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Old 16th Jul 2018, 10:36 pm   #1
Biggles
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Default AVO 8 mk3

Hi all, I have just unearthed an AVO complete with leather carry case from the depths of my shed, that I had forgotten about. The cutout button will not latch down, although when held in the meter seems to work ok. Is it a big job to repair or replace the cutout and are spares still available?
Alan.
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Old 16th Jul 2018, 10:39 pm   #2
Sinewave
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Default Re: AVO 8 mk3

These are usually easy to sort out. It's a nice mechanical cut out which will just need a good clean up.

You'll find there's a cam which moves to keep the contacts in, which will have gone sticky, so won't return free.
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Old 16th Jul 2018, 10:57 pm   #3
Biggles
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Default Re: AVO 8 mk3

Ah right, thanks for that. As yet I haven't had the chance to open it up. I just fancied an analogue meter again for tuning up. I will investigate further when I have a couple of spare hours. The carry case looks like it's never been used. Probably as it's so bulky.
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Old 17th Jul 2018, 11:41 pm   #4
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Default Re: AVO 8 mk3

There is a certain amount of adjustment in the cut-out platform, after the two screws have been loosened slightly.

One screw is obvious, the other is effectively a tall brass rod with a notch in the top, identified by the fact that is wears a bronze coil spring, and steadies the moving contacts as they open or close.

The likelihood that someone has already tried and failed to fix it is always there of course! Sometimes even a slight kink in the actuation rod is enough to allow it to bind and upset the cut-out operation and/or reset.

As far as spares go, it's a donor meter or nothing- just as well there are loads around.

I find the cut-outs are very easily upset if utmost respect is not paid when actuating them manually to test their operation and observe their action. The moving parts must not be manipulated further than absolutely necessary.

Once inside it is usually possible to reset the cut-out manually by intervention, but this is merely a dodge as next time it operates you won't be able to reset it from outside!

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Old 18th Jul 2018, 10:59 am   #5
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Default Re: AVO 8 mk3

Hi Alan,

I went through this exact problem a few Months ago on a
9 Mk II.
The Thread is here:
https://www.vintage-radio.net/forum/...d.php?t=145375

May or may not help with your specific problem, but my problem turned
out to be an incorrect previous repair.


Ian
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Old 18th Jul 2018, 12:19 pm   #6
The Philpott
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Default Re: AVO 8 mk3

Oh yes, now i remember!

Peter has also observed that the actuation rod, if removed from the assembly, becomes very well camouflaged against most backgrounds. Very easily mislaid.
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Old 21st Jul 2018, 9:45 pm   #7
Biggles
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Default Re: AVO 8 mk3

Sorry for the delay in responding. Thanks for the extra info. Other things have taken priority so when I get round to opening it up I will report back. It's likely the meter has been repaired before as it came from my previous employers' workshop where everything was fixed "in house" if possible. We will just have to see if the phantom fixer has been in already. It was never my meter when I worked there, I came by it in a job lot of ex equipment from them.
Alan.
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Old 22nd Jul 2018, 9:22 am   #8
Phil G4SPZ
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Default Re: AVO 8 mk3

The Avo 8 Mk III is one of my favourite meters; a delight to use when working properly, and not difficult to fix.

All Model 8s of that vintage can give intermittent problems, the majority of which in my experience turn out to be caused by dirty contacts on the cam-operated leaf switches behind the rotary switch knobs, but if all works well, leave them alone!

One other tip is to make sure that the battery contacts are pristine clean, particularly those for the D cell. I hope that battery corrosion hasn't occurred, always a risk when a meter has lain unused for years. The 15 volt BLR 121 batteries are available on the usual auction site, at a cost, or make your own from cheap button cells.
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Old 26th Jul 2018, 4:33 pm   #9
Ken G3XSJ
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Default Re: AVO 8 mk3

I have a question about the AVO 8 Mk3 and wondered if those with detailed knowledge might offer advice. My "new" one seems to work reasonably well but when set to Ohms on the right hand control, moving the left hand control through the 50 microamp range to reach another range causes the movement to kick backwards. Holding down reverse causes the pointer to reach about a third of full scale.

I wondered if someone had soaked the switches in conductive fluid but they appear dry. Any clues please?

Thanks, Ken
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Old 26th Jul 2018, 4:51 pm   #10
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Default Re: AVO 8 mk3

That needle twitch you refer to when switching through certain settings, is an aberration of the design and is normal. (IIRC It is a useful indication that a battery is fitted)

I can't remember the last time i used the REV MC facility- on what setting do you get that 1/3 of FSD deflection?

Dave
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Old 26th Jul 2018, 8:29 pm   #11
Ken G3XSJ
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Default Re: AVO 8 mk3

Dave,

Thanks for the information.

Just checked the conditions under which this backward needle kick occurs and it is only on the Ohms range and when switching from "Resistance" to 50 microamps and back. The severity of the deflection was checked by holding down the reverse coil button and it's actually more like a fifth of FSD.

I'm relieved to know that this is a design characteristic and not a fault with one specific instrument. Thanks again for your reply.

Regards,

Ken
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Old 29th Jul 2018, 8:53 pm   #12
buggies
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Default Re: AVO 8 mk3

Quote:
Ohms range and when switching from "Resistance" to 50 microamps and back
Just for confirmation...
I have have tried precisely that on my 8 mkIII (owned since new and never repaired) and get the same result.
With the reverse button held down, if switched slowly enough, the movement flicks past FSD.
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