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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 7th Aug 2005, 8:31 pm   #1
pmmunro
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Default AVO Meter Servicing

[continued from AVO Multimeter thread

The VC-1 voltage convertor was an accessory marketed by Avo during the 1970s which overcame a shortage of suitable batteries for the "ohms x 100 range" of the Model 8. It used current from the D-cell which supplies the "ohms divided by 100" and "ohms" ranges.

A suitable battery is available from RS Components (Stock No. 196-6715 - £6.19 + VAT in the latest catalogue I have to hand). I beleive that Farnell also sell one, which probably means CPC as well. Other suitable types appear on eBay fairly regularly but their freshness may not be assured. If overall dimensions are a problem it should be possible to devise a simple cradle to take up the spare space. Avo did market a moulded plastic sleeve for this purpose which may still be available.

Ebay advertisers often claim to be unable to test an Avometer because of lack of cell/battery. The only "classic" Avometer which needs a cell or battery to work on the current or voltage ranges is the Model 7 which uses the D-cell for rectifier bias on AC ranges.

Last edited by Duke_Nukem; 13th Aug 2005 at 5:15 pm.
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Old 12th Aug 2005, 5:26 pm   #2
Neil Purling
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Default Re: AVO Multimeter survey

The #8 Mk 2 I got turns out to suffer from magnetic degradation in the meter movement. All the ranges were innaccurate (30% low).
Does this happen with older AVOs? Makes one wary about buying a old AVO.
There's a spare movement on ebay fora Model 8, a replacement part. One will need to know how old that is or else it may suffer from the same sort of degradation.
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Old 12th Aug 2005, 6:09 pm   #3
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Default Re: AVO Multimeter survey

I think Perer Munro should really field this one.

In my experience, AVOs hold their calibration (and magnetism) almost indefinitely. What they don't like is (even mild) abuse. If you "go in" and start poking about with a normal screwdriver, you stand a good chance of degrading the magnets - I've done it myself in the innocence of more tender years - but left alone they should stand up well. I have instances from (at least) the mid 1930s with no problems AND UNBROKEN SEALS ! I'm sure Peter does too.

I don't know what can be done about it. As a child I remember seeing loudspeaker magnets rejuvenated, so I imagine something similar should be possible with AVO magnets in the right hands - though possibly not AVO's any more.

Peter ?
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Old 12th Aug 2005, 7:35 pm   #4
Neil Purling
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Default Re: AVO Multimeter survey

The investigation of my AVO was done by a proper technician who calibrates & repairs test equipment.
Im wary of bidding for the NOS movement (& dial plate)untill I know it isnt really old.
The poorly AVO displays no marks of violence. If it had been left near to something that produced its own magnetic field would that exert a degaussing effect on the meter movement?
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Old 12th Aug 2005, 8:07 pm   #5
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Default Re: AVO Multimeter survey

It would have to be a very powerful source, and the inverse-square law gives very rapid drop off. AVOs were well-designed for their expected environments: if shielding had been considered necessary, Douglas Street would surely have provided it. Of course, it all depends on your unit's provenance (big ?), and it would be improper for me to comment on your expert's opinion.

I can only express my surprise at the "30% low" verdict. Is this on ALL ranges (AC, DC and Ohms) and is it proportionate across the scale ?

But this Forum is not the place for "forensic" examination.
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Old 13th Aug 2005, 8:54 am   #6
Neil Purling
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Default Re: AVO Multimeter survey

He said that he took the shunt out of the circuit and inserted a multi-turn pot, ( I assume a wire-wound example). The reading was still incorrect with no shunt in the circuit, (on the lowest amps range I assume).

I am not a happy bunny because it looked perfect, with no sign of abuse and I made a long motorcycle journey to collect it and spare it the tender mercies of the Royal Mail.
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Old 18th Aug 2005, 4:57 pm   #7
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Default Re: AVO Meter Servicing

Quote:
Avo did market a moulded plastic sleeve for this purpose which may still be available.
This is what they look like, part no 5210-064, sorry but i haven't checked if they're still available.
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Old 19th Aug 2005, 1:45 pm   #8
pmmunro
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Default Re: AVO Meter Servicing

Neil,

As SPCH says, Avometers do not usually loose their movement calibration without external influence. The oldest I have is from 1924 and it still has the final inspector's seal to prove it has not been opened. This meter is still in calibration as is another from 1931 which, as far as I can tell has had little interference, although the case has been opened. Others, between then and now, show no general pattern of flux loss.

I did find that one much abused batch from a college needed some remagnetisation which, lacking accurate information, I suspect could be due to gross overload, dropping and other abuse. (A good test for general movement condition and freedom from abuse is to check the pointer balance which should be within 1%, horizontally and vertically - described elsewhere in the forum).

I would have to say that attempts to "recalibrate" by adjusting either the movement swamp or shunts is a "fiddle". The movement should be adjusted for correct sensitivity out of circuit; this will have to be done in a draught-proof box as even a slight air movement will deflect the pointer. (The movement also has to be accurately balanced). Up to the Model 8 Mark V, there is a magnetic shunt which will give about 10% adjustment. If this does not bring the sensitivity to the required level, the movement will have to be remagnetised, which can be done by most professional repairers. The magnet is saturated, which gives too high a flux, and then "aged" by an alternating field to reduce the flux to a lower level which can be retained indefinitely. Later, centre pole magnets, have to be magnetised and aged to the exact design value.

Once the basic sensitivity is correct, the next stage is to verify that the resistance of the moving coil plus swamp is correct. Only when these values are correct can the movement be matched to the shunt and multiplier circuits.

The Model 8 Mk V, VI or 7 has a thick film swamp and this can drift in value. It is possible to fit a cermet trimmer as a replacement and this should be adjusted for the correct overall moving coil plus swamp value. I know some repairers do use an adjustable swamp for fine setting of the sensitivity but this does assume that the movement will still match all ranges, which is not necessarily valid. It also means that the overall resistance of the meter is non-standard although how important this is depends on the appliction.

A desensitised movement is not a complete disaster - there are still repairers with the resources to remagnetise movements, it doesn't take long and, if you're lucky, the charge will be modest.

As SPCH says, these are sensitive instruments and need careful handling in servicing. Considerable knowledge, and above all the ability to handle delicate parts appropiately is essential. Unless the repairer is very experienced, it is also very time consuming. In any event, a few simple jigs and suitable tools will be needed, probably too much for a single casual repair to be viable.

It has to be remembered that the last "traditional" style Model 8 (MK IV) was made before 1972, over thirty years ago. Since then there has been plenty time and opportunity for abuse, arguably more in recent years as fewer people appreciate how a sensitive analogue instrument should be handled. How many 30 year old digital meters are still around? And yes, they were made 30 years ago.
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Old 22nd Aug 2005, 9:51 pm   #9
Neil Purling
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Default Re: AVO Meter Servicing

I would like to express my thanks publically to SPCh for being willing to attend to my AVO 8 Mk 2.
You will recall that it mysteriously was reading 30% low?
The man met me and recognised the strange symptoms were potentially due to a dirty DC Range switch meaning the Zero ohms pot was still in circuit when it shouldnt be.
Hopefully the good news he gave me turns out to be true.
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