AVO Multimeter survey

[Paul Adams]

Phil, PMM,

I also think the former, although the meter is in exceptional condition, with no marks to the ‘crackle finish’ and it still has the paint seal in the screw, oh and it works too.

Very reluctant to open it up and break the seal.



Paul.

[pmmunro]

Paul,

The condition certainly does look exceptional, especially the red markings on the scale plate.

I would be very reluctant to break the seal too if it is original - it will probably have some marking if it is.

PMM

[Phil G4SPZ]

Another Universal Avominor for the survey, serial number U15/7533-845, made in August 1945 I assume.

Now, here's a funny thing - this instrument has different voltage ranges from any that I've seen before. It has 5, 25, 100, 500 and 1,500 volts, instead of the usual 5, 25, 100, 250 and 500 volt ranges. The 1,500 volt range uses two bobbin windings in series, each of 200 kilohms, pretty impressive for a wire-wound component. Both were intact, although it's little wonder they tend to go open-circuit. The highest value bobbins of this type that I've seen before are around 60k.

I'd love to know how many variants of the Universal Avominor were made. So far I know of three, including the Model E.

Phil

[pmmunro]

Phil,

You seem to have a magnetic attraction for Avometers; what's your secret?

This does seem to be a "special" variant. Like you I only knew of the Universal Minor Models 1 and 2 and the Type 'E'. (1952 catalogue page attached). Is the 1500 volt range instead of the 1000V one?

I would agree that this meter was made in August 1945, which probably means it was made to a ministry specification, very probably the Air Ministry. It is just possible it may even be more special than just having a higher voltage range - have you checked the current it draws for full scale deflection?

You will be aware that there is an Avominor HR which has a "sensitivity" of 20,000 ohms per volt, i.e. 50 microamps fsd and perhaps your meter is also a higher sensitivity than normal and intended for radar work. (See page from 1939 catalogue).

I had once assumed that the indented markings on Avometer front panels were moulded in but I've seen quite a few special versions with alternative markings to the standard type so I wonder if the marking were engraved after the moulding process.

PMM

[Phil G4SPZ]

Peter,

It's no secret, I simply think that they represent a classic design and demonstrate British engineering at its best. In addition, each instrument is a little restoration project in its own right. Then there are the subtle differences between models, marks and between different instruments of the same type that make them fascinating. As you know, I didn't set out to be a collector...

This latest Universal Avominor is a good example. It's definitely based on a Model 1 and has a 2.5mA movement, but instead of the 250 volt range it has 500 volts, and in place of the 500 volt range it has a 1,500 volt range. Otherwise it's to all intents and purposes a standard Model 1.

Speculating (as we all enjoy doing) it is feasible that this was a late wartime special edition developed for radar work, and hence the 1,500 volt range which would presumably have been just sufficient for the scope-type display tube EHT potentials of the day. This would have made it more suitable and convenient for use in the field, rather than a cumbersome combination of Model 7 plus external multiplier. It has no Air Ministry or other markings, and the scale plate is identical to the standard Model 1. The instruction plate on the back is subtly but correctly different, though, and refers to the fitted voltage ranges and also describes how to measure resistance up to 30 Megohms (rather than 10M on the standard Model 1) using external source of voltage of up to 1,500 volts - not a test for the faint-hearted!

So, we seem to have four variants now; the Model 1, Model 2, Model E and "mine" which, for the time being and in the absence of any other information, I will call the "Model P"...

[Barnmead]

I have just been given a model 40 avometer serial 417599 1146. It is in poor condition, the movement works but the scale has cracked up and peeled away from the brass backplate (look at my pictures). The movement which has a horse shoe magnet has been repaired a number of times and looks as if it could be brought back to life.

This is a longshot, but does anyone out there have a scale from a dud meter or a scan that I could print out to make the job worthwhile?

Part number on the scale is 10945-34

Thanks in advance for any help or suggestions.

[Phil G4SPZ]

Originally Posted by Phil G4SPZ

So, we seem to have four variants now; the Model 1, Model 2, Model E and "mine" which, for the time being and in the absence of any other information, I will call the "Model P"...

The pictures below show, L to R, the Model 1 front and rear and the "Model P" front and rear.

[Phil G4SPZ]

Richard/Barnmead,

PM sent regarding Model 40 scale plate - I can send you a cleaned-up hi-res scan, taken from an Admiralty Pattern 47A, but I'll need your direct e-mail address. Please reply by PM.

Phil

[Phil G4SPZ]

Another Model 7 Mk 1 has found its way to my surgery, seeking attention for a variety of ailments. It was made exactly a year before I was born - serial number 33206-A-352.

[Sci_Curios]

HI Everyone,

I have just joined the forum. I have a number of Avos but my pride and joy has to be a precision Avometer MkII serial number 724-870. Its a beautiful piece of kit dated 21-6-1971. Still with its test leads packed as though they have never been used and with its plastic dust cover. Apart from some slight crackling on the walnut veneer around its sides it appears in mint condition. Anyone happen to know if this is rare or valuable at all?

[pmmunro]

Sci-curious,

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

Thanks for the information on your Precision Avometer. Unfortunately the cracking of the finish on the wooden case is common.

By their nature these instruments are less numerous than the ordinary meters whose calibration they were intended to provide. However, to judge by the frequency with which they occur on eBay, there are quite a few around. The selling price is not great - I would be surprised if they made much more than £65 - possibly because their function is largely superseded by high precision digit bench meters and because of their size.

PMM

[Sci_Curios]

Thanks for the information PMM. Well at least I don't need to take out special insurance cover on it! Theres always a silver lining

[Sci_Curios]

Hello again PMM,

I thought you might be interested in this little unit for the survey. It's an extension unit for the Model 8 Universal Avometer for the high and low resistance ranges which takes the low down to 0-2.5 Ohm and the high up to 0-200 Megohms

Apart from one of the connecting tongues being slightly bent, the outside two terminals (one of the high and one of the low) having some of the bakelite cracked away from the base, and a chip on the bakelite switch top, it looks in good condition. It even has what looks like its original batteries in situ and surprisingly no signs of corrosion or leakage from them. It's part of a batch of AVO gear that I rescued from the scrap yard when a local university revamped their laboratories a few years ago along with a couple of Avo 8's and a couple of Avo 8 Mk II's


Pictures attached

[Phil G4SPZ]

Yet another variant in the Universal Avominor range has arrived, with 500 volt and 1,500 volt ranges and the early case style. Its serial number is U41209-641.

I've started a separate thread about these variants.

Phil

[Phil G4SPZ]

Originally Posted by Sci_Curios

...It's part of a batch of AVO gear that I rescued from the scrap yard when a local university revamped their laboratories a few years ago...

Hi Sci. I have a Model 7 Mk 1, engraved "University of Liverpool" on the battery cover. I wonder if it came from the same scrap yard?!

[noble kiwi]

I had once assumed that the indented markings on Avometer front panels were moulded in but I've seen quite a few special versions with alternative markings to the standard type so I wonder if the marking were engraved after the moulding process.

PMM

Kia ora PMM,

Your knowledge in respect of Avo is unmatched and your willingness to pass this on in sincerely appreciated.

I have noticed aspects regarding engraving have been mentioned in a new thread and this has prompted me to add a comment.

A technique is applied in respect of bakelite moulding whereby economy is achieved, when only small changes are required between several models of an item involving a single basic structure. The dies are made incorporating removable inserts which are changed as needed. Careful examination discloses where this technique has been used.

As a result another interesting factor is provided when collecting Avo's. It becomes apparent that as time has passed Avo became aware of this technique and used it extensively.

Trevor.

[Sci_Curios]

Originally Posted by Phil G4SPZ

Hi Sci. I have a Model 7 Mk 1, engraved "University of Liverpool" on the battery cover. I wonder if it came from the same scrap yard?!

HI Phil,
Yes it will be. Did I supply that one to you? I didn't know anyone else had salvaged any. A couple of mine have the same engraving.

[Phil G4SPZ]

Not sure, Sci, I bought it a couple of years ago on a well-known online auction site.

Back to Universal Avominors, I can report two more serial numbers for the survey, and at the same time pin down the date of the change from "raised" to "indented" legends on the range switch with a little more accuracy.

Serial number U68536-744 (July 1944) has raised lettering.
Serial number U85135-1145 (November 1945) has indented lettering.

So the design (and the moulds) must have changed sometime between July 1944 and November 1945.

[jamesvalves]

Hi all,
I thought it was about time I listed all my avo's in my collection

Model 40 - SN: 8356-B-1152

Model 7 - SN: 4574-A-967-M

Model 7 - SN: 71046-32283

Model 8 - SN: 12510-C-353

Model 8 - SN: 95621-C-859

Model 8 Mk3 - SN: 15462.565

Model 8 Mk3 - SN: 37867.266

Model 8X Panclimatic - SN: 115951-C-1260

Model 8X Panclimatic - SN: 72530-C-358

Model 9 Mk2 - SN: 5000-466

Test Set 1 High Sensitivity - SN: 6625-99-105-7050

Heavy Duty - SN: 3907-567

All of these are in ok condition, the Panclimatics both have issues, meter is knackered on 1 and the case is corroded on the other. A couple of the glass panels have cracked and become loose, I have cleaned and tightened them up. The things that don't seem to last are the probes...

I also have a small square avominor somewhere in the workshop, I don't recall seeing it in the recent avominor thread, maybe its another one.

[micheal]

Hi a few more AVOs for your survey

DC AVOMINOR
95617-155

DC AVOMINOR type HR
404-79 (july 1939)

UNIVERSAL AVOMETER
type HRM 280-548 model 1
type HR2 2521-B-1050 model 2

HEAVY DUTY
3095-167
4281-954 British rail type
1271-1071 British rail type

Regards Mike