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Fixing the AVO

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Roger Hart07/12/2019 16:26:17
111 forum posts
27 photos

p1040333.jpg

Roger Hart07/12/2019 16:35:47
111 forum posts
27 photos

p1040337.jpgAfter 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.p1040339.jpgp1040336.jpgp1040335.jpgp1040333.jpg

john fletcher 107/12/2019 18:31:00
564 forum posts

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

Phil Whitley07/12/2019 18:54:04
avatar
997 forum posts
132 photos

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

Roger Hart08/12/2019 10:30:57
111 forum posts
27 photos

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.

old mart08/12/2019 16:29:19
967 forum posts
104 photos

Have you balanced the needle?

Ian P08/12/2019 16:48:10
avatar
2279 forum posts
93 photos

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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