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Old 14th Aug 2004, 11:45 am   #1
radiosaddo
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Default AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Further to my recent thread on alignment, I thought I would check out AVOmeters, with a view to buying one.
It would seem that there are a number of variants of the AVO 8.

Does anyone have any info on them?

Which is the better one to go for?
At some point AVO changed from using " 2.5 " and " 10 " as a multiplier to " 3 " and " 10 " (i.e. from ranges 2.5/10/25/250/1000..etc to 3/10/30/300/1000, etc)

Also the high range changed from 2500V to 3000V and then disappeared!

This is what I have found out so far......

Mk1, not seen one, was it just called " AVO8 " as it was the first??
Mk2, ranges based on 2.5/10/25/250/1000 plus 2.5kV, also " LR " and " INS "
Mk3, appears to be the same as Mk2, anyone know different??
Mk4, not seen one.
Mk5, All change, now plastic, and case re-designed. Ranges now 3/10/30/300/1000 plus 3kV.
Mk6, Still plastic, a 600V range has appeared. The 3kV range and terminals have vanished.
Mk7, current model, as in RS, etc. Similar to Mk6.

Also, I have found a " Test Set Multirange " which looks like a Mk2/3. but has the ranges of the 3/10/30/300/1000 plus 3kV. Was this based on a Mk4?

Also I have seen also, a " Test Set Multirange Mk2 " which looks like a AVO8Mk5.
I guess the " Test Set Multirange " versions are military, as they look more robust.

I'm sure loads of you have these in your workshops, which one do you have, etc.??

My questions, does anyone else know more about this (I dare say)

Which is the best one? to get; an early one or a later one?
Which range set up is easier/better?

Many thanks
 
Old 14th Aug 2004, 12:00 pm   #2
Sam
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Just got my AVO 8 out, and I assume it is a Mk IV (well, it says 8 IV on the dial!). It has the 2.5/10/25/100/500/1000v ranges for AC and DC; 2500v DC terminals; 10mA, 100mA, 1A, 10A AC; 50uA, 250uA, 1mA, 10mA, 100mA, 1A, 10A DC; Resistance (ohms, ohms x 100, ohms / 100); and L.R. and INS.

I dont really know what L.R. and INS are for, as mine has a metal back, and the printing has worn away and cant be read! So the Mk IV is not all plastic!

I would have thought the 1/2.5 based multipliers would be simpler. It is 'fraction' of 10, rather than having to multiply by 3. It just seems simpler numbers (and our Physics teacher never liked us marking graphs off in threes - always tens (or fives, or 2.5's, etc)!) I would reccomend an earlier one, just because of the multiplier values. They all take the same batteries, so the 15v cell is still as awkward to find!

Hope this fills in a blank for the Mk4. I didn't make a choise of a model to buy. I just found one for a tenner and got it! Never realised they changed the multipliers!

Sam

ps, do earlier ones have something above the cut-out button, as there appears to be an un-used circle on the plasic front
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Old 14th Aug 2004, 3:20 pm   #3
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Hi Radiosaddo

In all honesty I’d go for a modern DMM. If you shop around, you should be able to get one with inductance and capacitance capabilities as well. I have a Wavetek / Meterman 27XT that does the lot. This replaced a Maplin Gold DMM that saw good service for the best part of a decade before I had an “unfortunate” accident with it (I tried to measure the resistance of the mains supply!) :

To your question, I also have 2 AVO’s I have had for years but seldom (make that never!) use. If it weren’t for the fact that they were given to me by a family member I would have got shot ages ago.

The first one is an AVO 8 MK IV, the back of the thing is grey plastic, but the front looks like earlier models. 3KV range 10 Amp Current etc. The whole back has to be removed to replace the battery

The second is a model 9 MK 2 which is altogether a sturdier beast, again 3KV max range Amp 10 Amp max. Battery replacement for this model can be done through a separate hatch in the top. If you insist on going for these dinosaurs I guess older would seem to be better.
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Old 14th Aug 2004, 5:27 pm   #4
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Hi,

I have a Mk2 and a Mk4 .The Mk4 has a grey plastic back which has the battery teminals connected by wires to the meter not like the older models with the sprung brass tags which always seem to need cleaning to get the divide by 100 ohms range to work! The Mk4 has a easier dial to read.

David
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Old 14th Aug 2004, 8:26 pm   #5
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Hi Sam

LR & INS......

I've got the instruction book for a Mk5 and INS is " insulation " ie up to 200Mohm. This is done by using an external supply of 140-160v.

The Mk5 has no mention of LR but my guess is it stands for " low resistance " .

Joe
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Old 14th Aug 2004, 9:08 pm   #6
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

there are specs and service data for a number of AVO's here:

www.g1jbg.co.uk/service.htm

(Gratuitous plug for own website )

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Old 15th Aug 2004, 5:24 pm   #7
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

LR = low resistance - Ins = insulation.
AFAIR the Mk IV was the first model built on a PCB - avoid they are prone to cracks and DJs where the heavy current transformer is attached
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Old 18th Aug 2004, 1:05 pm   #8
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

The Avometer Model 8 was introduced in May 1951. The first version can be recognised externally by the black, knurled screw terminals. The ranges are in the sequence 1, 2.5, 10. The movement is based on alnico block magnets (unpainted) and the shunts and multipliers are a variety of wound wire resistances and component resistors.

The Model 8 Mark II was first produced in 1956 and had the then new red and black socket terminals. The movement used shorter alcomax magnet blocks, painted with red crackle paint, but was very similar the the 'Mark I' internally. Soon after introduction, a 500V (DC) range was added.

The Model 8 Mark III came around 1964 and is basically a Mark II with a fuse to protect the resistance ranges. More component resistors were used.

The Model 9 Mark II is very similar to the Model 8 Mark II but has internation symbol switch legends and ranges in the sequence 1-3-10.

Some people believe the Model 8 Mk III and 9 Mk II to be the best Avometers ever made - they prefer these to the later Model 8, Mks V, VI and 7.

The Model 8 Mark IV (c. 1968) is substantially redesigned internally and the function switches are in the form of printed paxolin discs and phosphor bronze contact fingers. The discs can be subject to silver migration problems. The case changed from bakelite to moulded thermoplastic but lack of design experience with this material resulted in stress around the case screw holes which are very often the site of serious cracks. The magnet material again changed and is painted in light blue crackle paint. The main objective of the redesign was to reduce production time to restore manufacturing profitability as otherwise Jules Thorn threatened to end Avometer production.

The Model 8 Mark 5 was a complete redesign around a centre-pole movement which was developed from a Taylor Instruments patent. Internal wiring uses flexible printed circuits which are quite fragile and need great care in handling and soldering - use a low-temperature bit. The design was in close conformity to a NATO specification which include severe vibration and drop testing. Despite this, there have been reports of movement bearing bridges breaking under only moderate forces. These meters are usually event more accurate than the previous models if they have not been mistreated. (All Avometers in good condition are usually well within the published figures). The Model 8 Mk V also replaced the Model 9. Ranges are in the sequence 1-3-10. The front panel is still bakelite (to resist soldering iron burns) and the case is a much improved thermplastic moulding. Push-button switch 'boots' are often perished but can be bought from AVO (Megger) at Dover or a repair agent. The overload cut-out on these meters is easily adjusted and is very effective when correctly set.

The Model 8 Mark VI (c. 1980) uses the same basic movement as the Mk V but is completely redesign internally to use a pair of rigid pcbs for fast assembly. This required re-arrangement of the range switches whic now partly mirror each other so that the current ranges are on the outside of both switches. The 2500 V ranges were deleted to avoid safety regulation problems and to save the cost of high-value precision resistors.

The Model 8 Mk 7 is the current production model - basically a Mk VI with additional fusing.

NATO variants were made for Model 8 MKs I to III mostly with 1-3-10 range sequences, potted components to withstand tropical use and external supplementary screening cans around the case.

The 'best' version to acquire is partly a question of personal taste as condition is probably more important than other considerations.

If acquiring an Avometer with a faulty movement, do not be tempted to change scaleplates from one movement to another. Up to MK IV they were individually hand calibrated and later the were matched to the movement using 12 basic scaleshapes. (Look for a figure at the bottom centre of the scaleplate).

Up to Mk IV, the last figues of the serial number, at the lower right edge of the scaleplate are the month and yaer of manufacture. From Mk V the date is on a self adhesive serial number label on the right hand centre of the case, repeated inside the battery compartment.

Apologies for any slight inaccuracies of memory concerning exact dates.
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Old 18th Aug 2004, 4:27 pm   #9
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Thankyou all for your replies and thankyou Mr. pmmunro for your fascinating reply. I presume that you are/were and employee of AVO

Quote:
Some people believe the Model 8 Mk III and 9 Mk II to be the best Avometers ever made - they prefer these to the later Model 8, Mks V, VI and 7.

NATO variants were made for Model 8 MKs I to III mostly with 1-3-10 range sequences, potted components to withstand tropical use and external supplementary screening cans around the case.

The 'best' version to acquire is partly a question of personal taste as condition is probably more important than other considerations.
So which is best to go for??
An early Mk3 or a later NATO version?
(Assuming condition ok)

One question I have is why did AVO change from 1-2.5-10 range sequences to 1-3-10 range sequences?
WAs this due to manufacturing costs or in order to meet MoD requirements?

Many thanks

Last edited by Paul Stenning; 29th Dec 2004 at 1:24 pm.
 
Old 19th Aug 2004, 1:05 pm   #10
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

The choice is largely a matter of personal preference, usually based on past experience of using one of these meters, ergononomics and aesthetics. All Model 8 Avometers will still fully meet the requirements of their intended applications, although the later ones, Mk V, VI and 7, have slightly higher accuracy specifications.

Only the Mk VI & 7 are now " supported " by the manufacturers; these are distinguished by the lack of 2500V terminals and the " mirror image " layout of the range switches. If you are considering a fairly recent ex-NATO meter this may have some significance.

To check the general condition quickly, if not comprehensively, verify the accuracy at all ten cardinal points of the 50 microamp scale. Watch for sticking at any point on the scale.

Pointer balance is a usefull check of how the meter has been treated. Set the mechanical zero with the meter lying horizontally - (with its feet on the bench). Now hold the case in the vertical plane and so that the pointer should be first vertical and then horizontal. A variation of greater than 1% of full scale shows there is some imbalance of the moving coil and pointer assembly, often an indication of overload or dropping.

Rebalancing a movement is by no means impossible but needs acquired skill and is best done in a draft-proof box with a transparent front - even convection currents set up by the heat of the hand will disturb the pointer position. Avo always recommended returning movements for exchange if they had any significant fault but this is no longer possible for older meters.

Many other repairs are possible with care and adequate information if you do not insist on complete historic accuracy. For example, Westinghouse copper-oxide rectifiers can be replaced by germanium diode bridges.

The main reason for changing the range sequence was to enhance export sales where Avo believed the 1-3-10 sequence would be more acceptable; it may also have been specified for the NATO versions. There was even a range of Avometers specially designed for export, Models 14, 15, 16 & 20 - all similar to the EA113 in appearance, mostly with grey/blue front mouldings and a " landscape " format which do not seem to have been available for the home market.

I never worked for Avo or had any connection with the company, but I have been researching their history for some years, a significant part being practical research by repair/restoration of meters.

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Old 20th Aug 2004, 8:57 am   #11
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Mine is a Model 8X Panclimatic. It has the metal outer case, and on the RH side of the dial is 'C-658' so I assume it is a MkII variant from June 1958. I have had it since the late 1960s
The ohms pots are getting a little rocky (can you clean them?) but otherwise it is in good order
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Old 20th Aug 2004, 1:55 pm   #12
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

On the Model 8 Marks I to IV, the ohms and ohms x 100 pots. are sealed types, Morganite from memory. Unless you know a technique for resealing these, cleaning is probably not practical as they would have to be cut open. However, the most troublesome pot. is often the ohms div by 100 and this is an open wirewound track. To clean it the movement has to be taken off the front panel.

With Model Eights, you have to be careful to avoid any sideways force on the spiked studs which make contact with the battery compartment 'fingers' as the studs are mounted on the bakelite plates which support the adjustment pots. and undue force will break the mouldings which do not seem to be amenable to gluing. I have found that a good repair can be made by bonding two thicknesses of 0.125 " pcb material together and drilling and cutting to the template of the original. Cut off the moulded feet for reuse as spacers.

A non-magnetic screwdirver (brass or a suitable grade of stainless steel) is recommended for unscrewing the two 2BA screws which retain the movement to the front panel but a piece of plastic sheet of the right size can be used to prevent contact between a steel screwdriver and the magnet. Knocking the magnet can reduce the flux and require re-magnetising. DO NOT on any account loosen the magnet clamp nuts.

The pot. track wire can be cleaned with isopropyl alcohol and a cotton bud. It is probably best to clean the wiper arm and spindle in the same way and to lubricate the spindle bush with a film of petroleum jelly at the same time.

When the movement is refixed to the front panel, the cut-out will have to be readjusted as the clearance between the leaf spring on the movement and the wire pushrod will have changed. The cut-out table can be rotated a few degrees when the two clamp screws are released and should be set to trip on an overload of around 12 times and NOT to trip on an overload of 4 times. (The exact figures vary with different versions of the meter and service manuals but 12 times is a reasonable figure).

While working on the meter, cleaning the cut-out button shaft is a good idea as it is often sticky due to dirt. However, work on the cut-out mechanism is quite involved.

I hesitate to give too much information in case I give the impression that work on these meters is easier than it really is. A combination of skills similar to clock repairing and instrument building and the correct data and equipment are all needed in addition to general electronic skills.
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Old 20th Aug 2004, 4:14 pm   #13
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

Hi pmmunro
Thanks for that - I will have a go at cleaning the divide by 100 pot - it is indeed the worst one.
I am no stranger to clock repairing - been doing it since I was about 13 so should not have too many problems
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Old 13th Nov 2004, 10:47 am   #14
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Default Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc)

The 1 - 3 - 10 is for people use the decibel scale
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