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 Model 7 Mk2 s/n 53854-A-255
This model 7 which has a 1 mA movement is housed in a black crackle
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.
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
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
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.
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:
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