AVO Test Set Multi Range No 1

The AVO Test Set multi range No1  was a military version of one of the most ubiquitous multimeters in the AVO range which started life back in 1923, and although it was initially a DC-only instrument many of its features remained almost unaltered right through to the latest analogue models which ceased production in 2008.  In particular the mirror scale, the "smiley" meter scale, the two switches and two terminals. AVO multimeters are renowned for their reliability and robustness, the early incorporation of a mechanically operated cut out linked to the meter movement and the two switch range selection system have endeared these meters to generations of electrical and electronic engineers. By 1965, the company had already created over one million AVOmeters. 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. Though known for their Avometer general purpose multimeters, they made a wide range of test gear including valve testers, oscillators and light meters. 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.

AVO Test Set Multi Range No 1 High Sensitivity (NATO part number 6625-99-105-7050) s/n 12195-8-71 with leads clips and probes working

This, an ex-Royal Air Force instrument last calibrated in 1989 and unopened since cost me 2.00. In appearance it looks very similar to the Avo Model 8 though on closer inspection we see that the scale markings are clearer and less cluttered, the dB range has been omitted and the ranges are in multiples of 1 and 3 rather than 1 and 2.5 on the model 8. The Low Resistance and Insulation measurement ranges have been omitted. Apart from this Test Set No 1 has a steel protective casing, is fitted with an internal fuse and the internal components are potted in epoxy resin. Electrically the circuit which also has a 37.5 uA movement is almost identical to that of the model 8 (apart from the values of the resistors). I have been advised by a visitor to this page who owns both the AVO 8 and the AVO 9 that this instrument is a military version of the AVO 9 MK 2

Battery Compartment You can see that one fuse is a spare and unconnected. The negative contact of mine is a bit green and you will see that I have split open the case of the original battery to refill with new button cells.

I was surprised to see that the Bakelite case which had been hidden by the steel outer screening was a dark brown colour rather than black as I had expected. You will note that the current transformer and some of the wire wound resistors have been encapsulated in epoxy resin and other components have been varnished. Apart from this the internals are much the same as the Model 8Mk2.

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How old is it? Most 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 No. 12195-8-71 would mean that this one was made in August 1971.

Weight Of the four Avo multimeters featured on this site this is the heaviest by far at 3795 grams.

circuit diagram (jpg file) click here for diagram

For further information have a look at my pages on the model 8 AVO and Model 7 here and here