Remembering the Avo 7

The remarkable Avo Mark 7 moving coil multimeter remained in production from 1936 to 1986. No self-respecting engineer was without one.

It doesn’t seem that many years ago when the average radio engineer or technician had a very limited arsenal at his or her disposal for identifying and correcting problems.

High-class workshops might have had an oscilloscope running at 10 MHz with a single trace and costing a fortune. There was that mysterious instrument known as a valve voltmeter that largely presented a high impedance load to the circuit under investigation. And there were signal generators and watt meters, and more specialised instruments such as the grid dip meter, field strength meter and the SWR meter.

But apart from the ubiquitous small screwdriver in the top pocket, the tool most technicians relied on was the moving coil multimeter … and if it was worth its salt at all, it had to be one from the Avo range, the first of which appeared in 1923 as a seven-range instrument.

The model most well known to radio engineers was the Mark 7. Specifically designed for radio work, it was promoted as a ‘high sensitivity device for radio and electronics’, and stayed in production from 1936 to 1986.

With 1000 ohm/volt sensitivity, a current range to 10 A and a voltage range extending out to 1000, the Mark 7 also had power factor and wattage units available as add-ons to increase its versatility.

Each Avo (amps, volts, ohms) model had a very long production run, and some were quite extraordinary - the Model 8, for example, was introduced in 1951 and continued until 2008 (the year of the company’s demise).

Even the earliest models had the large, distinctive, kidney-shaped viewing area, which was to last until the end. They all had knife-edge pointers and a parallax correcting mirror.

These meters, although regarded with great affection by their thousands of users, were eventually overtaken by technology the makers did not wish to follow up. The digital multimeter (DMM) became master of the field and instruments such as the valve voltmeter faded into irrelevancy.

Having said that, there are still many people out there who prefer the comfort and familiarity of a moving coil meter and its needle that swings across a scale. Precise measurement may be a little more of a challenge, especially in the low ohms range, but it is the skill of using an instrument with a Rolls-Royce pedigree that is part of the attraction.

While today’s analog multimeters have great sensitivity (sometimes up to 100,000 ohm/V) and a wide selection of switchable ranges, many of them have relatively small display scales that make it difficult to get an accurate reading. Compare a DMM whose overall accuracy may be 0.8%, with an analog struggling at 3%, and it becomes obvious why Avo has slipped from the lexicon and the workbench.

There is little doubt that the DMM currently reigns supreme - a very high impedance that puts minimal load on the circuit under test, and its great accuracy, have ensured its universal adoption.

Oscilloscopes, too, have advanced out of sight. Once they were a desktop fixture. Today they can be carried in the pocket like a multimeter. Speeds have gone through the roof, there are colour displays, huge data storage and multiple traces. As specs have risen, prices have fallen.

So we’ve come a long way from the Avo 7. They are now collectors’ items with asking prices far above the original cost. But hopefully, the much loved and sought after instrument will not become as extinct as the Tasmanian tiger.