AVO Multimeter survey

[ubrigens]

Recently obtained an AVO Model 8 and it was suggested that I add it to this survey.
Serial number is 806-C-451.
Plate on battery cover is missing---what functions appeared on this plate??

[Pilotltd]

Originally Posted by pmmunro

The Model 8 Mark III differs from the Mark II in having a fuse, in the battery box, added for the protection of the resistance ranges,

Cant see any fuse? Here's some more pictures that might help identifying it.

Steve

[pmmunro]

Ubrigens,

This is a first generation Model 8. The battery cover on these was a phenolic resin (Bakelite) moulding which had a channel for a clip-on probe and a rectangular recess for to rigid probes. The channel was wider at one end to accommodate the body of the clip-probe. There was a pressed metal turnbuckle to retain the probes.

Later versions were supplied with a pair of hook-on probes and the battery cover was changed to a plain piece of sheet aluminium. I posted a drawing of the simple cover some time back.

I have sent you a private message.

PMM.

[pmmunro]

Steve,

It's definitely a panclimatic model as shown by the encapsulated transformer and multiplier resistors. As it doesn't have a screening can, it should be a Model 8X.

The movement is a Model 8 Mark III because it has a thermistor for temperature compensation, so the battery box should have a pair of fuses between the clips for the 15V battery, one active and the other spare - see attachment.

The lack of a fuse suggests that the back case of yours suggest is not the right one for the meter. It's possible that the original case, if this is not the right one, may have been screened so it may have started out as a Model 8SX.

I don't have a commercial 8S or 8SX from this period, but I do have a Test Set No. 1 Multirange, High Sensitivity from November 1967 whose interior is very similar to yours (2nd attachment). The battery box photograph is from this meter.

Perhaps someone else might have some thoughts on the matter?

PMM

[Pilotltd]

It may well be the back is not original as it doesn't sit tightly against the large O ring seal that runs around the body. The movement, magnet and screen all have the serial number pencilled on. Looks like it's a hodge podge of various parts. Whether that was some sort of factory rebuild or somebody else's repair it was well done, and from the soldered joints, not recently, but makes the story of it being bought new in 71 seem unlikely.

It's just a pity that it doesn't zero most of the time on x10 but otherwise it works very well.

Steve

[ralph2472]

Here are some AVO numbers for your collection:-

Model AM D14579-944
Model 40 563-B-248
7X Panclimatic 63204-A-356 (M.O.D. number ZD02995)
Model 7 2376-A-167
Model 8 Mk III 27280-1065
Model 9 Mk II 15891-168
Model 9 Mk IV 75495 94/9/72

I also have a Model 8 Mk III which has had the serial number missed off. It doesn't look as if it's been removed. Please see the photograph.

Regards

Ralph.

[pthomas]

I have two Avometer 8 MK III
No 56847-467
No 19340-665

Both in excellent working order and overall condition. The first one has a back plate with instructions marked Mk iii which refers to copper oxide rectifiers even though on inspection it has germanium! Both well inside specification i.e. <1% compared to Fluke 87.

From what I remember one has more of the blue resistors than the other so internal construction very slightly different

I have a movement Part no 13574-3.A No 125367-C-861 from a written off Model 8 MK II. The movement is fine with good balance and FSD at a measured 37.5 micro amps.

What are the results of this survey?

[pmmunro]

Thanks for the information Pthomas.

Your experience seems common with carefully kept Avometers.

Your meters were both made around the time AVO opened their new Dover factory but as production continued in London for some time it is possible that they were made in different plants. For many years, only wire wound shunts and multipliers, supplemented by carbon resistors, were sufficiently stable to meet specifications but around when your instruments were made, adequate component resistors were becoming available and gradually replaced many of the wire windings.

This survey was begun by the late SPCH and was intended to give some measure of the production rates for each model. It has also been helpful in tracking some of the design changes over the years and has given a better insight (to me at any rate) into when models and variants appeared.

Eventually I hope to collate the information for our general enlightenment.

PMM

[pthomas]

I have a third Avometer recently acquired to add to the data collection
Details are:

Test set no 1 on rear panel
Vauxhall Bridge Road Manufacture
9 SX on front panel
metal screened enclosure with earth terminal
Green Carry Case
Ohms Scale on lower part of dial
dB scale included
Scales 1-3-30 (like Avo 9)
No fuse protection.
Panclimatic (components are potted with large number of wire round bobbins)
Dial plate says Multimeter type 12889 5QP/17447
Accuracy BS 89 IN
ACW R 11.64
Ser No 1529- IS (or 15)- 961
This of course works well and accurate with the only work required being some battery compartment corrosion and small adjustment of the trip. Its very heavy

Does anyone know the background for this one - Army or Air force? NATO? Why the screened shield?

[pmmunro]

pthomas,

Thanks for the information. Not many of this model have been registered with the survey so it is good to have. Serial Number evidence suggests this model was produced between 1961 and 1965.

You will see that we had quite a discussion of this Avometer model in the thread entitled "Is there an AVO Model 9 Mk 4" from May 2012.

I had wrongly thought that the first reference to a Model 9 was the commercial Model 9 Mark 2 which has international symbol markings and was the first civilian meter to use the 1 - 3 - 10 scale sequence.

There are two variants of the Model 9SX or Type 12889; both have simplified scales with the voltage/current scale outermost, which is logical for greatest accuracy on the most frequently used ranges. Earlier version have an inner 'decibels' scale but this is omitted on later versions. (My April 1963 example has the dB scale - others might care to report please)?

The Model 9SX designation seems to have been dropped by 1967. The instruction plate of the 12889 has the title "Test Set Multi-Range Pan-Climatic No. 1 (High Sensitivity); the 1967 version has neither the 9SX button on the front nor the word Pan-Climatic in the instruction plate title.

This meter is described in Air Ministry Publication AP 4343J Chapter 1 where the steel enclosure is stated to provide magnetic screening. I don't know if this was to screen the meter from external fields or the other way round but magnetron magnets come to mind and it is very likely that this instrument could have been used for radar work. Perhaps someone who worked on radar could comment? It would also be interesting to know the experiences of anyone who used such meters in their original application.

Screened Model 8/9 variants were discontinued when the Model 8 Mark V was introduced in 1972. The Mark V centre pole movement is inherently screened by its construction and should have a very small external field. There is some restriction on carrying instruments containing magnets on commercial aircraft, although I don't know how long this has been in force. (I have presented a 1932 Avometer packed in a cut-down wine case at Paris Charles de Gaulle airport security as hand baggage some years ago and it was passed without comment).

The AP suggests that the meter design was to an Air Ministry specification but I don't suppose that there was any reason, other than inter-departmental pride and rivalry - never to be underestimated - why it could not have been used by other services.

The Mark V was designed to replace all Model 8 variants and was adopted as a NATO standard.

I have never seen an advertisement for the Model 9SX which probably means it was a ministry-only production.

PMM

[Paul Adams]

Two more HR Avo’s have found their way to me, ( 4 of this type now) these were in a bit of a sorry state as someone had milled out the front and engraved their initials into the front.

If that wasn’t enough, the ‘Low ohms’ push button was seized and so was the zero ohms pot, on both meters.

Both of these had liberal amounts of ‘WD40’ applied using a cotton bud and were left overnight which resulted in both sets of push buttons and pots working the next morning.

I filled in the damaged area using ‘Black Milliput’ and whilst the filling material was still soft, used a ’home made impression tool’ pushed into the soft Milliput for the leather finish of the front of the Avo, made previously from Milliput, and using 3in one oil wiped on to the face of the tool, acting as a release agent.

The repair is not completely invisible but much better than looking at what it previously was, when I got the meters.



Serial Numbers, are: 1560 - 150 and 1565 -150.



Oh and if your wondering, I now have well over 200 Avo’s.

And I still haven’t found that item for ‘threeseven’ as it’s amongst the 200 Avo’s, which I am putting all of the serial numbers onto the computer for later reference.

Paul.

[Red to black]

Avo 8 MK V S/N: XXX01 8V/1/78

[pmmunro]

Red to Black

Thanks for the information.

Paul,

Congratulations on an ingenious repair technique.

Did these two HR meters with such close serial numbers come from the same source? As you probably know, these meters were not widely advertised although I recently acquired a catalogue, which I believe is from 1950, which includes the attached page.

At the time when your meters were made and sold, the design of the Model 8 must have been well advanced - such are the hazards of progress.

PMM

[Paul Adams]

PMM,

Many thanks for the congratulations, The two HR Meters did come from the same source, together with a couple of 9mkIV's unfortunately the rear cases on the 9MkIV's were so badly damaged, I ended up replacing them from some surplus cases an ex work colleague was selling, these rear cases are the more modern plastic versions (the same as the 8mk5, but without the lower two fixing holes).
Because the old cases had the serial numbers on stickers, these were long gone.

One day I will stop collecting these

Paul.

[Alistair D]

Just acquired a heavy duty AVO serial number 9558-459.A.

Al

[Little-Freddie]

Hiya!! New member, not sure if I can help with your survey, but here goes.

I have an AVO ETM (Electronic Test Meter) think it's a Mk4, but look as I can, I can't seem to spot a serial number, .

Anyone tell me where to look?

Cheers, Fred G4ZWI

[pmmunro]

Fred,

Welcome to the forum which I hope you will find useful and interesting.

From your photograph, it looks like your meter has grey/green wrinkle paint - I believe that all Mk IV versions had sliver/grey hammer finish.

The earliest version of this meter had the movement at the bottom of the front panel and used two EF50 valves. Later versions used a 6SN7 twin triode and the Mk IV had a 12AU7.

The serial number will probably be marked at the lower edge of the translucent scale plate and should be repeated on a model plate, either on the bottom edge or the back of the casing.

PMM.

[Little-Freddie]

Hi, PMM, thanks for that, all interesting stuff!!! popped the case, and sure enough, it uses a 6SN7 (smells lovely inside!! ), As for the Serial number, on the meter face, {LH side} it says "ACWEECO RT No: 1203`-1ETI", and on the RH side it says "No 4257-U-951", which I guess is the serial number.
Only other data I can find is a small label 'pop riveted' onto the bottom, which says :-

" AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO.LTD,
WINDER HOUSE, LONDON SW1 "

still lights up when powered up, and the meter zeroes/moves with the front pots, but I don't have a set of test leads with it, unfortunately!!!! {Not sure if I can vouch for its calibration, though!!}
Found a circuit diagram for it having searched the forum, so will have a play!!
Thanks for the welcome, sure I will find lots of useful stuff on the forum, & hope I can add some input.

Cheers, Fred

[Phil G4SPZ]

Hi Fred,

These Avo electronic testmeters are actually very good instruments, capable of measuring a very wide range of resistances and voltages, at high input impedance. I've repaired a couple. Correct zeroing most probably indicates correct operation, but there are a couple of capacitors that can become 'leaky' and will cause the zero to drift over time. Leave it on for a few minutes and see what happens. Then get some test leads and try it out!

For the survey, I should like to add my latest acquisition, a kind donation by a friend of a Model 8 Mk II, serial number 160282-C-663. This instrument served its original owner well during his 40-year career in the TV trade. It still looks good, but sadly it's rather poorly now and is on the waiting list for attention to its leaf switches which, in my limited experience, seem to be the cause of most problems in early Model 8s.

And greetings to Peter, hope you're well.

[martiR2R]

Just picked up another Avo today, (complete with invoice from Avo for repair in April 1965).

(Why do I need 3 of them!)

AVO 7 S/N 7544-15508

Martin