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Old 16th Dec 2004, 12:12 am   #28
pmmunro
Octode
 
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default Re: AVO Multimeter survey

Brian,

From the serial numbers, your Model 8s were made in September 1961 and April 1958. That makes both Mark IIs.

The main difference between these and the Mark I is that the movement magnets are Alnico in the Mark I and Alcomax in the Mark II. The Alcomax magnet is crackle red painted and is shorter than the Alnico magnet. Externally, the Mark II has red and black socket terminals rather than the knurled black terminals of earlier meters.

You will probably find that the 1958 meter has an unused gap on the selector switch between the 250V and 1000V (DC) ranges. On the later meter there will be a 500V range.

Avometer Mark and Model mumbers are neither entirely logical or consistent. This could perhaps (this is speculative) be because, in the 1950s and 60s the company was run with some degre of autonomy, I have heard it described as autocracy, by Hugh S. MacAdie, son of the original patentee, Donald MacAdie.

For example, there never was, as far as I know, a Mark I or Mark 3 Model 9. The first Model 9 was produced in parallel with the later Model 8 Mark II and was called the Model 9 Mark II. The Model 9, Mark IV was very similar to the Model 8 Mark IV.

Avometer 'Marks' on came into use in the late 1950s; model numbers were not used until 1936, and then somewhat uncertainly. The early Model 7 (from August 1936) was often advertised as the " 46 Range Universal Avometer " with Model 7 almost as an afterthought.

When the Model 7 got socket terminals it became the Mark II. Otherwise there was little change. Earlier however, a more significant redesign which replaced the original movement which had a horseshoe, nickel-iron magnet with the more modern alnico block type (c1947) went unmarked in catalaogues or advertisements.

The Model 7 Mark II slowly evolved internally to use fewer wound wire resistors and more component resistors throughout its production up to the 1980s. None of the changes were thought sufficient to call for a change of mark. The huge block paper 1 microfarad capacitor which lived alongside the scaleplate of the early Model 7 became an, almost dissapointingly small, film dialectric capacitor in the final versions.

The addition of a fuse, a thermistor and a different grade of alcomax magnet were enough to distinguish the Model 8 Mark III from the Mark II (1964).

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