Avo model D Meter schematic

[wave solder]

Hello,

Having had a good look around the internet and not finding any info, does anyone have the schematic diagram (with values) for the Air Ministry AVO Model D ?

I have one with quite a few burnt shunts and some of the ranges have suffered and knowing what the correct values are would save a bit of head scratching.

[Brian R Pateman]

The Model D is a variant of the Model 40.
No diagram, but there is some information here:-

https://www.vintage-radio.net/forum/...ad.php?t=12012

As I have just archived the thread this afternoon!

Regards,

[hilitevr]

Hi, See thumbnail.

Cheers
Trevor

[pmmunro]

The following are some notes which I wrote some time ago as part of my research on Avometers, including the type 'D'.

Please note that the information is based on examination of actual instruments and the point about the application of this meter to motor vehicle and aircraft circuits is SPECULATIVE. I do not yet have evidence to confirm this. A search on the internet will find an Australian site where the writer implies that this information is known to be correct. Since he got the information from me, and ignored the caveat about its being speculative, I am not too pleased about his using it without acknowledgment or qualification.

Avometer Model D - Interim Information
(Subject to amendment after further research)


The Avometer Model D (c.1938) appears to have been a special design to Air Ministry specification, originally 10A/10610 and later 10S/10610. The earlier version was a modification of the 36-range Universal Avometer Design and the later instrument was adapted from the Model 40.

Meters to the same specification were also made by Salford Electrical Instruments Ltd and GEC, whose Type D was labeled “Selectest IV”. The survival rate of the SEI and GEC types would suggest that, like the Avometer, they were made in large numbers.


Original Type D

The original Type D was protected by a fuse, similar to other contemporary Avometers but with the letter 'D' moulded into the cap. This would suggest that the Model D fuse had a different resistance to other meters. Ranges and features were otherwise similar to the later type which replaced it, probably at the start of 1940.

Later Type D

The later Type D was produced in the war years and beyond, possibly from 1940 to 1947. From known serial numbers, between 4000 and 5000 were produced in 1944. Significant numbers were sold on the surplus market up to the early 1960s.

The construction and circuit are generally very similar to the contemporary Model 40 but with different component values to suit the different scaling. The D is possibly unique in five features;

The scales and ranges are based on multiples of 75 and 150 - all earlier Avometers used scales based on 12 and 120 except the Model 7 introduced in 1936, with decimal scales.

The movement has a basic sensitivity of 2.5 mA fsd

There is a dual sensitivity switch but it is in the form of a two-position rotary switch rather than the divide-by-2 push-button fitted on all other meters which have this feature.

There are only two resistance ranges, requiring only a single dry-cell, a potentiometer 'P' and rheostat 'R'; no 'Q' adjuster is provided. (The Admiralty Pattern types 47A and 48A also use only a single dry cell)

The highest direct current range, 15/30A, is much greater than normal - 10 A on the Model 7 or 12A on the Model 40

The scale values, and the client, suggest that this model was intended for work on the electrical systems of motor vehicles and aircraft. The 15/30V ranges on the DC switch would suit the nominal 12 or 24 V systems on motor vehicle and the maximum direct current of 30A would cover most vehicle generators of the period. Used for these purposes a divide-by-2 button would have been inconvenient. The AC ranges would cover some aircraft supplies and mains supplies in buildings. Radio work would be more specialised and for this the Model 7 and the Universal Avominor would be more suitable. The D would also have been suitable for Army vehicle work (RAOC or REME?) but there does not appear to have been any evidence of War Office examples.

Most Type ‘D’ meters have a crown with the letters “A” and “M” on either side at the top of the scale plate. Others have a broad arrow (feon) in place of the crown. This may be a possible indication of the War Office branch purchasing the meters but all have the same type of instruction plate which states that the meter conforms to an Air Ministry specification.

[End of 'D' Notes]

The attachments are drawings and diagrams which I have made from the original ACWEECO drawing and actual instruments. I claim copyright to these diagrams and they are intended for private use only.

I hope the quality will be adequate for study or repairs. The only means I had to hand to get them into a suitable file format was through scanning.

The diagrams apply only to the ACWEECO version of the meter; the Salford and GEC circuits will be entirely different, only the specification being the same.

I would be interested to have any additional information about the the 'D' Avometer from verifiable sources.

I recently made up a reproduction 1.5V cell for this meter. This too is speculative, as no information was available to me on the correct No. 6 cell, 5S/2000. There are some pictures posted on a thread using the cell type as the title. If even this limited degree of authenticity is unnecessary, other 1.5V cells can be easily adapted for the resistance ranges.

Peter M. Munro

[wave solder]

Thank you all for your input, that is a great write up Pmmunro, and the information provided has answered all of my questions. thank you for that.

[wave solder]

I have just about got my Avo Model D working as it should do.
Quite a few of the wound resistors were quite a way out in value compared to the information gleaned from Pmmunro's posting.
I have also modified an existing diagram to show a full schematic of what I found. The only range that I have not got working satisfactorily is the high ohms range although the low ohms range works perfectly. Also, the transformer for the AC range in my meter is different to that shown in pmmunro's posting. (see schematic).