Fixing the AVO

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Fixing the AVO

This topic has 7 replies, 5 voices, and was last updated 8 December 2019 at 16:48 by Ian P.

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7 December 2019 at 16:26 #440665
Roger Hart
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@rogerhart88496
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7 December 2019 at 16:26 #31276
Roger Hart
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@rogerhart88496
7 December 2019 at 16:35 #440666
Roger Hart
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@rogerhart88496
After deciding to press on and rewind my AVO coil (see last posting) I took the coil apart and stripped off the wire and measured. Then built very simple coil winder for 50swg wire (a bit fine). Put on 1200 turns from calculated value plus 10%. Then made jig to re-glue bearing pivot plates and re-assembled. Disappointed to find read about 25%. Didn't fancy winding again so fitted some bigger neodymium magnets plus a bit of jiggy-pokery with shunts and swamp resistors. Now reads correctly. Was it worth it – no, cheaper and easier to visit eBay. But I've started so I'll finish…… Sorry about order of photos, not got the hang of that.
7 December 2019 at 18:31 #440677
john fletcher 1
Participant
@johnfletcher1
You will find a lot of information regarding AVO meters on Vintage Radio, it is free to join, and there are many pages of AVO info. I haven't heard of anyone carrying out movement coil rewinds before. Good for you and its now working. John
7 December 2019 at 18:54 #440682
Phil Whitley
Participant
@philwhitley94135
You sir, are a hero! that is not an easy job to tackle! I have an avo service manual, if you would like one, but it is 5.9 meg so would have to use dropbox etc to get it to you, it may even be available at vintage radio as a download, thats probably where I got it from!

Phil
8 December 2019 at 10:30 #440733
Roger Hart
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@rogerhart88496
Thanks for the responses. I looked at VintageRadio, very handy info. The bottom line is that I used 50 SWG wire which is 0.001 dia and requires 78,750mm for 2608 ohms resistance. This leads to 1078 turns approx. This IMHO turned out to be too few for the magnet block. Ampere-turns is what sets the sensitivity.

Had I used 49 SWG .0012 dia and requires 113,550 mm for 2608 ohms which leads to 1554 turns approx I would probably had the right result. But being a bit dim, although the wire sample I measured did measure a bit over 0.001 I took the extra to be the enamel. That 0.0002 makes a lot of difference – cunning chaps those AVO people.
8 December 2019 at 16:29 #440782
old mart
Participant
@oldmart
Have you balanced the needle?
8 December 2019 at 16:48 #440788
Ian P
Participant
@ianp
You have my admiration for rewinding a moving coil meter!

When I've examined some meter movements (probably smaller than AVO size) the whole moving assembly looks to use watch sized components like tiny jewels held in microscopic collets held together with shellac or similar. I would never have thought they were repairable.

Ian P
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