serial number 01311 73A/1/79
This is an updated more versatile and better protected version of the earlier model 71 and model 72 meters.The basic construction and circuitry is much the same but the meter movement is protected by a number of small neon tubes and a 3.15A fuse which has a neon and resistor connected across it to indicate failure. The versatile moulding for the meter and batteries has holes for additional sockets so that it can be used for the other meters in the range. This particular insrument was made for Thorn Television Rentals (TTR) part of the Thorn Group of companies as was AVO at the time. When Thorn bought Radio Rentals in 1968, the two businesses were merged to form Thorn Television Rentals.
You will find more information here about these instruments and other similar models. AVO produced a range of the smaller multimeters such as this which are described here
Blue resistor modules M1 M2 M3 (from left to right) note the incorrect replacement diodes adjacent to the fuse
Underside of printed circuit Blue resistor modules M4 (top) and M5
I was given this meter which formed part
a collection of many instruments collected by Bob Evans by his
daughter Alice Kirby. As can be seen the negative battery connector had
corroded and the diodes had been replaced. I found that the instrument
worked correctly on the DC volts and current ranged but on AC the
readings were about 5/6 of what they should have been.
I also found that the 3Amp AC shunt was open circuit and that there was
a dry joint on one of the connections to resistor module M1. The open
circuit shunt was replaced with a short piece of resistance wire and I
checked through all the switch positions and sections of the resistance
modules, all seemed OK. Only when I removed one of the diodes did I
discover that it was labelled IN4040, a silicon device. When I replaced
them both with a couple of 0A81s that were the closest to the specified
0A95s the readings on the AC ranges proved to be satisfactory.
This what the meter looked like when I
received it, I discarded the case and the glass stuck to the front and
repaired the broken Bakelite with two part epoxy mixed with black
filler before cutting and fitting a new glass inside the meter.
AVO produced the "70 series" range of analogue pocket meters with 2000 Ohms/Volt sensitivity and 3 inch scale lengths at the beginning of the 1970's. The following is from a 1972/3 trade catalogue.
For further information have a look at my pages on similar AVOmeters here