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Old 3rd Jun 2017, 6:10 pm   #1
The Philpott
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Default Boosting meter magnet with Neodymium

I know this has been done before, but having an older Avometer that was 6 or 7% below par (and which could not be improved enough by turning the rotary shunt bar) i tried the Neodymium 'button' magnet method.

With experimentation I got (volts) correction at 65% of FSD, was surprised by how little non-linearity was experienced lower down the scale. The error really was negligible at 10% fsd, which was not what I was expecting. I have since tried the ohms scale at various deflections and that is reasonable also.

The magnet in question was set in place using araldite, which it must be said wasn't too eager to make friends with the shiny surface, but ultimately it did hold it.

The trick seems to be to get the booster magnet as 'close to the action' as possible- see pic.

Noted that often the rotary shunt bar on these particular meters can't be rotated very far before it clouts the cut-out rod and 'twang'- the cut-out trips and will not reset until the shunt is retracted slightly.
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Old 4th Jun 2017, 4:07 pm   #2
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Default Re: Boosting meter magnet with Neodymium

Is this a Model 40, AM Type D or Admiralty 47/48A from 1940 to 1947?

Well done for trying anyway. Have you checked the scale shape (linearity) towards fsd? I would have been concerned that the auxiliary magnets would have too strong an influence as the moving coil got closest but perhaps the additional flux is distributed around the magnetic circuit. Is there room for a second magnet diagonally opposite the one shown?

PMM
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Old 4th Jun 2017, 7:55 pm   #3
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Default Re: Boosting meter magnet with Neodymium

Well spotted that man, it is an AM Model D' from early 1945. It had a loose (aluminium) back-box, a loose resistor, and missing case screws, so I knew something would be amiss. There is indeed room for a second magnet in the diagonally opposite position from that shown, but bearing in mind the initial result I hadn't considered this yet. A second magnet might stop the shunt rotating for future adjustment, but we'll see.

The casing screws, incidentally, are in different positions from normal, and there is an extra (larger) hole to the lower right which (reading between the lines) looks to have been for a top-hat washer, grub screw, and black 'anti-tamper' composition seal over the top of it all. There are no other differences found as yet. Possibly a shadow factory product, certainly drilled in a different jig from the standard article.

I haven't checked full scale deflection but will do so shortly- watch this space

(P, you missed out one, it could have been a 47AS, ie a variant of the 47A which is able to use it's 100kohm range without external voltage, courtesy of a 9v power pack! i only found out these existed recently; and there are a few around)
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Old 4th Jun 2017, 8:46 pm   #4
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Default Re: Boosting meter magnet with Neodymium

Now rejoining the real world from the shed:

Result at 65% deflection is accurate. (ref voltage 2.048V)

Result at (around) 98% deflection and a reference voltage of 1.476v, is 1.465v, ie an under-read of 0.75% of the reading.

Result at (around) 10% deflection and ref V of 1.477v, is 1.5v, which is an over-read of 1.55% of the reading.

I am quite pleased; it's not quite as it should be but certainly not all over the shop.
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Old 4th Jun 2017, 11:05 pm   #5
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Default Re: Boosting meter magnet with Neodymium

Good fix. I have a 1944 Air Ministry Model 7 that reads 4% low, so I might try the neodymium magnet. Where did you source yours from?
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Old 5th Jun 2017, 6:11 pm   #6
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Default Re: Boosting meter magnet with Neodymium

Phil, I got them in a local hardware store (Staines) but the supplier is Centurion. They seem to be a distributor for nuts, washers, hooks, hinges etc.

www.centurioneurope.co.uk might show up a retailer closer to you.

The one I used was in a box of 2, stock code CA85P. 10mm Diam. 4mm Thick.

They do an oblong one 24mm/9mm/2mm under code CA86P but this proved to be too large to even experiment with.

I suppose an assortment of smaller sizes would be ideal then you can pick and choose. This job was quicker than it might have been since I previously made a live testing chassis from a badly warped AVO back box. It has large access ports all over it.
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Old 5th Jun 2017, 6:33 pm   #7
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Default Re: Boosting meter magnet with Neodymium

Test chassis (lack of battery box gives very good access to the top of the meter)

I have to remember to insert casing screws top and bottom to avoid the front and back parting company during handling.. (This avo was an oddity and not a single hole lined up so had to rely on a big rubber band)
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Old 5th Jun 2017, 9:17 pm   #8
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Default Re: Boosting meter magnet with Neodymium

That looks like a good way to protect the long hexagonal pillars of Model 8s and 9s which make contact to the battery box.

Looks like a bit of careful marking out and filing to achieve nice rectangular apertures.

PMM
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Old 5th Jun 2017, 9:17 pm   #9
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Default Re: Boosting meter magnet with Neodymium

Now, that's a novel idea!

Thanks for the magnet info. I was amazed to find so many styles and sizes available on eBay at very low prices. These magnets are being sold for the price of sweeties and aimed at the 'crafting' market, to make fridge magnets, gift cards etc. And there was me thinking that Neodymium was a scarce element, in global short supply...

I saw some small 6mm square magnets which may be easier to fit in the space available. The only thing that worries me a little is the strength of the stray magnetic field. My workshop is quite small, things are in close proximity and I'd hate my large collection of audio tapes and cassettes to be damaged.
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Old 5th Jun 2017, 10:10 pm   #10
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Default Re: Boosting meter magnet with Neodymium

Thanks Pete, the dimensions were a guess as to how much 'meat' to leave, but it's actually very rigid. Used pilot holes followed by a loose hacksaw blade - needed to back off or slow down promptly when the drill or blade squeaked. I only had one shot at it so i wanted it to be right.

Phil, my shed roof has dexion stiffeners which keep any spare magnets up and out of harms way!
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Old 6th Jun 2017, 8:38 am   #11
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Default Re: Boosting meter magnet with Neodymium

Phil.

Is the 47AS your own designation or is that the official one? Looks like my collection is short of an Avometer type - action may be required.

There was a discussion on the Avometer survey a few years ago about 47/48A resistance ranges. It might be a better place to discuss these variants so that the information can be traced later.

Best regards,

PMM
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Old 6th Jun 2017, 5:26 pm   #12
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Default Re: Boosting meter magnet with Neodymium

Good plan; i will put something on the survey thread this evening. (have just found an old scanned document which refers to the sub-type mentioned)

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