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Old 21st Jan 2020, 9:29 pm   #1
flyingtech55
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Default Avo Model 40 MkII

Dear all

I have a really nice Avo 40 Mk II, date code March 1968. It all works, and I use it for electrical/automotive work but one fault that really annoys me is that the needle is slightly bent. It was like this when got it. The needle is bent in a forward direction as follows: if I zero the needle on the centre scale, it is about 1mm (or slightly less) inside the zero of the upper scale arc and the same amount outside the zero on the lower scale arc. When the meter is at approx. half scale deflection, it is possible to see the slight 'forward slope' of the needle. The question is, is it possible to bend the needle straight by hand or will it break. If it is possible, how do I remove the meter movement to perform the operation. As I say the meter still works with very acceptable accuracy and I would hate to break it trying to repair a minor fault. It is very handy to have a 1000Ω/volt meter when digital meters are almost all 1MΩ or higher. If anyone can help with this it would be appreciated; PMs if necessary. Unusually, the 'P' knob is perfectly intact and working. Also, any idea if the coloured terminal caps are still available. On my meter it looks as if they've been tightened using a pair of pliers and the pliers have slipped. This is simply an enquiry; if parts or services are requested I shall use the relevant sections of the forum.

Thanks in advance

TimR
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Old 21st Jan 2020, 11:47 pm   #2
Superscope
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Default Re: Avo Model 40 MkII

The simple answer is Yes, it is perfectly possible to straighten a Bent Pointer.

However, any work on the Movement needs to be carried out with the greatest
care. It is a fragile component when removed from the protective environment of
the Meter.

The Meter Movement (Including Swamp resistor) is held in place by Two Brass
Screws. These will be fairly obvious when the Case is removed and there are
Two or Three connecting Wires.
It is very important that no ferrous Screwdrivers or Tools are allowed to come into
contact with the Magnets, as there is the possibility of loss of magnetism and hence
meter sensitivity.

The method I use is to locate the exact location of the bend and place
Two supports equal distant from the centre point on the inside of the Bend, then to apply
pressure to the central point on the outside of the Bend.
This tends to straighten the Pointer nicely, but it can put pressure on the Pivots so
you need to be very careful.
Hope that makes sense.

The pointer is very flexible and does not take much effort to bend back into shape.

If the Pointer is bent, it almost certainly means the Movement has been out of the
Meter housing at some time and probably been placed face down accidentally on a worktop.




Ian
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Old 21st Jan 2020, 11:56 pm   #3
Superscope
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Default Re: Avo Model 40 MkII

I forgot to mention, the Terminal Caps are no longer available.

They do come up for sale on Ebay occasionally and a 3D printer
file is available on this Forum on another Thread if you have access
to a 3D Printer.
Not sure what colour options are available for 3D printed items though.



Ian
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Old 22nd Jan 2020, 12:30 am   #4
flyingtech55
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Default Re: Avo Model 40 MkII

Dear Ian

Thank you for your speedy reply. The seal on the side of the metal rear case is completely empty so I presume the casing has been opened at some time in the past. I've looked inside the instrument and have observed two relatively large screws each side of the magnet assembly. There is some slight to moderate marring of the screw slots which I would imagine would not be present if the last time they were accessed was by the Avo assembly folk. Are these the screws which secure the movement? If I remove the movement, is it simply a matter of screwing it back in place or are the holes slotted for adjustment purposes?

Also, do you know if the coloured terminal caps still available?

Thanks in advance

TimR
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Old 22nd Jan 2020, 11:32 am   #5
The Philpott
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Default Re: Avo Model 40 MkII

Tim, the screws you mention are indeed locating the movement/magnet/scale plate assy.*
To prevent any minor distortion of the operating rod the mechanical cutout should be carefully triggered before removing the movement*. It should be in this triggered state when the movement is replaced in the meter- it can then be reset and adjusted if necessary. The cutout is on such a hair-trigger that it is likely it will need adjustment once the movement is reinstalled.

The only place to get NOS coloured terminal caps is on buying/selling sites, usually ebay- some did come up last year. Alternatively i recall someone has scanned them with a view to 3d printing them so this is worth looking into.
The NOS caps are vulnerable to splitting where the edge of the mould created little flaws in the plastic, so they need to be heated a little before pushing onto the splines.

Note the mounting screws for the movement do not locate through a slotted hole. There is nevertheless a slight amount of slop in them. A screwdriver with insulating tape around the shaft (just leaving the tip bare) is a good way of mitigating the risk of it hitting the magnet as you work.

The only thing i can add is that Pmmunro (peter munro) explained exactly how to remove and reinstall an avo movement quite recently, and he generally remembers aspects that i forget when summarising it- so it's worth doing a search in the forum search box.

I guess the only other thing to remember when straightening the pointer is to avoid the tools leaving black metal marks on the scale plate- which can be problematic to remove.

Dave
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Old 22nd Jan 2020, 6:29 pm   #6
The Philpott
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Default Re: Avo Model 40 MkII

....and check that the glass is (clean, and) firmly glued in place once the movement is out of the meter. In order to settle meter readings there is a tendency for users to tap the glass rather than rock the entire meter on it's tripod feet....and over time this can loosen the glass.

As the movement is the most sensitive part of the meter, once it is out there is also an opportunity to blow, tap and shake loose debris- shellac flakes, solder balls, etc out of the meter without causing damage. On one occasion this liberated an extraneous screw left in there during previous maintenance!

Dave
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Old 24th Jan 2020, 1:37 pm   #7
pmmunro
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Default Re: Avo Model 40 MkII

Just to add a little to what Ian and Dave have written, the pointer could be bent due to a secere overload.

With care straightening is not too difficult but you will probably have to rebalance the movement afterwards. As your meter is a Mark II, it will probably have the type of cut-out with the die-cast components. The cut-out pushrod in this type is just a straight length of spring wire - which is very easy to lose. The sensitivity of the cut out should be checked, and if necessary, adjusted each time the movement is removed from the front panel.

PMM
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Old 25th Jan 2020, 10:40 pm   #8
flyingtech55
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Default Re: Avo Model 40 MkII

Thanks to Ian, Dave and Peter for your very helpful and knowledgeable replies. My gut feeling is that the meter has been subject to a severe overload and the pointer bent due to bouncing off the end stop. It's certainly bent in that direction. I will inspect the operation of the mechanical trip. I think Peter (pmmunro) wrote a piece on the operation of the mechanical cutout which I shall read when I locate it.


TimR
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