Products > Test Equipment
Cannot zero offset error on Unigor A43 analog multimeter
<< < (3/4) > >>
wasedadoc:

--- Quote from: Gyro on June 28, 2022, 09:55:02 am ---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).

--- End quote ---
Yes, that friction is what I asked about in one of my posts above.
Gyro:
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.
pbs74:
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 pm --- 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.
--- End quote ---
According to manual it is even as low as 8.8microAmps



--- Quote from: wasedadoc on June 28, 2022, 09:08:15 am ---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?

--- End quote ---
Correct, 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 am ---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 to rotate the screw until it fits.
--- End quote ---
Thanks 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 pm ---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.
--- End quote ---
Good 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 am ---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.

--- End quote ---
It 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 pm ---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?
--- End quote ---
This 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 am ---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 construction 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.

--- End quote ---
There 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?
Kleinstein:
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.
Gyro:

--- Quote from: pbs74 on June 28, 2022, 08:09:23 pm ---
--- Quote from: Gyro on June 28, 2022, 09:55:02 am ---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.

--- End quote ---
It 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.

--- End quote ---

It'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.


--- Quote ---There 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?

--- End quote ---

I'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.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod