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Old 31st Mar 2018, 9:01 am   #1
Superscope
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Default Troublesome AVO Model 9 Mk II.

Hi Chaps.

I have just started work on a Troublesome Model 9 Mk II that I picked up last year Serial No: 10131.767 which is in pretty nice condition and well within Spec.

Problem is, the Cut Out won't stay in. I suspect this has been like it for years and hence the Meter has had very little use. I had it out, and it all looks fairly simple to understand how it works, but it just doesn't. Looks can obviously be deceiving!

The Latch doesn't seem to swing far enough over to capture the restraining Bar even if fully adjusted. Very occasionally you can get it to latch, but rarely.

My thoughts are, that it seems like the Rod from the Meter Movement might be to tight, thus restricting the free movement of the Cut Out Mechanism. But this is only a guess at this stage.

Anybody had this problem? Any thoughts out there on how I can tackle this?

Would prefer to hear from anybody with more experience than me, before I start bending things about.

Ian

Last edited by Station X; 3rd Apr 2018 at 7:18 pm. Reason: Posts moved to new thread.
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Old 31st Mar 2018, 10:39 am   #2
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Default Re: AVO Multimeter survey

What occasionally happens is one component within the cutout gets changed, which fails to give a result due to all the components within a particular cutout being closely matched to work together. The clue to this is often tool marks on the various screws, (visible under x2 magnification, probably)

I like to mend rather than replace, but there was one occasion where i resorted to swapping out the whole cutout (including knob/shaft assy and wire rod) due to lack of patience and wanting to finish the job the same day!

I did have one assembly where the swing arm (with jewelled foot) was too tightly screwed down to the plate to swing freely..but P Munro has messed with cutouts far more than me and he would be able to give a far more comprehensive analysis of the mechanism as a whole.

The whole thing is very averse to contamination and if the slightest trace of oil or grease is suspected a cotton bud soaked in isopropyl alcohol can be used to dab it. Care is required however as cotton buds are of course 'hairy' and it is possible for it to inadvertently snag onto summat delicate.

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Old 31st Mar 2018, 12:52 pm   #3
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Default Re: AVO Multimeter survey

Thanks Dave,

Yes, like you, I prefer to fix things if I can, but in desperation last night
I did swap out the whole mechanism from an old Scrap Model 8 I use for parts.

Guess what? Same thing still happening, so I put the original back in!

Which is why I was wondering if the Rod from the Meter movement might
be causing the problem. It does seam a bit tight, but thought I would ask
around before doing any damage.


Ian
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Old 31st Mar 2018, 1:54 pm   #4
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Default Re: AVO Multimeter survey

Sometimes with these frustrating problems i do wonder if the whole bakelite meter facia is slightly distorted- not that there would be anything we could do if it was..

Did you change the wire rod as well as the other bits? If not.. I wonder if it's a slightly different length..

I have one meter that, once tripped, would not reset unless i opened it up and reset it manually. I swapped out the entire cutout but the problem persists even now. The distortion theory mine is arguably more likely with my one as it is a wartime moulding.
(I still use it as otherwise the meter is fine) Thinking about it this one was dishwashered by a previous 'owner' so distortion/contamination of the facia is all the more likely.

I am not confident enough about cause and effect to say 'nip a bit off the end of the rod' but i admit i have been tempted.

The only other thing i recall about rotating the cutout table for adjustment, is that more than one screw has to be loosened in order to do this.

Do let us know how you get on.
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Old 31st Mar 2018, 2:03 pm   #5
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Default Re: AVO Multimeter survey

Additional: There is a small amount of 'slop' in the two big screws that locate the meter movement in the facia. If these were loosened slightly and the movement persuaded away from the actuation rod (without touching the magnet with anything ferrous) this might give an extra 10 thou clearance.
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Old 10th Apr 2018, 11:52 pm   #6
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Default Re: Troublesome AVO Model 9 Mk II.

It might be worth removing the movement (trip the cutout first) and checking that the push rod isn't bent, kinked or otherwise sticking in the passage through which it slides. Also, while you have the movement out, check that the cut-out actuator pad below the front of the moving coil is free to move and not distorted, and retracts fully. There are two small L-shaped cranks, one at each end of the coil's travel, and they should both sit in symmetrical positions.
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Old 17th Apr 2018, 10:33 pm   #7
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Default Re: Troublesome AVO Model 9 Mk II.

Hi Phil, it's funny you should mention that!

The fault turned out to be the Push Rod was not travelling through the tiny Hole in the Metal Bracket which guides it to the Meter Movement plunger.

Having never played with the Cut Out mechanism before, I didn't know there was a tiny Hole in the Metal guide Bracket. It looks like a previous owner had incorrectly placed the push rod on top of the Metal bracket, not through it. I merrily copied his (or Her) mistake.
I just happened to notice this tiny Hole under a magnifying glass, and took the back off another AVO to check how it was supposed to be which is when the Penny dropped.

This also explained why a replacement Cut Out taken from another Meter which I tried still had the same fault.

With the Push Rod slightly under tension because it wasn't travelling straight, it was just tight enough to prevent the Cut Out Pawl from freely moving far enough to Latch the Cut Out Spindle.

I spent a good few Hours understanding exactly how the mechanism works, but in the end I got it working, adjusted and Tested. I now have a fully working Model 9 MkII and in Spec to Boot!

Flushed with this success, it's now time to turn my attention to a non working Cut Out on a Model 8 Mk IV.
This Cut Out is quite a bit different in construction, but works on the same principles.

The Mk IV is a real Pain of a Meter to dismantle.

Ian
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Old 17th Apr 2018, 11:16 pm   #8
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Default Re: Troublesome AVO Model 9 Mk II.

Great news, Ian! Well done. Glad you managed to find the cause and fix the fault. It's often the case that the first time you encounter a particular fault, it takes a lot of time and research to sort it out, but the next time a similar fault crops up you'll get it done in no time. It's particularly significant that someone else had worked on it before you, and put it back together incorrectly. If I had a pound for every time I'd seen that, I'd be a rich man...

I don't recall seeing inside a Model 8 Mk IV so I can't offer any advice on that one, but isn't it much the same as the Mk I, II and III? I know the Mk V, VI and VII are totally different though, and I've never tackled one of those either! I have a Mk V but luckily it works perfectly.
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Old 18th Apr 2018, 9:47 am   #9
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Default Re: Troublesome AVO Model 9 Mk II.

Fixed! It's of great help to have an similar meter to refer to- not always possible but a good excuse for acquiring a duplicate..

The 8/IV has the old facia but newer guts, Phil- (printed circuit boards.) Not one i have looked at either- but at least you have a head start regarding faulty cut-outs Ian!.

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Old 18th Apr 2018, 9:10 pm   #10
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Default Re: Troublesome AVO Model 9 Mk II.

The two Mk 4's I have don't have PCBs, but maybe the later Mk4's do?...the ones which have the rear case of the Mk5 onwards, just before going onto the Mk5.
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Old 19th Apr 2018, 9:23 am   #11
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Default Re: Troublesome AVO Model 9 Mk II.

The Mk 4 is partly assembled on a Synthetic Resin Bonded Bonded Paper board (SRBP) as used for the substrate of early PCBs but there are no copper tracks. The function switches have printed discs with phosphor bronze contact fingers. These replace the relay style switches of the earlier meters. The tracks are silver plated and can be subject to whisker growth.

The Mk IV was not in production for long, less than 2 years, and there was a lot of trouble with cracks propagating from the screw holes for mounting the front panel to the case. Eventually, a Mk V style case was offered as a replacement spare part. It would seem unlikely though that it was ever used in production as manufacture of the Mk IV would probably have stopped as soon as the Mk V was available.

A Mark V back case can be adapted to fit a Mk IV by cutting down the internal pillars which are intended to support the front panel. I think its only the lower pillars which need to be modified.

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Old 28th Apr 2018, 11:21 am   #12
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Default Re: Troublesome AVO Model 9 Mk II.

Two posts split to a new thread: https://www.vintage-radio.net/forum/...d.php?t=146024
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