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AVO 8 Mk2 meter wire size

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Roger Hart30/10/2019 17:06:09
118 forum posts
27 photos

Having decided no more weird projects I am now tackling the repair of a Test Set Multirange No1, basically an AVO8 Mk2. Very nice instrument that tempted me into thinking all that was wrong was a loose glass.....

Not so simple, this one has an open circuit meter coil. I have tried all the simple fixes and am looking at a rewind. This has a 37.5 microamp movement which should be 3333 ohms including the swamp. This one should be 2608 ohms for the coil alone.

However having unwound part of the coil I am having trouble deciding whether this wire is 49SWG or 50SWG. It measures just over 0.001 inch. Now one step in gauge makes a lot of difference to the number of turns. So does anyone have an idea what wire AVO used or the correct number of turns?

Too nice for the bin but I don't want to wind the coil twice....

Emgee30/10/2019 17:17:34
1492 forum posts
217 photos

Hi Roger

If you know the value of the swamp can't you just calculate the coil resistance required for the total 3333 and then thickness of wire is not such a problem if you know the resistance per metre or other measurement.


Simon Williams 330/10/2019 17:25:12
508 forum posts
80 photos

Hi Roger

I've got a Mk3 in the shed, and it's been sat on a shelf for about 30 years. I pulled it down to admire it not long ago and realised that the needle was sticky. Pulling the back off shows very fine iron filings aggregated to the magnets in the coil gap.

On one of the radio/electronics forums I was advised not to take the magnets off the movement, as cycling them through the magnetic remanence path of removing them and replacing them will change their strength. These are red eclipse pot magnets before the days of rare earth.

It may be an old wife's tale, but if so it's well entrenched in Avo folklore that removing the magnets is a one way trip.

If you know different or have a cunning work-around do tell!

Best rgds Simon

Mike Poole30/10/2019 17:33:47
2575 forum posts
60 photos

The wire will have a coating on it which will need to be stripped before measuring. We used to just burn it off in a flame and then clean before measuring for motor rewinds. A fine wire can very easily melt in a flame so take it steady. A chemical stripper may be more gentle but I am not sure what would be best.


KWIL30/10/2019 18:02:06
3239 forum posts
63 photos

Meth spirit flame temperature is quite low, thats what we used to use.

Georgineer30/10/2019 18:18:34
354 forum posts
16 photos
Posted by Simon Williams 3 on 30/10/2019 17:25:12:

... It may be an old wife's tale, but if so it's well entrenched in Avo folklore that removing the magnets is a one way trip...

It certainly wasn't an old wive's tale when I took the works out of a Sturmey Archer Dynohub without a keeper when in my teens. I would certainly err on the side of caution with any permanent magnet device.

George B.

Edited By Georgineer on 30/10/2019 18:21:35

john fletcher 130/10/2019 18:45:36
586 forum posts

For any thing AVO I suggest you join Vintage Radio it is free. Members have a vast know of any thing AVO, circuit diagram, manual etc are readily available. I think you are on a looser attempting to satisfactorily rewinding the very delicate moving coil. To remove iron filings use a wooden tooth pick and some blue tack. You are correct for the resistance and current value. John

Roger Hart31/10/2019 07:01:59
118 forum posts
27 photos

Thanks for the replies and advice. I figured out in the end that although 50 gauge wire has an od of 0.025mm it has a coat of enamel as well. This adds around 0.003mm depending on the class of enamelling. The upshot is that 50 gauge wire will mike a little over 0.001 inch, so I will go with that.

I avoided disassembling the magnet block and put the central core back in, so I hope to avoid problems from that area.

This chap seems to have done something similar, quite interesting, don't know how to do links, so

restauration eines US-Roehrenpruefgerates TV-7 A/U

should find it.

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