AVO Model 7 Mk2

serial number 53854-A-255

The AVOmeter model 7 was a development of the original AVO multimeter 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 meter shown here.  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 7 Mk2

AVO Model 7 Mk2 s/n 53854-A-255

This model 7 which has a 1 mA movement is housed in a black crackle finish aluminium case. It is an early model and perhaps one of the last before the terminals were changed  to the more familiar red and black ones. When I received it in early 2015 it was very dirty, had several screws missing, the rubber feet had completely been worn away/lost and the threaded piece to accept one of the battery cover screws was absent as was the cover. The cut-out had clearly caused problems to a previous owner and would only very reluctantly stay closed. Apart from all this the insides looked to be in great condition. After some fiddling and bending of the cutout spring I was able to get it to operate correctly and to trip with a meter overload. The Q potentiometer needed a little cleaning and lubrication before it would rotate freely.

After the meter was tested on all ranges and found to be OK I removed the battery compartment to gain access to the rivets which held the rubber feet in place. Fortunately I had three black ones of more ore less the correct size which I was able to fix in place with self tapping screws. A small piece of brass tapped with two 6BA holes was made and screwed and super glued in place to accommodate the second battery cover retaining screw. Using a cover from one of my other meters as a template I made a new cover from aluminium and painted it black to match the case.

AVO 7 Mk2


avo

Batteries The model 7 used a rectangular 1.5 volt cell with a brass terminal and a flying lead for the negative connection. For the highest resistance range two 4.5 volt cycle lamp batteries with brass spring connectors were used. These can still be purchased but will be a very tight fit in the battery box. You may find that you need to remove the labels and/or file away some of the casing to get them to fit!

As a replacement for the latter you can wire in a lead with a push fit connector to suit the more commonly available and cheaper 9 volt PP3 battery. There is a small hole in the battery compartment between the contacts for the 4.5 volt batteries through which the wires can be threaded. The 1.5 volt battery can be replaced with a single cell size D to which soldered connections are made.

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. 53854-A-255 would mean that this one was made in February 1955.

What is it worth? Mine cost me 2.00, so not much! The model 8 being more sensitive is probably a better investment, you should be able to buy a model 8 MkII for 10- 20.

What is the difference between this model and a MK1 ? As far as I know the Mk2 is electrically identical apart from the PF sockets and resistors, though you will note that the markings on the front panel of the MK2 are engraved and filled with white.

7mk2diagram

The original AVO meters date from 1923 but the model 7 was the first with with auto cut out and decimal scale and has the basic design on which the model 40 and model 8 was based:

Model 7-1936
Model 40-1940
Model 7Mk2-1948
Model 8-1948
8 Mk II-1956
8 Mk III-1965
8 Mk IV-1970
8 Mk V-1972 [new design]
8 Mk V1-1984
8 Mk VII-1992-2008

Weight 2350 grams.

For further information have a look at my pages on the model 8 AVO and Test Meter No1 here and here