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Old 29th May 2013, 05:42 PM   #1
Seraph is offline Seraph  United States
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Default Help with AVO MK IV Meter

Greetings everyone,

I just purchased an AVO MK IV yes off eBay. The meter has no continuity across the meter electrodes/is open circuit. The meter is also stuck just past the zero mark--in spite of its described free movement which lead me to think the meter is OK and I can go through the electronics.

I have searched the forum and I am aware of retrofitting a Simpson 29T but I was wondering if anyone could refer me to an outfit or person in the US who may be able to rebuild the original meter. I have already contacted Herts Meter Co and was told by a gentleman that since the meter has no continuity across its terminals, its moving coil is probably open and that renders the meter irreparable because the coil has many turns of tiny little windings of very fine insulated wire. I was told replacement coils for the meter are no longer made or available.

Would appreciate any feedback.

Regards

Seraph

Last edited by Seraph; 29th May 2013 at 06:43 PM.
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Old 29th May 2013, 05:59 PM   #2
Mooly is offline Mooly  United Kingdom
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If the movement is physically damaged (stuck) then there's little hope tbh.

There's a wealth of information on AVO's here,
Vintage Test Gear and Workshop Equipment - UK Vintage Radio Repair and Restoration Discussion Forum
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Old 29th May 2013, 06:19 PM   #3
Seraph is offline Seraph  United States
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That is indeed a rich source of information on all AVO meters and testers. Some members of Vintage Test Gear community have made substantial efforts in compiling and publishing their well documented findings. That is commendable.

Thanks very much.
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Old 29th May 2013, 09:09 PM   #4
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Mechanical faults can be repaired.

There are companys rebuilding the meter completely mechanical.

But if you got an open (check if there´s a bad connection between the actual coil and the terminals of the instrument) then it´s a lost case.
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Old 29th May 2013, 10:53 PM   #5
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Pretty much what everybody else here has said - if teh meter coil is open, it's toast. The windings are made of extremely fine wire, and quite easy to burn-out especially if the meter was in Ammeter mode and somebody put the probes across a power supply to measure voltage (don't ask - I will NEVER admit to having done that myself )

In testing the meter coil, be especially careful as teh winding is very low ohms and even an ohmmeter across the meter terminals can slam the needle briefly to full-scale before the meter coil opens and the meter makes one last, slow drift back to zero... (again, don't ask...) Use a series resistor to limit the current to test the meter.

It's a very nice looking meter - probably the equivalent of the Simpson 260 here in the States. The dream meter that every kid wanted when I was a kid. I see them on eBay all the time, but opted to go for a VTVM to supplement my DVMs here - sometimes, it's good to have that analog meter to see "how things are" or to tune (or dip) a particular circuit. DVMs just don't do that for me, and in those cases I am not particularly concerned about the exact voltage - looking for relative stuff.

Good luck with the meter - maybe you can find another meter on eBay and build a single working Mark IV if that is your dream.

Sam
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