AVO 20 range Model 3

The AVOmeter model 3 introduced in 1932 was a development of the original AVO DC only multimeter which started life back in 1923. 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 Model 3 112-685  Given to me by Patrick Allfrey

This is an updated version of the 13 range instrument with the added AC capability using transformer and copper oxide meter rectifier rectifier to provide 7 additional ranges. The mechanical design is the forerunner of the later model 7 and model 40 series of meters.

DC 0.12, 1.2, 12, 120, 1200 volts - sensitivity 6 mA
DC 12, 120 mA, 1.2, 12 A
AC 1.2, 12, 120, 1200 volts
AC 120 mA, 1.2, 12 A
R   1k, 10k, 100k 1M ohm (12.5, 125 ohm center)


This is what it was like when I received it, note the bent meter needle and non original terminals and V3 painted on the rear of the scale plate

As received the instrument was very dirty and was missing the instruction plate,battery cover and all the 6BA countersunk securing screws. The front panel was washed with floor cleaner and a tooth brush and the knob engravings filled with off white emulsion paint which was rubbed off whilst still slightly damp. A light coating of oil brought back a shine and more cared for look.

When I removed the meter from the case to straighten the pointer I found that the movement was not securely fastened to the front panel because the threads for the two securing brass  screws had been stripped. After straightening the pointer I found two slightly larger metric ones which although steel now do the job. The the wooden battery housing was removed and the black crackled finished aluminium case was coated with black enamel from a spray can which hid the unsightly bare patches. The terminals had been replaced with two dissimilar ones one with a 6BA thread and the other with a 4BA one. As originally manufactured the they would both have been male 6BA ones. As I had some 4BA female ones I drilled and tapped the negative terminal boss to take a threaded insert.

Surprisingly the meter movement is fine and is now almost straight. Three soldered joints on the shorter resistance board were remade and it then worked fine on the current ranges and on the lowest two voltage ones but one of the resistors for the 12V range had four very fine stray wires which when sorted out and connected correctly together gave a resistance just short of 1000 ohms rather than 1050 expected which then got the meter working on all DC ranges but the AC ranges were reading very low. I re-soldered a couple more joints, replaced a burnt out 96 ohm resistor, dismantled and cleaned the contacts on one of the leaf switches which was not shorting out a 700 ohm resistor which should only be in circuit on the DC ranges. When this was corrected I found that on all AC ranges other than the two highest current ranges there was no resistance between the meter terminals and after a lot of head scratching discovered that one section of the current transformer primary was open circuit. With this meter it is important to return the two switches to the neutral positions to avoid discharging the 1.5 Volt cell for the resistance ranges, later models have additional contacts which avoid this problem.

batteries This model used a rectangular 1.5 volt cell with a brass terminal and a flying lead for the negative connection. Surprisingly AVO retained the casing design long after the rectangular batteries were unavailable.

How old is it? Early AVO meters such as this made during the 1930's can be dated from the first (3 or 4 digits) of the serial number under the right hand end of the scale. For example this one was made in November 1932.

What is it worth? Not much use today but fairly rare, expect to pay £40. The model 8 being more sensitive is a better investment, you should be able to buy a model 8 MkII for £10- 20 which you will find equally as good as the later models which often sell for much more.

For further information have a look at my pages on the AVO range of multimeters HERE