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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 22nd Feb 2024, 10:12 pm   #1
Techman
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Default AVO 8 MK V not working and totally un-damped needle.

I went to my local garage to help the garage owner's son with an electrical/electronics problem (amongst other things that are off topic for on here) and I'd popped my small, cheap DMM in my pocket just in case. It turned out that he already had a good Fluke DMM, but for what we were doing I said it would have been better if we had an analogue meter such as an AVO and that if I'd known I would have brought one with me. He then said that he'd been given an AVO a couple of years ago and hadn't been able to make anything of it. So off he went to find it and it turned out to be a very nice condition AVO 8 MK V, complete with leather case, leads, probes and original instructions. Unfortunately it didn't work. It still had the 1.5 volt cell fitted, which tested good, but the 15 volt battery was missing. The meter showed no deflection on the low ohms range with probes shorted together, which would normally be expected with the 1.5 volt cell in place and the fuse tested good. It doesn't read on the voltage ranges and I suspect that the movement could be o/c, a common problem, especially as I noted that the needle swung undamped across the scale when the meter was 'rotated'. I took the meter away with me and said I'd have a look at it for him, but told him that I thought it looked like the movement was probably open circuit and that they're now unobtainable. He then asked if it could be rewound, but I told him not a chance, unfortunately.

It happens that I have the very same model and mine is well damped and the needle doesn't swing across the scale undamped when rotated like the meter in question here does. I also note that the cut-out push button rubber dust seal on his meter seems to have degraded and is possibly stopping the cut-out from re-setting, but can't confirm that this is the case at this stage.

So the question I'm asking before I dive in and pull it to bits and possibly waste a lot of time on a lost cause is what do the AVO experts think - does it sound from my description like it's got an open circuit movement?
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Old 23rd Feb 2024, 10:45 am   #2
pmmunro
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Default Re: AVO 8 MK V not working & totally un-damped needle.

Your assessment is correct; if the pointer swings freely without damping, either the moving coil itself is open circuit or the shunt chain which should be connected across the moving coil is disconnected.

If the fault is outwith the moving coil it could be due to the Reverse Moving Coil switch, a fault in one of the selector switches or part of the shunt chain being open. Setting the right hand switch to 'DC' and the left-hand switch to 'OFF' should short out the movement for transport. If the pointer is not damped in this setting, the trouble is within the immediate circuit around the movement. In the diagram, you will see that contacts 1 & 17 and 2 & 18 should be closed.

As you suggest, if the Reverse Moving Coil switch was in some intermeidate position, this too could result in an open circuit.

If you need a copy of the service manual or a replacement switch boot, send me a private message.

PMM
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Old 23rd Feb 2024, 11:09 am   #3
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Default Re: AVO 8 MK V not working and totally un-damped needle.

Setting the two knobs to both DC and AC on most AVOs shorts the meter circuit to damp the movement for transport. The reverse MC switch will have to come after this, so a bad contact here would leave an undamped swinging pointer.

David
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Old 23rd Feb 2024, 11:14 am   #4
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Default Re: AVO 8 MK V not working and totally un-damped needle.

Potential solution?
https://www.vintage-radio.net/forum/...d.php?t=208773
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Old 23rd Feb 2024, 2:07 pm   #5
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Default Re: AVO 8 MK V not working and totally un-damped needle.

Doesn’t the Mk 5 use a flexible PCB which is prone to broken tracks? If so, add that to the list of things other than the movement itself which could be faulty.

Anything other than an open circuit movement should be repairable. You can test the movement by connecting a battery in series with a high value resistor as close to the movement as you can get. The resistor needs to be over 40k with a 1.5V cell, and over 240k for a 9V battery. You don’t need to measure the resistance of the movement, any deflection in the movement, which you can see even with the AVO face down, will tell you if it isn’t open circuit.

Stuart
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Old 23rd Feb 2024, 3:12 pm   #6
pmmunro
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Default Re: AVO 8 MK V not working and totally un-damped needle.

Quote:
Originally Posted by stuarth View Post
Doesn’t the Mk 5 use a flexible PCB which is prone to broken tracks? If so, add that to the list of things other than the movement itself which could be faulty.

Stuart
Yes, it certainly does. The trouble is that the flexibility is limted and the tracks are easily broken when handling the meter as any access to the movement or other parts covered by the flexible printed wiring requires the film to be flexed. Gentle flexing with an almost rolling motion should be all right. If the flexible printed wiring is damaged, a conventional wire alongside the track is a possible repair. If several tracks are damaged, it might be better to make up a complete replacement with a conventional wiring harness.

The printed wiring substrate will be very easily damaged at normal soldering iron temperatures (e.g. Weller No. 6 Tip - use No 5 and low melting point solder).

PMM
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Old 23rd Feb 2024, 7:19 pm   #7
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Default Re: AVO 8 MK V not working and totally un-damped needle.

Thanks for all the info. and pointers.

I found that I'd got a copy of the service manual (28 pages if that's the one) that I must have downloaded at some time, probably when I got my MK 5 from a junk sale a year or two back. I've just noticed that mine doesn't have those dust seal boots on the switches and I didn't realise that they're supposed to have them at all until I had my hands on this particular one - perhaps not all the models had them?

I've opened it up and it looks clean inside with nothing untoward to see, although I noticed that the whole meter and its carry case smell quite musty, so although it doesn't look to have been in damp storage, the smell would perhaps indicate otherwise.

Checking for any current flow through the movement, as advised, and then cautiously measuring the resistance across the points 1 & 2, which is as near a test point to the movement as can be got to without further dismantling, I measured low ohms (as in short circuit) with the meter set to DC & OFF and around 8K with the meter set to other voltage and current ranges, with absolutely no deflection of the meter coil whatsoever. So guessing that other than a possible, but unlikely fault with either the reverse MC switch or R27 swamp resistor, I'd say that it's looking like an o/c meter movement.

This AVO is a rather 'modern' meter to me, as I'm more used to working on the older Model 8 types and the earlier Model 7 types, hence why I thought I'd ask a few questions before getting too involved in it. I remember at a place where I once worked a lot of the earlier Model 8 meters were used and it was known to be a stock fault with the movements going open circuit even back then and when replacements became unavailable a lot were presumably scrapped, although I wasn't in the particular department that handled the repairs, so don't actually know where all these broken meters eventually ended up, although I did get given four very early Model 7 types and managed to make up two good ones out of the four at the time.

Using an AVO minor MK 5 on the X100 ohms range, I compared the resistance via the probes on the Model 8 and there was a varying reading on all voltage and current ranges on both the meter under test and my own Model 8, with my meter giving very slightly lower ohms readings to take into account of the working movement and being careful regarding the 0.05 milliamp range on my working meter, as this was giving just past full scale reading with the current from the AVO minor. I think this test shows that the rest of the meter under test is working on all its ranges with just the movement that is faulty.

The question is, is it worth removing the component board and digging further? I suppose there's nothing to be lost at this stage. I think with it looking very much like an o/c movement that I'll take it back to the garage when I next visit and ask him what he wants to do with it.

As Nick pointed out in the link in post #4 - I had actually already spotted them and they are a gift if I'd have been going to that event, but it looks like there's other interest in them anyway, so not to worry.

Thanks all for the comments and advice so far.

Picture of the 'innards' below:-
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Old 24th Feb 2024, 6:52 pm   #8
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Default Re: AVO 8 MK V not working and totally un-damped needle.

I wouldn’t give up hope until you’ve checked a bit closer to the movement, as you say the Rev m/c switch and swamp resistor could be faulty, and have you excluded the (known to be problematic) flexible pcb?

I can see from the photo that access is poor though.

Stuart
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Old 25th Feb 2024, 2:44 pm   #9
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Default Re: AVO 8 MK V not working and totally un-damped needle.

As far as I can tell I think there's only the red and black ordinary wiring after the points 1 & 2 and doesn't pass through any more of the flexible, or what the manual calls 'film' wiring.

I may delve a bit further, but this business with o/c movements in all the model 8 MKs is a very well known fault. I remember being told a story by one of the old timers very early on in my working life that there was a young lady on the production line who handled the movement coil assembly at that time and she always had an apple in her lunch box. Apparently, the problem with all the movement failures was eventually tracked down to the fact that she had her apple at lunch break and didn't wash her hands afterwards and the acidity on her fingers from the apple got onto the fine copper wire when she returned to her work station after her lunch break and caused the wire to eventually corrode in a vital place. This may be why some AVOs such as my own still work and will probably carry on working, while others, such as the one in question here, failed in their early life after not having had a lot of use.

I've actually got a couple of nice earlier model 8s myself that have o/c movements, one definitely and the other suspected of doing it intermittently, and they're in near mint condition having obviously not had a lot of use. So I think this tells us that all the AVO 8s made before lunch break at AVOs works went on to have a long and useful life, but all (or most) of the ones made after lunch had a short life and needed a movement replacement early on in their life while they were still available. So this one lady supposedly very nearly brought the company to its knees - of course this story could be a complete urban myth, but it does seem to be a fact that the meters that turn up with o/c movements all seem to be in immaculate condition, not having had a lot of use, whereas the ones that work fine and go on and on, seem to often show signs of a long life of heavy, or regular use.

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Old 25th Feb 2024, 5:15 pm   #10
pmmunro
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Default Re: AVO 8 MK V not working and totally un-damped needle.

Techman,

You seem to have had a most unfortunate experience with Model 8 Avometers and a far higher failure rate than my experience would suggest. The smaller movements, as in valve characteristic meters, do seem to fail much more readily and the predecessor to the Model 8, the HR Avometer are much less reliable. Other Models do not seem to fail much with O/C movements.

The story of the acidic fingers is quite plausible but not one I'd heard before. I knew a BBC technician who had naturally acidic skin; every ferrous tool he touched was covered in rust. I don't know what effect he had on copper.

Another technician in a US fcatory had a very higher rate of failure in soldered joints until it was discovered that he was using a reel of solder which he had rescued after it had fallen in a bucket of oil.

PMM
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Old 27th Feb 2024, 10:03 pm   #11
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Default Re: AVO 8 MK V not working and totally un-damped needle.

I thought the very common failure of the meters in the valve testers was more to do with a lack in design for any meter protection against certain valve faults or operator error.

I've never known, or heard of, a Model 7 or 40 having an o/c movement.

I worked as a technician for some years in a further/higher educational establishment (hence my username) where a lot of AVO 8s were in use at the time, literally dozens and dozens of them and a lot went out of service with o/c meter movements. I wasn't a 'techman' in that particular department for most of the latter part of my time there, so only heard of the problems from a distance. You could argue that student abuse gave these meters a hard time, but no type of abuse (incorrect range settings etc.) would actually kill the meter moving coil itself.

I did remove the component board, but it didn't give me any more access and would really require total dismantling of the whole thing, so I'm going to leave it at that for the time being and I've put it all back together now and I'm going to tell its owner the situation and see what he wants to do next, if anything. I'm pretty confident that the meter coil itself is open circuit from my tests and what's been found with these in the past.

I have to say that it's a real shame, as this meter is in almost pristine condition and I think it didn't get a lot of use before the movement failed. The one odd thing is that the 1.5 volt 'D' cell is in-date with a 2027 date stamp, so either someone fitted it recently, thinking it would make it work, or it could indicate that the meter was in fact still working and in use relatively recently - I'll have to ask and get a bit of the back story on it when I return it.

I'm not used to these more modern AVOs and am more at home with the earlier models. The first one of this type that I've had internal dealings with was my own example, which I bought for a fiver at a junk sale a couple of years or so ago. I had opened mine up when I got it, so was familiar with what the innards looked like and we also opened up the meter in question here at the garage to have a quick look inside for anything obvious, but it was as soon as I handled it and noticed the pointer swinging around undamped that I didn't hold out much hope for it.

Looking at this AVO has got me looking at another of my own AVOs, which is an early Model 8 with the non 4mm plug socket terminals and has a C-952 date code. It was given to me by a chap who was clearing out his late fathers house (I noticed there is a post code written in felt pen on the inside of the battery lid, which locates to the exact place where his father lived near Lincoln) and he decided not to keep the meter himself as it gave random readings when used. I did at the time have a go at cleaning the switch contacts, which seemed to sort out the random behaviour, but what I notice now is that the meter reads consistently low, ie, a 9 volt PP3 battery with a terminal voltage of 9.3 volts, reads 8.6 volts on the AVO, so something else to investigate on this one.

I notice that the garage man's meter has a date code of 9/78, while mine has a date code of 1/74, I also note that it has a Nottingham post code scratched onto it - they've all got some history!

Another couple of pictures below of the under component board and another of the complete meter reassembled with all its accessories etc. It was the case and paperwork that smelt musty rather than the meter itself, but this seems to have largely cleared now - I've heard others say that the cases can have a certain smell to them.
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