AVO Model 7 s/n 6412-A-748 with leads clips and probes
The model 7 which has a 1 mA movement is housed in an aluminium case. This model is renowned for its reliability and robustness, the 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 radio and television engineers. Indeed many service manuals for radios include voltage values based on such a meter.
This instrument had several faults when I received it. The meter pointer had at some time been broken and a piece of pointer from another meter had ben fixed to the stub of the original one.The glass also appears to have been replaced. A date for December 1967 is pencilled on the back of the meter scale. The result of this repair was that the pointer failed to zero except when the meter was horizontal. By carefully removing the meter movement and experimentally adding weight (a piece of copper wire) it was possible to balance the movement.
The movement now worked and the meter appeared to give correct readings on the AC and DC voltage scales. I noticed that a wire was missing from the capacitor and that the bakelite strip resistor bank was unsupported at one end and that one of the wire wound resistors had got hot at some time in the past.
Using the circuit diagram I tested through the circuit for the capacitance range and could find nothing wrong, the reason for the missing wire is a mystery. As the 17.7 ohm resistor in the lowest ohms range measured 11 ohms I cut this out and replaced it by a 33 and a 39 wired in parallel.
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.
As a replacement for the latter I have wired in a lead with a push fit connector to suit the more commonly available and cheaper 9 volt PP3 battery.The 1.5 volt battery was replaced with a single cell size D to which I soldered connections.
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. 6412-A-748 would mean that this one was made in July 1948.
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 betweet this model and a MK2? As far as I know the Mk2 is elecrically identical though you will note that the markings on the front panel of the latter are engraved and filled with white.
The origial 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 Of all the AVO multimeters featured on this site this is the lightest at 2350 grams.
For further information have a look at my pages on the model 8 AVO and Test Meter No1 here and here