Avometer Movements

[Sinewave]

With all of the repairs and restorations I'm currently involved in with Avo's I thought I'd make a thread regarding the movements, as I have some questions and other may also find this thread becoming useful, I hope.

I'll kick off with this question.

The Avo Model 40 horse shoe magnet movement, is there any known safe adjustment to compensate for any loss of magnetism?

I believe these are 3mA movements and I rehomed an old 1943 model 40 which takes about just under 4mA to reach FSD - this was done connected direct to the movement. It's otherwise in ok visual condition.

This could also apply to any mark 1 Model 7's which are 1mA horse shoe magnet movements, of which I do have some which have lost some sensitivity.

Thanks.

[pmmunro]

Sinewave,

As you state, the basic sensitivity of the Model 40 movement is 3mA. I'm sure you know the sensitivity of these movements is set by means of the magnetic shunt, but for the benefit of anyone finding this thread in future, I'll describe these.

For horseshoe magnets the magnetic shunt is in the form of two flat steel strips clamped to the narrow sides of the magnet by a single long screw. (1st Attachment)

For block magnet movements, the magnetic shunt is as shown in the second photograph, actually of a Model 8 X, but the construction is the same.

If the movement sensitivity cannot be brought to the correct value by the magnetic shunt, the magnet should ideally be remagnetised and "artificially aged". The process saturates the magnet then reduces the flux to give the required sensitivity. There are probably still meter repairers with the necessary equipment but finding them may not be easy.

Some people have reported some success by using small auxiliary disc magnets attached to the main magnet, but there is a risk that the field between the pole shoes may be uneven which could affect the scaleshape (linearity) of the movement.

PMM

[Sinewave]

PMM, thanks for that info.

I'll take a look at that and see that I can get this 40 in spec. It has a few other minor issues which prevent any ranges from working at all, but basically the movement is working.

It's one I picked up recently and instead of putting it into my big pile of things to do, I thought well why not just do it?

[The Philpott]

The early type horseshoe movements have a magnetic shunt with quite limited effect- once you turn it past 45 to 55 degrees the influence actually starts to decrease, so you are unlikely to see one where the shunt has been moved beyond 45 degrees.

The Model D movement pictured was found lacking by around 6 to 7% (judged by needle deflection on the 3 Volt DC range), well beyond the trimming capacity of the swivel shunt. It was experimentally boosted with a Neodymium (10mm diam) magnet. From memory this method of placing the booster magnet on the concentrator rather than on the magnet proper may have been casually called the Japanese method, due to being popularised on the web by a Japanese chap on his website.

I have seen a case where success was achieved by strategically placing less powerful grey magnets around the horseshoe in order to get linearity, but having had success with the Japanese method i would try it again as it took very little time to get the Neodym shuffled to a good position, then araldite it in place. I don't know quite what tool I would use if I ever needed to shift it...something bronze probably. Note that in my case the Neodym now prevents the swivel shunt from rotating. If i had been able to source a slightly smaller booster magnet this might not have occurred. (But not a problem anyway)

My latest source of (very)miniature Neodyms was a pair of old specs. that had a tinted overclip with lozenge shaped magnets at either end. A very useful size that might even allow an Avominor to be given the booster treatment in future.

Dave

[The Philpott]

...I have just noticed that PMM's first picture shows a shunt type that is 'very early'...and from what people have said, is even less effective than the intermediate one I have shown.

[Sinewave]

Indeed I see what you mean about the limited affect the shunt has on the horse shoe type movement. I've just moved it on an early Model 40 Mk 1 and I'm still only reaching approx 87.5% FSD.

Even on the later type, there's not a massive amount of movement.

[The Philpott]

My magnet was from the local hardware/ironmongers. (Manufacturer/distributor is www.centurioneurope.co.uk but their website looks at first sight like it might be trade only..) Ferrite/ceramic disc magnets will not be powerful enough for this, but Nd will do the job. I found that the magnetism is your friend as very small adjustments can be made while setting up, without the booster shifting around of it's own volition. (i think i used blu tack to wipe away any debris between the mating surfaces)

I got lucky insofar as the size and power of the booster was ideal. Any larger would not have fitted, and much smaller might have required a sister magnet over the other side of the concentrator.

[Sinewave]

Revisting this as I'm going to be making an order up soon and I know CPC sell rare earth magnets, so was thinking I could have a go with this.

They have some 3mm and 8mm wide ones. I'm wondering if the 3mm is too small and I should go with the 8mm magnets?

[Sinewave]

Or maybe removing the magnets from the movement and using a process as described here?

https://www.wonkeedonkeetools.co.uk/...an-old-magnet/

I use to disassemble old hard disks to use the Neodymium magnets for other uses. They are quite strong, flat an small you can also divide them in 2 or more parts very easily.


Another way to increase the strength of the magnets is using a non metal against one of the poles, the force is duplicated on the other pole.


Looking to the magnetic field produced by a horse shoe magnet, i think you should need at least 2 neodymium magnets for each pole (north with south and / south with north) to increase the strength

[Sinewave]

I'm just having a little think about what else I may need before I put an order in.

I certainly think it's worth ago with these magnets. There's that model 40 and somewhere in my Avo Store there are a few more which could use a remagnatising.

One of my NATO job lot meters from back in October needs doing perhaps.

[The Philpott]

My Model D which suffered 7% shortfall needed (only one) Nd disc shaped magnet, it was 10mm diam. and 4mm thickness. This was a meter with the horizontal type horseshoe magnet, there was not a great deal of room for manoeuvre- just enough.

If you have a 12.5% shortfall perhaps one of these either side..? (adjusting them may be tricky, but at least you don't need to glue them in place before testing..)

If i was buying them again i would get an assortment of sizes.

Incidentally I have yet to find a Model 8 or 9 (or indeed anything with later type block magnets) that was beyond adjustment.

Dave

[pmmunro]

Sinewave,

I can't quote you magnetic field strength figures but I think it would be most inadvisable to dismantle your Avometer magnet assembly and attempt the magnetising method in the link you give. It just wouldn't create an strong enough field even to get you back where you started.

Dismantling the magnet assembly will certainly result in very considerable loss of field strength.

PMM

And what about with a coil around the magnet and a few volts?

[The Philpott]

I'm with Peter on this one. You have everything to lose by dismantling, but nothing to lose by experimenting with Nd booster magnets.
Dave

[Sinewave]

I'll try the booster magnets, rather than taking a movement apart.

Just racking my brains out over what else I need to order from CPC.

Maybe a few more glasses of red wine will help me decide.

[Sinewave]

I've just taken a delivery of rare earth magnets, so I'm looking to have a go with this either tonight or tomorrow hopefully.

[The Philpott]

Coincidentally i have just been working on a Type E Avominor where adjustment of the shunt is no longer able to compensate for loss of flux. I ended up removing the shunt altogether which resulted in a gain of 2.5%, which brought the basic calibration up to a good level.

Do let us know how you get on with booster magnets, (particularly if you manage to get a linear correction across the scale.)

Dave

[Sinewave]

I've yet to try this, maybe this evening!

Where literally did you put your magnet Dave?

[The Philpott]

I didn't need to boost the Avominor but the Neodym in the Model D Avo is shown in Post No.4 above. If i ever need to move it i am faced with finding something non-ferrous to chip away the Araldite with! (But..judging by the photo i left the shunt in there so that could be removed altogether for an additional boost.)

D