AVO Multimeter survey

[pmmunro]

Nicko,

If you look back through the thread you should find a full explanation of the structure of Avometer serial numbers as we understand it.

The most significant part of your meter's serial number (depending on your interest) is the last group of four figures, indicating the month and year, December 1942. The first part, the 40, is a slight mystery, but the 2471 is the number within the sequence of production. The two together would be about right for a Model 40 of the period, but probably too low for the 47As I know about.

There are several possibilities. For example, your meter could be a 47A which has had a faulty movement replaced by one from a contemporary Model 40, or a Model 40 which has inherited the case from 47A. A photograph of the front panel would help.

The 47A has provision for just one 'D' cell for the resistance ranges, the Model 40 also takes two 4.5 volt batteries, now known as Ever Ready 1289 or MN1604 - but that goes with the case.

Many wartime Air Ministry Avometer type D meters were built up on Model 40 front panels, but I don't remember this being done for 47As, possibly the Admiralty were more discriminating. In any event working meters were probably more important than such details at the time - and who knew how long the would need to last?

The Model 47A often comes in its own wooden case with a second copy of the instruction plate inside the lid. It is quite likely that this is how they were supplied to the Admiralty. This could explain the good condition of your meter's case. When supplied with a larger wooden case containing a range extending shunt and current transformer, the same instrument was designated 48A.

PMM

[VK7ZL - Bob]

Avo Model 8, Mk 3, S/No 18584.665
It is in excellent condition with original leather case, operating instructions and probes.

[Nicko]

Originally Posted by pmmunro

The most significant part of your meter's serial number (depending on your interest) is the last group of four figures, indicating the month and year, December 1942. The first part, the 40, is a slight mystery, but the 2471 is the number within the sequence of production. The two together would be about right for a Model 40 of the period, but probably too low for the 47As I know about.

There are several possibilities. For example, your meter could be a 47A which has had a faulty movement replaced by one from a contemporary Model 40, or a Model 40 which has inherited the case from 47A. A photograph of the front panel would help.

Its definitely a 47a movement - it has the red dot on the needle and the scale markings are 47a not 40 when compared with photos on flickr of "known good" models. Its odd, as the movement clearly says "Model 40 AVOmeter", but the scale and needle are obviously not!

The outer leather case is from a later AVO, I'm sure - its light tan and in VGC - too good to be contemporary I would think.

[pmmunro]

Nicko,

The Model 40 on the Flickr page was made in 1975, so it has a machine printed scale chosen from a range of 12 types for the matching linearity characteristics of the scaleplate and the movement.

Until the mid 1960s, all full-size Avometer scaleplates had only the arc lines ready ruled; the cardinal radial lines were ruled in by hand after the main points (12 in the case of a Model 40) had been marked with a centre punch, above the top scale, when compared to a standard instrument. The intermediate lines were then ruled in by interpolation using a form of pantograph. Up to the 1950s, all the scale numbering was also done by hand, probably until it occurred to someone that the numbers will always be close enough to where the radial scale lines will lie if they are ready printed.

Attached are scans of the scaleplates of a 1944 47A and a 1945 Model 40. You will see that, allowing for the differences in the hand written markings they are very closely similar. The 47A has lost its red disc from the pointer, which would require the movement to be rebalanced. The mark where it has been is just visible on the pointer.

I have another Model 40 from 1945 which has a 47A case and instruction plate - almost certainly to replace the original broken case of the Model 40, or because someone who was repairing both types was not too particular about which meter went back in which case.

I still think that your meter is a 47A which had a faulty movement and was given a transplant from a Model 40, but please try to persuade me otherwise if you do not think this is the case.

Note the letter 'J' at the end of the voltage scale on your meter and my 47A. I don't know the significance of this. It could be "Operator J" relating to reactive loads or perhaps something entirely different.

The Royal Navy still measured capacitance in "Jars" until at least the late 1930s, but that would not explain why it was inscribed on a Model 40.

In case the instruction plate of your meter is difficult to read, I have also attached a scan of a good plate - there is no mention of the "J" marking, unless I have missed something.

PMM>

[Nicko]

Originally Posted by pmmunro

...
I still think that your meter is a 47A which had a faulty movement and was given a transplant from a Model 40, but please try to persuade me otherwise if you do not think this is the case...

I'll defer to your infinitely greater knowledge of these things! I really appreciate the background info.

Since you mentioned it, the meter has got a Model 40 instruction sheet on the back, but the front clearly says Model 47A (see photo above).

Bit of a dog's breakfast, methinks!

[radiomobile nz]

Two working model 7's here, both differ so a few questions for the experts to date.

7146-24581
Thumbscrew terminals, no PF terminals.
Model 7 numeration under meter zeroing screw.
The Working Instructions are all but unreadable, but I see
Automatic Coil Winder & Electrical Equipment Co Ltd
London

3284-154
Plug and Screw terminals, PF terminals.
Model 7 numeration on lower RHS of meter scale.
The Working Instructions are in good condition.
AVO Ltd Dover England
A Member of the Thorn Group

Avometer 8 MkV 12xxx 8V/4/74 needs little explanation or image.

Is there a suitable thread for discussing accessories?

[radiomobile nz]

3284-154

Oops, sorry, that's the meter movement #.
Having done some cleaning the # is 75xxx 7/9/72 on a now indistinct label RHS of case.
The 75xxx # is also lightly hand etched atop the case .

[pmmunro]

Radiomobilenz,

Your older Model 7 was made in January 1946 - the other meters' serial numbers have the date of manufacture as the last two digits.

Several threads have discussed accessories for Avometers, but if none of these covers your questions, why not start a new thread?

PMM.

[Colinaps]

Model 8/6, 1161 N91115 11/86

TS1Mk4 1050 M923130 02/88 (Navy issue 8/6 I think)

Just youngsters!

Colin.

[noble kiwi]

Not being able to resist what appeared to be a good buy, I have just acquired another two model 8 AVO meters, both mark 2.

87239 - c - 359. and 2163 - C - 463D.

One had a hardly used near immaculate leather case and a user and a service manual was included, which was the main attraction. However on close examination I find that the Bakelite meter enclosure has been split asunder at some time, but has been very carefully glued together. It represents evidence as to the abuse the AVO will withstand and how an extensive repair was a viable option.

The second is fitted with an adaptor numbered 5210-064 which takes a tubular Varta V75PX battery. I gather that this is a common accessory. The glass was loose on this one and had bent the pointer slightly, but both are now restored to good order.

[mark pirate]

Another one for the list, model 8 mk3 - 86679.1068
I thought it was worth £3

Mark

[Andrewausfa]

AVO 8 (no Mark), serial 5433.1164

My first AVO. This one seems to have been repaired in 1976 when I assume a later model shunt winding board has been added. Unfortunately the .45ohm shunt winding has melted in one place and the rest of the wire looks charred. I carefully re-connected the wire with the tiniest amount of solder so it's back in circuit and at least the AVO 'works'. It's also missing the 'D' cell earth end metal connector in the battery box. I expect it will need a professional repair before I can use it

Andrew

[Alistair D]

Andrew

I have a scrap 8 mk2 here. You are welcome to any of the battery box components for the cost of p&p. Let me know exactly what you need.

Al

[Paul Adams]

Hello, I am a bit new to this forum but i have a number of AVO's and if it's serial numbers you want well here goes: 71143-11440 model 7. 6625-99-105-7050 number 1 high sensitivity. 81224-A959 model 7. 1779-8-467-M model 40 mk2. 15158-1180 heavy duty mk 5. (i have two of these but only one has a number) Also i have a EA113 Serial: 5396. also various digital ones including an AVO DA116:05636 116/7/78.
Plus a Multiminor (no S/N)
Hope this helps.

Regards
Paul.

[migette]

Hi had pleasure in picking up Avo 7 at Bootsale yesterday for 4 pounds, leads probes and crocs well used but there. No battery leakage corrosion and far as Ive tested appears to be ok, would say it belonged to someone who looked after there equipment no cracks or smeared screws a good piece of test gear. Numbers on it 39-A-1147 Nov 1947 I beleive. I was but 3 yrs old makes me feel very old Peter

[pmmunro]

Migette,

Congratulations on your purchase. The serial number is quite interesting as the first part is very short.

It was around 1947 that AVO (actually ACWEECO at the time) changed from the then traditional horseshoe magnet movement to a design using alnico blocks. It may be that you meter is one of the first block magnet types. If you have occasion to take it out of its case (carefully, of course as SPCH would have cautioned), please let us know.

PMM.

[Paul Adams]

Just bought a AVO Minor serial number: U.46934-542 and the leather case has AM and a crown stamped on it, could this be Air Ministry ?

See attached photo's

Paul.

[pmmunro]

Paul,

The serial number of your Universal Avominor shows that it was made in May 1942, so it is not at all surprising that it is an Air Ministry one. I believe they were also used by the Royal Signals and possibly REME too. Usually the meter itself would also have some AM identity as well as the case, often in the form of a broad arrow or pheon.

PMM

[Paul Adams]

Hello again,
Thanks for that info, and just to update everything, I bought another AVO today at a local car-boot sale for £5.00 it's an AIR MINISTRY one as it's got that on the scale plate (see photo's).

I have tried the meter on volts and it seems to have held it's accuracy, but the battery compartment is puzzling me (photo attached) it seems to have been made out of wood and not the usual bakerlite material, the wood has all been very neatley dovetailed, and the internals of the meter look as good as new.

I have took a picture of the serial number for the forum, that's fifteen AVO's now, I need looking at !

Paul.

[pmmunro]

Paul,

Your new meter is an Air Ministry type 'D', 10S10610 made in May 1941. I have two near contemporaries, from the same year (D1631-241 and D2539-641) which suggests that production was running at between 100 and 500 per month for this model. That's quite a few Model D Avometers per annum, not to mention other types of Avometer. As you will see from previous post, other manufacturers were also supplying meters tot the Type D specification, although these were entirely different in construction.

The next newest I have is D7866-143. If the serial numbers are consistent, that would suggest that over 6000 were produced in 1941 and 1942.

The wooden battery compartment was used on all Avometers of the early 1930s but by 1939/40 had been replaced by the Bakelite moulding.

You will notice that your Model D has a Model 7 front panel. My 1941 'D's have Model 40 front panels. Both have aluminium cases which were standard on Avometers up to this time but during the Second World War Bakelite cases came into use, probably due to aluminium being needed more urgently for aircraft production.

I would be interested to know what form the highest voltage range multiplier resistance is. Some 'D's had a glass cylinder wound with very fine (and very delicate) wire, around 52 swg. Other used a group of high stability resistors mounted on a SRBP rectangle.

The spurious front panels and the reversion to wooden battery boxes would suggest that ACWEECO were using whatever materials they could get in order to meet ministry orders, despite material shortages. In 1941, who knew what the future held?

Beware the Avometer bug - once you get past 20 the addiction is serious!!

PMM.