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Old 21st Apr 2022, 11:59 am   #1
russell_w_b
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Default Fixed My AVO 9 II

I'm sure this is a well-known fault, but hey... My AVO 9 II was reading around 10% low on all a.c. ranges; everything else was fine.

Measured the bridge rectifier and sure enough, one of the diode stacks was leaky and was dropping a couple of volts in reverse bias. It's one of those arrangements where the diodes are individual discs, like tiny sweeties, held firm with tiny springs against metal contact plates. I've only ever seen such-like in a Beethoven shot-firing exploder (more elements; larger) and that had gone leaky too.

Made up a bridge of 4 x OA90 diodes and shoehorned it in. Voila! All good and accurate on all ranges!
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Old 21st Apr 2022, 1:36 pm   #2
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Default Re: Fixed My AVO 9 II

Always good to see another Avometer repaired.


Sometimes, just re-seating the Discs can restore full functionality to the Bridge.

I've managed to repair a couple Bridge Rectifiers that way.

They can also be re-stuffed, if originality is important, but the small ones
like on the Model 9 are obviously a bit more tricky due to their size.



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Old 21st Apr 2022, 1:42 pm   #3
The Philpott
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Default Re: Fixed My AVO 9 II

*Someone actually managed to restuff one of those copper oxide rectifiers with diodes! I think the worst copper oxide i had was registering 240v mains as 180v. The knackered ones are sometimes seen to be corroded on the outside.

There wasn't an Avo 9/III as far as we know, but if there had been, logically it would have had the upgrade that you've fitted!

*Crossed

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Old 21st Apr 2022, 1:45 pm   #4
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Default Re: Fixed My AVO 9 II

Quote:
Originally Posted by Superscope View Post
Always good to see another Avometer repaired.


Sometimes, just re-seating the Discs can restore full functionality to the Bridge.
That's interesting to know, Ian. How long do they last if you do that?

Quote:
Originally Posted by Superscope View Post
They can also be re-stuffed, if originality is important, but the small ones
like on the Model 9 are obviously a bit more tricky due to their size.
I re-stuffed the ones on my Beethoven exploder (long paxolin tubes), but the AVO 9 one was too tight for the OA90s. I can see what you mean, though. I'm not fussed about authenticity in this case as it's my 'desk' meter I keep handy on my bench and it's a practical working instrument
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Old 21st Apr 2022, 1:49 pm   #5
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Default Re: Fixed My AVO 9 II

Quote:
Originally Posted by The Philpott View Post
*Someone actually managed to restuff one of those copper oxide rectifiers with diodes!
If I'd located some smaller germanium diodes I might've done that as it's a good way of securing them. The pink sheet sandwich is a large Hellerman sleeve cut in two, and offers enough grip to keep the diodes secure and padded.
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Old 21st Apr 2022, 5:21 pm   #6
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Default Re: Fixed My AVO 9 II

If I can dig out the Meter with the re-seated Rectifier I will test that it
still works.
But I have no reason to think it won't last as long as it did originally.

The one I repaired had gone Short Circuit on one or Two of the Rectifiers
as far as I can remember.
I think what happens is, after Decades of vibrating around the Discs can tilt
slightly under spring pressure and the edges can touch because the Copper Oxide
doesn't always go right to the edge of the Disc and the Cu2O layer is very thin.

In your case, it sounds like the Cu2O Junctions may just need cleaning.

The Discs can be stacked and tested outside of the rectifier body first in
order to test whether a re-seat will work.

The re-stuffing Dave mentioned was probably me.
You can read about it here:
https://www.vintage-radio.net/forum/...=160057&page=2 Post #26

Although, mine was a much bigger variant as found in the older Model 7.
I used BAT 49 Diodes and there were no problems with the accuracy.

I have also had success with IN4148 silicon signal diodes without any
adverse effects. Not for re-stuffing, I hadn't tried that at the time, but just
as a replacement like you have done.
The 4148's are very small. Theoretically I didn't expect them to work, but
they seemed fine.



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Old 3rd Jun 2022, 10:01 am   #7
FrankMCG
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Default Re: Fixed My AVO 9 II

What a great job you did! Can you advise me on some manuals or so for the similar job?
All the best
Frank
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Old 4th Jun 2022, 11:16 am   #8
The Philpott
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Default Re: Fixed My AVO 9 II

The thing is, Frank, there are no manuals for this sort of job (That is to say that this website is probably the closest you will get to a manual at this point in time)

Every year someone finds a new workaround (or 'hack' if we're down with the kids) to keep old equipment functioning and faithful. The search box at the top of the page is your friend.

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