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Old 9th Jan 2019, 7:40 pm   #1
Cathode Ray
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Default Avo 8 MK3 switch alignment issue

Hello folks,
I have a nice copy of a Mk3, everything is working fine.
But I had an issue with AC voltage which I traced back to switch contacts o-p (see diagram below) which remianed closed in AC.
A gentle adjustment of the berillium copper springs enabled them to open agains and all was well.

Here is my concern, the air gap on these contacts about 0.5mm, far less than all the other contacts which seem to be about 1.5mm to 2mm.

Firstly, why would this contact develop this peoblem? I mean is there another issue causing it?

Secondly, should I try to make the gap larger given this is ACV switch and the meter can potientially develop 2500v across it?

Thanks
Raymondo!

PS sorry if its not allowed to upload diagrams, but this one is feely available on dinternet!
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Last edited by Cathode Ray; 9th Jan 2019 at 7:50 pm.
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Old 10th Jan 2019, 12:30 am   #2
The Philpott
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Default Re: Avo 8 MK3 switch alignment issue

...I believe that these switches only have to deal with minimal current & voltage, hence the size of the contacts. All the volt-dropping happens 'before' these switches.

The clearances can look very small, sometimes it's down to wear in the contacts or the spacers, and sometimes it's because someone has taken a nail file to the switch contacts in frustration. (This is a very bad idea as it ruins all the tolerances; all that's needed is a clean, and, sometimes in addition to this, a slight bend in the leaf is required- as you have found!)

This is the Achilles heel of this meter, but i have only ever found one that was so badly worn down that the whole leaf stack was beyond use. (and that was a much older meter) As long as your 0.5mm is obtained consistently, it might be OK. I have never measured my switch gaps, just done it by eye. Often the gap looks more sensible when seen through x5 magnification!

Dave
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Old 10th Jan 2019, 12:38 am   #3
Cathode Ray
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Default Re: Avo 8 MK3 switch alignment issue

Cheers Dave, thanks for that.
Before I fixed it, the AC voltage range was reading about 50% low.
Probably because the shunt resistors were still in circuit.
But as you say,if there isn't any high voltage at the contacts then I'll just leave well enough alone. The meter is reading perfect on all ranges.

Regards
Ray.
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Old 10th Jan 2019, 1:38 pm   #4
The Philpott
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Default Re: Avo 8 MK3 switch alignment issue

That's exactly it! When the switches are maladjusted in such a way the current takes a longer route through the meter than it should do, like a taxi driver who's fallen on hard times..

So when AC is 55% low or DC is 35% low on this model, you can be almost certain where the problem is.

I did sketches at the time to remind myself of this, as i had two meters that both said my AC mains voltage was 95v- and the arrow signifies where i applied some force. (i have always used a strip of the cardboard from inside loo rolls soaked in isopropyl alcohol if the contacts just need cleaning up.)

Dave
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Old 10th Jan 2019, 1:45 pm   #5
Cathode Ray
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Default Re: Avo 8 MK3 switch alignment issue

Bingo !
It's like you drew my exact problem.
I also bent the lower spring upward as it was easier.
Now I wonder why this problem occurs?
The black plastic lift cam on mine isn't worn down.
Perhaps it's the metal cam which is attached to tbe lower spring which is bending over time .

Anyway, good to see were aligned.

Cheers
Ray
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Old 10th Jan 2019, 5:59 pm   #6
The Philpott
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Default Re: Avo 8 MK3 switch alignment issue

For the sake of completeness...If the DC readings should ever drop around 35% the problem is likely to be one of the contacts shown below!

I commented elsewhere that the silver alloy of the contacts, while corrosion resistant, will easily blacken when near decomposing rubber (ie closed up in a box with smelly old test leads!) Silver Sulphide is not an aggressive corrosion, and can be polished away, but it will certainly stop conduction of electricity...Hence the aforementioned problem of misguided owners simply grinding metal off the ends of the spikes.

As for meters that have not been abused i can't really resolve why these problems happen- but i do notice that some of the original quality control within the leaf switches was marginal even as late as the '60s - spikes misaligned and only just touching the edges of their contact pads, etc.
I know that phosphor bronze has a 'grain'- and wonder if this might be the case with beryllium copper/bronze, and if so whether this material can distort over the years.

The brass bushes that the main rotary selectors sit in, (in the meter facia) have to be a fairly snug fit, and if they ever got slack (like the valve stems in an old cylinder head) this could cause sloppiness with the leaf switches.

Dave
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Old 10th Jan 2019, 8:48 pm   #7
Cathode Ray
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Default Re: Avo 8 MK3 switch alignment issue

Thanks dave, you obviously have done a lot of debugging on these, fair play to you.
There was a sign that someone else had performed some re-forming on the leaf spring, but I suspect it was many years ago.
My meter still had a black wax seal on the side screw hole and a very faded repair sticker dated '78 or '68 it looks like. Performed by A.D.L Technicare Ltd in Maidenhead, phone number 71101

I'm just glad my eBay purchase worked out ok. At least the Mk3 is is tip top condition now.

But my Mk5 is in need of a meter.
Ive done watch repairs in the past so I'm half considering taking a look at the bobbin winding to see if I fix it... tall order I know. I could throw the calibration completely out if the windings were not exact. But Ive nothing to loose as the movement is a right-off.

Cheers
Ray
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Old 10th Jan 2019, 10:00 pm   #8
The Philpott
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Default Re: Avo 8 MK3 switch alignment issue

On the metallurgy side of things i read that the conventional process for makings strips of CuBe alloy is to stamp them out from sheet before ageing/heat treatment. This makes the stamping out easier, but the subsequent heat treatment can cause distortion. If the lattice structure is inconsistent (or some parts were treated for longer than others) i can see how this could cause some alignment problems. It's predictably difficult to find information on how the passage of 50 years affects this alloy however!
I still prefer this switching system to later ones..

Dave
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Old 11th Jan 2019, 12:25 am   #9
Cathode Ray
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Default Re: Avo 8 MK3 switch alignment issue

Interesting points Dave
From what I can see on the internet, CuBe is ideal for electrical spring contacts and us widely used in the industry.
So I would tend to think it's able to keep its shape over a long period.
I'd be more inclined to go with your previous comments about AVO quality control. And perhaps the gap closes over time because if other mechanical issues were not yet aware of. For example I noticed that the two switch banks on my DC switch are not aligned in the same plane. The rear one is tilted up slightly, this might be the cause of my particular issue.

Ray
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