AVO AD. PATT. Model 48A

ser no 15085 744

The Admiralty and the Air Ministry procured many thousands of AVOmeters during WW2. These were substantially the same as the commercial instruments of the time, but modified to suit their particular requirements. The  Admiralty Pattern model 48A used by the Royal Navy was a variant of the AVO Industrial Test Set which included a model 40 meter and accessories in a wooden box. Two versions of the Industrial Test set were made:

Model 1.
A test set comprising a Model 40 Avometer with the following
4,800V Multiplier, 120A Shunt, 480A Shunt and 60/240A Transformer.

Model 2.
As an alternative, a test set can be supplied comprising a Model 7 Avometer with the following:
4,000V Multiplier, 100A Shunt, 400A Shunt and 50/200A Transformer.

The Admiralty pattern instrument was very similar to the Model 1 but has:

3,600V Multiplier, 120A Shunt, 480A Shunt and 60/240A Transformer.

The serial number on the box indicates that it was made in July 1944

The meter in this box is a model 40 Mk2 manufactured in June 1970 the other items are original

The multiplier resistors on glass tubes are housed in a ventilated wooden box.

Shunts and current transformer

This was given to me by Warren Jackson. It belonged to his father, Fred Jackson who worked as a maintenance electrician at Searles refrigeration near Gosport. It was in need of a good clean and and some repairs to the box, the meter worked on all ranges but was dirty and had been modified to include a fuse and a home made resistor. The front panel was washed with detergent and a tooth bush, rinsed and washed again and allowed to dry. The engravings were filled with off white emulsion paint and the panel then given a thin coat of light machine oil. A fuse holder had been fitted to the left hand side of the case and this was wired in series with the home made resistor which measured 17 ohms. Both were removed, the latter with a selected 5% tolerance 15 ohm half watt resistor and the hole left by the fuse holder with a two part epoxy resin mixed with black powder intended for the colouring of cement. The label giving the serial number was almost unreadable, though I was able to establish that it was made in June 1970 and the date codes on the internal blue Welwyn resistors lend support to this.

I imagine that Fred Jackson may have purchased the box (maybe without the original model 48A meter) at a government surplus sale many years ago and subsequently obtained the much later AVO 40 Mk2 and removed the instruction plate which would have been screwed in the lid.

The shunts, current transformer and voltage multiplier were cleaned but have not been tested, though the resistance of the latter was found to be 5200 ohms.

The firm also produced a range of smaller multimeters, the AVO Minor and later the AVO Multiminor as well as a number of special instruments such as the Heavy Duty model. Production of the iconic model 8 meter ceased in 2008. You will find more information here about these instruments and other similar models. AVO also produced a range of smaller multimeters which are described here.

Batteries The model 48A used a rectangular 1.5 volt cell with a brass terminal and a flying lead for the negative connection this can be replaced with a single cell size D with soldered connections.

How old is it? Many AVO meters can be dated from the last 3 or 4 digits of the serial number under the right hand end of the scale. These define the month and year of manufacture. For example this one was made in July 1944.

What is it worth? One in this condition with leads, probes and case is unusual so maybe £30. The model 8 being more sensitive is probably a better investment, you should be able to buy a model 8 MkII for £20- £30 which you will find equally as good as the later models though without leads and probes.