
content: 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

author: wasedadoc
author_rank: Super Contributor
timestamp: June 26, 2022, 09:41:50 pm
content: 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.

author: bob91343
author_rank: Super Contributor
timestamp: June 26, 2022, 10:57:42 pm
content: 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.

author: pbs74
author_rank: Contributor
timestamp: June 27, 2022, 08:45:00 pm
content: 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 pmI 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 pmMight 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 pmI think this would have been ideal, had these "ears" been of metal and not something resembling thin pcb.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 27, 2022, 11:16:16 pm
content: Quote from: pbs74 on June 27, 2022, 08:45:00 pmThere 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 the6 o'clock3 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?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2022, 07:33:11 am
content: With very sensitive meters the restoring force can be from just the torsion of the wire holding the coil in place and providing the current.

author: F64098
author_rank: Regular Contributor
timestamp: June 28, 2022, 08:41:33 am
content: Hello Peter,as i see, there is missing a part for zeroing by small screwdriver.In the hole, marked with <0> has to be a special screw with an excentric pin on the other side.This pin changes the position of the fork-like wing from hard paper.With open instrument, you can use a big screwdriver and put it in the gap and move it slightly to zero the instrument.Best regardsFrank

author: wasedadoc
author_rank: Super Contributor
timestamp: June 28, 2022, 09:08:15 am
content: No, it is not missing. It is clearly visible in the photo of the front of the multimeter in the first post.However, now that I have looked at that photo more carefully I realise that the zeroing screwdriver slot is displaced to the right of the centre of the meter. So my mention of 6 o'clock position on the forked arm was incorrect. Can the spigot on the adjustment screw not engage in the fork slot when the forked arm is in the position which moves the pointer to zero on the scale?

author: F64098
author_rank: Regular Contributor
timestamp: June 28, 2022, 09:34:14 am
content: Okay, i see it in the first phot, my mistake.You have to "lock" the pin of the adjustment screw into the fork,then you can zero your instrument.So you have to change the position of the fork slightly ond to rotate the screw until it fits.Maybe the previous owner changes the fork position accidentally when opening the unitand so the pin of the screw rotates outside the fork.Best regardsFrank

author: Gyro
author_rank: Super Contributor
timestamp: June 28, 2022, 09:55:02 am
content: Quote from: Kleinstein on June 28, 2022, 07:33:11 amYes, from the photos that's a taut band movement, there are no hair springs or pivots to be adjusted.Yes, it is easy to get the offset pin in the adjustment screw misaligned with the slot in the adjuster - a good indication though, is that the needle can be adjusted in one direction but stays there when you try to adjust it back (onno opposite side of the slot to pull it back).Do notattempt to bend the pointer, as somebody suggested! The pointers are made of very thin walled Aluminium tube and will kink if you attempt to bend it. You will almost certainly snap the suspension band too. Bending the needle will also throw the movement completely out of balance, meaning that the zero position will vary wildly with the meter orientation.Try moving the meter into various orientation and see if the needle 'zero' point remains reasonably constant. This will indicate whether anything has been bent in transit and upset the balance of the movement.All movements have a means of adjusting the zero adjustment range. This is in the form of an adjuster on the bottom end of the suspension - either the bottom hairspring mounting, or in this case, the other suspension band support. This is also moveable (although it doesn't have the same long adjustment lever. The correct procedure is to set the top zero adjuster to mid position, and then rotate the bottom one to bring the pointer to approximate zero position.The ease of adjusting the bottom adjuster depends on the constuction of the meter. You will probably need a long needle (non magnetic), or may even need to remove the scale plate to get access. One of the movement connecting wires will be soldered to this bottom adjuster - the wire getting accidentally tugged can be one reason for the problem occuring.EDIT. On some movements, there is a fricton fit between the adjustment lever and the actual spring / band mount. Looking at the photos, I notice that the lever is phenolic, so it may be possible to move it in relation to the top band mount. This could have go misaligned by the a previous owner (in fact, it looks to be the case).

author: wasedadoc
author_rank: Super Contributor
timestamp: June 28, 2022, 11:04:52 am
content: Quote from: Gyro on June 28, 2022, 09:55:02 amYes, that friction is what I asked about in one of my posts above.

author: Gyro
author_rank: Super Contributor
timestamp: June 28, 2022, 03:16:58 pm
content: Ah yes, I missed that part of your comment.Looking at the 5th photo, you can clearly see that the forked lever is actually made up of 2 sheets of phenolic, one above the other, with the brass (more likely phosphor bronze) sheet holder for the end of the taut band sandwiched between them. A sure sign of a friction coupling. By holding one of the two small brass tabs (not the one tensioning the the band), and moving the forked levers, keeping them parallel, they will move relative to each other.I had the same situation with a single spring electrostatic voltmeter a few weeks ago and it wasn't immediately obvious that there were two parallel metal forked levers.

author: pbs74
author_rank: Contributor
timestamp: June 28, 2022, 08:09:23 pm
content: Thanks for all the good comments, getting closer. Let me reply on a few, and fill in some of the gaps so far.Quote from: wasedadoc on June 27, 2022, 11:16:16 pmAccording to manual it is even as low as 8.8microAmpsQuote from: wasedadoc on June 28, 2022, 09:08:15 amCorrect, that is the issue now, it seems I was not clear earlier. I do indeed have the adjustment screw (see new image 11 showing backside tap on screw). However, I took it out to show how it will not fit correctly in the adjustment slot once the adjustment lever is a position where the meter is zeroing. This is my main issue at this point (in first post, I could not zero it, but that is at least possible now, but will not allow using screw). I've updated image 4a showing the adjustment lever outline and the travel of the adjustment screw.Quote from: F64098 on June 28, 2022, 09:34:14 amThanks Frank, unfortunately screw will not fit the fork once latter is set to zero meter, see above.Quote from: wasedadoc on June 27, 2022, 11:16:16 pmGood with a confirmation, that is how I interpreted it as well based on looking at needle and wire while turning zeroing arm. Only heard of spirals before, never worked with analog meters inside.Quote from: Gyro on June 28, 2022, 09:55:02 amIt does not. With the original "zero" at 5 on 100 scale lying flat on table, putting it on one side would move needle to 0 and putting it on the opposite side move needle to 10. Not sure if that is enough for it to be out of balance from transit, or it is just very delicate/accurate. It is made for use only lying flat.Quote from: wasedadoc on June 27, 2022, 11:16:16 pmThis is exactly what I already tried: gently holding back either the left "ear" or the top part that wire is solder on to with a nail while turning. See updated image 3a with arrows. Nothing moved, but I did it very gently to not break anything so might not have used enough force?Quote from: Gyro on June 28, 2022, 09:55:02 amThere is in fact a screw on the backside of the meter scale, inside the multimeter - see new images 9 and 10. It is made for adjustment, as there is a hole in the pcb to reach it. It has been fixed in position with (white) lacquer. Looking at the screw there is a something going through it, it might very well be an anchor for fixing the torsion spring. I did not touch this screw at I did not know what it was for. If indeed used to set zero range for the front zero adjustment, should I try turning this screw, or instead apply more force on the front adjustment lever while holding back the metal parts, to overcome the friction hold?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2022, 09:04:40 pm
content: Given that the meter is quite low current the sensitivity to orentation does not look too bad. It does not nned that mcch extra weight at the pointer or counter-weight to cause that imbalance.For the srew at the back, it may be the point for a coarse zero adjustment, but could also adjust the position / hight and worst case hold the wire / spring. At least I can't tell for sure.

author: Gyro
author_rank: Super Contributor
timestamp: June 28, 2022, 09:36:15 pm
content: Quote from: pbs74 on June 28, 2022, 08:09:23 pmIt's probably close enough, balancing is seldom perfect and yes, it will be intended to operate lying flat. I suspect that the suspension band would probably snap before anything bent during transit anyway.QuoteI'm nervous of the screw if I can't fully see what it is attached to. It may be fixed to the bottom band attachment, but it looks quite a big screw. A more likely possibility is that it presses on the bottom attachment, raising and lowering it to adjust the clearance between the needle and the scale (possibly tension too). If the laquer isn't disturbed then it obviously hasn't been disturbed to cause the problem.My bet is still on the friction hold between the lever and the top attachment.Avoid Hold#1!The most likely result would be snapping the suspension band, changing the tension, or misaligning it from the vertical. Hold#2 is the correct place, it is the same piece of metal but can only rotate around the axis. Look for signs of white laquer (hopefully disturbed). One thing you could try is gently moving one of the forked levers relative to the other (don't snap the forks) that might free things up a bit - Hold#2 can only follow one of them, that's the side that's sticking and you can then hopefully ease that more easily.Sorry, it's a lot easier to do first hand (and know when I'm applying safe pressure) than to describe to someone else - I would feel bad if a tool slips and you destroy the movement.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 29, 2022, 06:34:19 am
content: One normally does not have to adjust the mechanical zero of the movement very often. With age the zero should become even more stable. So one may be able to live without the easy adjustment from the front.

author: Gyro
author_rank: Super Contributor
timestamp: June 29, 2022, 09:29:29 am
content: Yes, that's certainly a viable option if things don't free up easily and you don't want to force them. I'm not sure off-hand whether a taut band suspension will have a more stable zero than a traditional one, it only has one restoring element involved rather than a pair of coil springs, and of course no bearing friction.I can't see from the photos whether it would be possible to replace the zero adjustment screw in a position where there is sufficient clearance for its pin. You certainly need to protect the hole against dust and swarf if not.