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Old 22nd Oct 2021, 12:54 pm   #1
HBWOODY
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Default Avo Mk2 VCM Meter Balancing

Have just about finished calibrating but I am unable to adjust "S" pot to balance the meter with the reading, taken at the link, can only reach 85mA when link reads 50mA.
The meter has FSD 0f 455uA and a resistance of 114 ohms.
Could this be due to weak magnets in the meter?
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Old 22nd Oct 2021, 11:26 pm   #2
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Default Re: Avo Mk2 VCM Meter Balancing

I think the meter is meant to be 440uA at approx 110 ohms - probably the magnet has lost some magnetism - I had a similar problem with a MKII that I was trying to bring back from the dead, but in my case the movement had been replaced with a non original 470uA 185ohm movement.

I made a post about it here https://www.vintage-radio.net/forum/...d.php?t=165034
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Old 24th Oct 2021, 1:08 pm   #3
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Default Re: Avo Mk2 VCM Meter Balancing

I re-magnetised the meter magnet and now the meter is fully working over the whole scale.
I'd like to thank retailer for directing me to his very informative post.
I have another meter to do on my last Mk2, hopefully re-magnetising will work on that too.
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Old 24th Oct 2021, 1:29 pm   #4
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Default Re: Avo Mk2 VCM Meter Balancing

Nice!

What did you use to re-magnetize the meter magnet with?
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Old 25th Oct 2021, 2:14 pm   #5
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Default Re: Avo Mk2 VCM Meter Balancing

I used the method laid out on this site https://magnetpartner.com/blog/how-t...gnetize-magnet
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Old 25th Oct 2021, 2:23 pm   #6
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Default Re: Avo Mk2 VCM Meter Balancing

Thanks, I have seen that text before.

Do you have any tips & tricks to share on how you did this with the AVO meter, so that I and others can learn from your success?

Did you take any photos while performing this procedure that you can share?
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Old 26th Oct 2021, 2:06 pm   #7
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Default Re: Avo Mk2 VCM Meter Balancing

I literally followed the instructions. The dimensions of the magnet I used was 50mm x 5mm. Obviously great care was taken to avoid damaging the meter mechanism when touching the poles of the meter magnet. Rather than just touching the poles I did so 20 times for each pole. Sorry not really set up to photograph the task. The instructions on the web link are very clear. I did not think it would work.
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Old 26th Oct 2021, 4:23 pm   #8
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Default Re: Avo Mk2 VCM Meter Balancing

I have read the article to which HBYOODY referred to, re magnetising magnets, very interesting. Did you simply touch the magnet as the article said or wipe them with neodymium magnet? Prior to neodymium magnet being around I have re magnetised magnets by a wiping action, using two large ex speaker magnet. Maybe a touch would be better ? Ted
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Old 26th Oct 2021, 6:04 pm   #9
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Default Re: Avo Mk2 VCM Meter Balancing

Ted
Thought if one touch does no harm a few more should not be a problem so I lightly touched each pole 20 times. Did not wipe them (that may also work).
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Old 26th Oct 2021, 6:43 pm   #10
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Default Re: Avo Mk2 VCM Meter Balancing

It would be interesting to know for how long the re-magnetized magnet will keep its magnetism, have anyone seen any figures on this?

If possible it would be very good if you could report back here some time in the future (a year from now or so) on how well the meter works.

All of the companies manufacturing magnets and meter companies, both those refurbishing meters as well as those manufacturing new meters, that I have spoken to have always said that this method of re-magnetizing a magnet will only hold for a short time, they have usually answered that it will work for under a year before the magnet returns to its original magnetism. They claim that this is due to the magnet that you re-magnetize this way really isn't re-magnetized in a proper fashion, it only has its internal structures "re-positioned" for a short while until they will return to their scrambled positions again. They also claim that proper re-magnetizing of this type of magnet found in meters is done by first re-magnetizing to a much higher flux value than the final flux value, which is then reached by re-magnetizing at lower and lower flux values in steps until you reach the desired flux value. This is how it is done when producing the magnets, some magnetic materials also need to be heated to considerable temperatures to really keep their new magnetism.

I've never tested using a Neodymium magnet in this fashion even though I have several different types that I have used to get old Trochotron tubes up and running when they have been missing their original Alnico or Ferrite magnets.

I really hope that this way of re-magnetizing an old AVO meter magnet works out well! Maybe a Neodymium magnet is strong enough so that the original meter magnet reaches the higher flux value that the manufacturers claim is necessary for it to keep its magnetism, at least for a short while when the Neodymium magnet is in contact with it. It is a lot easier than repairing the rest of the parts of a meter like hairsprings, pivots and the rest of the moving parts.
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Old 26th Oct 2021, 8:55 pm   #11
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Default Re: Avo Mk2 VCM Meter Balancing

Reading these posts through has got me thinking. In practical terms, the movement magnets are horse shoes, correct? I will assume so.
Take a 100 micro amp meter which reads low. Pass the FSD current and take a reading, say 60% FSD.
Take a small neodimium magnet and hold near the outer end of the horse shoe. With that FSD current still flowing, attempt to position the magnet to bring the reading up to 80% FSD. At this point you may need a larger or smaller magnet. Mark the position with a fibre tip, apply super glue to magnet and refit, holding it until secure.
With a second identical magnet,find the 100% FSD level with the magnet outside the other end of the horse shoe and secure.
Now apply a small amount of araldite to both magnets to ensure permanency.
Fine tuning can still be done with the original shunt on an AVO VCM.
Will it work
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Old 26th Oct 2021, 9:00 pm   #12
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Default Re: Avo Mk2 VCM Meter Balancing

It does work by applying small magnets glued to the magnet, like in this photo: https://www.vintage-radio.net/forum/...3&postcount=14 where you can see one small round Neodymium magnet on each side just below the white scale.

It has been used for several years in getting AVO meters, and other meters, showing correct FSD.
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Old 27th Oct 2021, 7:31 am   #13
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Default Re: Avo Mk2 VCM Meter Balancing

Looking at these posts I feel more confident in the attaching of two small magnets. I will hold on to the MK2 VCM (meter re-magnetised with touch method) and check the meters performance in a years time.
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Old 27th Oct 2021, 6:22 pm   #14
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Default Re: Avo Mk2 VCM Meter Balancing

Martin, Yes, I was aware of the use of neodymium magnets on moving coil meters. What I was putting forward was the idea of getting it right first time. IF the method works. i.e., in my example, one magnet halves the error (60% to 80%, with second magnet getting it "spot on", rather than just try it, move it, try it move it ad infinitum.
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Old 30th Oct 2021, 12:47 pm   #15
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Default Re: Avo Mk2 VCM Meter Balancing

Quote:
Originally Posted by HBWOODY View Post
I literally followed the instructions. The dimensions of the magnet I used was 50mm x 5mm.
I bought a couple of those magnets as I want to try remagnetising a Hallicrafters S Meter.

I was surprised to find they were not magnetised in the "NORMAL" way. I expected one end to be the North Pole and the other the South Pole. However one SIDE is North and the other South, the dividing line, if you can call it that, being along the long axis This is most noticeable when rolling the magnet along a magnetic surface. It's difficult to roll, but will suddenly flip over by 180 degrees about the long access. Also the magnets will stick together any way you like ie side by side or end to end in either orientation.

I think this could make remagnetising a meter's movement difficult.

EDIT.

Apparently the correct term is "Diametrically Magnetised".
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Old 30th Oct 2021, 2:13 pm   #16
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Default Re: Avo Mk2 VCM Meter Balancing

Hi Graham

The magnets I used had a normal magnetization, on flat side S and the other side N, it's impossible to put them together the "wrong" way.
I bought it from an Swedish site, magnetvaruhuset.se, but I'm sure there are many sites that have similar magnets.

I didn't glue them either, the stuck really hard, so no risk of unwanted movement as I see it.

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Old 31st Oct 2021, 9:06 am   #17
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Default Re: Avo Mk2 VCM Meter Balancing

My intention is to remagnetise the existing magnet by tapping or stroking it with a more powerful bar magnet. If that doesn't work I'll try using small magnets in the manner you describe.
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