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Old 15th Nov 2017, 7:46 pm   #1
Sinewave
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Default Avo 8 movement

Quick question regarding the 8 Mk5. I've just been handed one today which doesn't work. The movement is very free as if something has gone open circuit, lack of dampening. I believe the meter very recently was subject to some shock whilst in its case, but a visual looks good.

What should I be looking for first inside apart from ensuring a connection hasn't been shocked off?

Thanks.
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Old 16th Nov 2017, 12:34 am   #2
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Default Re: Avo 8 movement

Where I used to work it was always accepted that the movements in this model of AVO where notorious for going open circuit. There are many examples of this model with O/C movements (I've even got one myself) and there's absolutely no chance of finding a replacement as everyone is looking for one and all the spares have long since been used up.
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Old 16th Nov 2017, 1:01 am   #3
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Default Re: Avo 8 movement

Who knows what's happened with this one. I suspect it's been thrown around and bounced off the walls. I've spent quite a while on it this evening, got nowhere.

When I first took a look at it, the cut off had been operated, it didn't seem very positive when I pressed it home, but it's stayed in. I'm somewhat certain I saw dampening return for a moment afterwards, but now nothing. So I suspect something is not very good in that mechanism.
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Old 23rd Nov 2017, 5:12 pm   #4
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Default Re: Avo 8 movement

Also, another question, regarding another instrument, an 8 Mk 6.

When measuring VDC, the displayed reading whilst increasing the voltage is different than when decreasing.

For example, when increasing voltage, the readings may be 1, 2.....7, 8v

when decreasing, there's a difference of 200mV, so 9v can be 8.8v, 7 would become 6.8v.

What would cause this do you think?

Thanks.
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Old 23rd Nov 2017, 8:02 pm   #5
The Philpott
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Default Re: Avo 8 movement

Hmm. Are you working from a known accurate voltage source? I am interested to know whether the correct readings are occurring as you escalate the voltage up the scale, or when you diminish the voltage. The situation i am familiar with is a slightly lazy movement which underreads as you escalate, but shows (better) accuracy as you take the voltage down in stages. This, generally, i attribute to excess mechanical resistance between the jewels and their sprung pivots. I haven't, however, messed with anything later than a Mk III so will leave it to someone else to comment further on this.. and what you might be able to do about it.
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Old 23rd Nov 2017, 9:51 pm   #6
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Default Re: Avo 8 movement

It is a known accurate voltage source confirmed with a Fluke 787. I'll have to perform the test again to see if it was accurate going up, or coming down - or completely inconsistent.
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Old 24th Nov 2017, 5:02 pm   #7
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Default Re: Avo 8 movement

Most of AVO's competitors switched to taut band movements to eliminate the pivot friction problem and give better shock resistance. Does anyone know why AVO stayed with pivots - are there any advantages of pivots over taut band?

Stuart
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Old 24th Nov 2017, 5:38 pm   #8
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Default Re: Avo 8 movement

Avo movements should be well damped when the switches are in the Off positions. But for another test to see if a movement itself is open circuit, locate the leads inside that go to the movement and short them out with a croc-clip wire.
If the movement is still swinging very loosely when the Avo is moved about a bit, the coil is probably open circuit. If the movement now stays close to zero there's probably a fault somewhere in the switching or cabling inside. Mk5 Avos have a flexible pcb-type cabling system inside that can go open-circuit at the ends or crack somewhere along a track.
Good luck
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Old 28th Nov 2017, 4:54 pm   #9
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Default Re: Avo 8 movement

With regard to the Mk5, can I ask a possibly daft question, why not disconnect the movement leads from the rotary switch circuit bd ( 2 solder joints) and measure the resistance between the lead ends?
The only problem I have encountered with this is that you need to be careful about the current that the measuring meter puts out.
Once the movement assembly is removed from the meter it is, from memory, quite easy to isolate just the movement coil assembly (hair springs and actual coil etc.) and check continuity through it.
With regard to the cutout, it might be worth checking to see if the cut out's moving contact carrier is broken, I have one where this was in 2 or 3 pieces but remained in situ and, at first glance, seemed to operate correctly but did not create continuity.
I was able to glue it back together with Araldite but clamping the pieces together in the correct positions whilst the glue set was extremely awkward.
In the process I made what I came to think was a mistake and glued the actual contact piece in situ. This was partially due to the difficulty of clamping the pieces in place and the contact piece appearing to be an convenient 'splint'. The gluing worked and the reassembled cut out tripped and latched etc. but there was no continuity though it, I think I ended up taking a needle file to the contact points on the contact piece. I now suspect the contact piece is meant to have some degree of freedom of movement i.e. some float.
Re the Mk6, if the coil assembly is carried in a plastic (polythene?) 'frame' visually check that frame for cracks/breaks. They can crack or break near the screws, this seems to allow the frame and coil to shift slightly with respect to the cylindrical core and the coil may catch on something.
The photos shows my first attempt at a repair, the break was in the 'leg' with the copper wire strap. The 'strap' cinches the break together but I think the frame's position need some fine tuning.
The piece mark "bridge" is removable and the strap is run underneath it however it might be better to run the strap over the top of the bridge but I need to investigate that.
I would love to know how these things were aligned during manufacture!
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Old 5th Dec 2017, 5:59 pm   #10
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Default Re: Avo 8 movement

Thanks for the information everyone. I might get round to doing something with them some day.
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Old 21st Dec 2017, 4:35 pm   #11
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Default Re: Avo 8 movement

Quote:
Originally Posted by Sinewave View Post
Also, another question, regarding another instrument, an 8 Mk 6.

When measuring VDC, the displayed reading whilst increasing the voltage is different than when decreasing.

For example, when increasing voltage, the readings may be 1, 2.....7, 8v

when decreasing, there's a difference of 200mV, so 9v can be 8.8v, 7 would become 6.8v.

What would cause this do you think?

Thanks.
Just looking at this again, what I've noticed is that in the 50 micro amp range, the movement goes to 50 and back to zero all day long, fine. I believe that when in the 0.05ma range, we're pretty much connected directly to the movement, with no dampening circuitry at all.

In any other range where there is some dampening, it's as if the dampening is engaging every so often, causing the needle to stick. A very (and I mean very) gentle tap on the case frees the needle up again.
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Old 21st Dec 2017, 5:52 pm   #12
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Default Re: Avo 8 movement

On 50uA you are connecting directly to the 'ends' of the movement assembly but there is also 8kΩ in parallel with the movement which perhaps? gives some damping. The attached in for a Mk5 but I think it is essentially the same as the Mk6, the green is the 'off' connection.
I know it's a crude method but I find that 'tracing' out the circuit as shown helps me follow it
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Old 21st Dec 2017, 11:08 pm   #13
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Default Re: Avo 8 movement

Damping doesn't make the needle stop in the wrong place, it just causes it to settle more slowly.

Stiffness in the pivots could cause the needle to settle in a different place depending on whether the applied voltage (or current, or resistance) was moving slowly upwards or slowly downwards. Such stiffness could be caused by dirty or over tight pivots, but if the meter has had physical shocks, one or other of the pivot pins could have popped out of its little cup.


Dirt in the gap between the moving coil and the core/magnet could cause the needle to stick at some parts of the scale. So could problems with the hair springs.

A static charge on the meter glass can cause erratic behaviour, but is easily checked by breathing on the glass to steam it up.

Does any of these behaviours match your meter?
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Old 21st Dec 2017, 11:20 pm   #14
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Default Re: Avo 8 movement

It doesn't move slower in any particular direction. I've ran it through some tests again and it's behaving as it should. I'm wondering if it's anything to do with it not being used for a long time. Earlier today it was moving fine on the 50 micro amp range but sometimes sticking on others, now it's doing alright
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Old 22nd Dec 2017, 12:31 am   #15
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Default Re: Avo 8 movement

It has been said that they can sulk for a short time after extended inactivity. A recovery can sometimes be hastened by applying a gently fluctuating current or voltage of appropriate magnitude. (I think i used a 6v flashing road beacon to exercise a slightly lagging movement)
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Old 22nd Dec 2017, 6:25 am   #16
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Default Re: Avo 8 movement

Dirt and debris in the magnetic gap between rotor and field causes intermittent sticking I find. A good work out as suggested usually shakes it out. If its had a bit of a rough ride, this is the most likely cause. ---Very--- gently blowing it out, avoiding the hair springs and whacking the movement, usually works.
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Old 23rd Dec 2017, 1:15 pm   #17
Sinewave
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Default Re: Avo 8 movement

Thanks for the information so far.

Having another play with it, it's sticking again today, though whenever free it's still actually reads bang on accurate.

I'd love to open it up and see if there's something I can do, but I'm terrified of making any mistakes with it.

Last edited by Sinewave; 23rd Dec 2017 at 1:23 pm.
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Old 23rd Dec 2017, 1:27 pm   #18
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Default Re: Avo 8 movement

(Might be) a tiny flake or spike of ferrous metal that gets knocked around when the movement responds, but never manages to get entirely free of the magnetic field. Quite difficult to see, especially if they're 'end on' to a jeweller's loupe. Blu Tack is sticky enough to grab them with a bit of care, especially if you have massaged it beforehand to increase the tackiness. Most troublesome artefacts can be seen with a x10 loupe if the light is good enough.
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Old 23rd Dec 2017, 1:35 pm   #19
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Default Re: Avo 8 movement

I have an Avo 8 Mk III whose movement stuck randomly when I first got it. Cleaning debris out of the magnet gap fixed it:

https://martin-jones.com/2013/07/24/avometer-8-repair/

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Old 23rd Dec 2017, 10:51 pm   #20
Sinewave
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Default Re: Avo 8 movement

Quote:
Originally Posted by The Philpott View Post
(Might be) a tiny flake or spike of ferrous metal that gets knocked around when the movement responds, but never manages to get entirely free of the magnetic field. Quite difficult to see, especially if they're 'end on' to a jeweller's loupe. Blu Tack is sticky enough to grab them with a bit of care, especially if you have massaged it beforehand to increase the tackiness. Most troublesome artefacts can be seen with a x10 loupe if the light is good enough.
I will have to invest in a good loupe.
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