AVO Model 7, 8 & 9 history and development.

[Radio_Dave]

I've never understood the AVO model 9 .

I think I'm right in believing that the model 8 succeeded the model 7? Therefore, logically, the model 9 should have replaced the model 8... but, obviously, it never did.

Why was the Model 9 made and what are the difference between it and the Model 8?

Thanks
David

[newlite4]

Briefly, the model 7 was intended more for electricians and power engineers. The model 9 was developed for technicians and electronic engineers. The model 8 was a general purpose meter designed for most users. It is not a case of one succeeding the other. The model 8 is still available, the MKVIII is over £600 .
Neil

[Radio_Dave]

Thanks for the explanation, but what are the differences between the 8 and 9? They seem to look the same to me

[newlite4]

As I recall Dave, the model 9 has greater sensitivity than the model 8, as such it is more suited to electronic work.
Neil

[SPCh]

Here's the facts, full frontal:

The Model 9 was the "export"/international version of the Model 8.
The ONLY difference between the equivalent Marks of Model 8 and 9 are that the 9 uses international symbols (rather than "English" words) on the controls, and has voltage scales based on a 1-3-10 rather than a 1-2½-10 sequence.

Apart from the resistor value changes required by the 2½/3switch ,
the Model 9 Mark II is internally identical to the Model 8 Mark II, and
the Model 9 Mark IV to the Model 8 Mark IV.

It really is that unromantic.

And just to screw things up a bit further, there never was a Model 9 Mk I or a Model 9 Mk III.

[Brian R Pateman]

The model 9 (Mk IV) doesn't have a dB range whereas the Model 8 has.

This seems to be the only difference apart from the internatonal symbols as already pointed out.

I have a 9 MkIV which is my main meter day to day, reliable and highly recommended.

Regards,

[SPCh]

To be strictly correct, NEITHER the Model 8 nor the Model 9 has a dB RANGE.

The early Model 8s (up to Mark IV) have a dB SCALE for use with the AC voltage ranges, but this is of limited accuracy because it asssumes that the meter does not burden the source. In fact it does; and variably.

The meter terminal impedance at 25, 10 and 2½ volts AC is 6260, 100 and 62½ ohms respectively, which will impose horrendous error variations on a load of 600 (or even 75) ohms fed from a matched source.

[SPCh]

Correction to my previous post: the load values are 6250, 1000 and 62½ ohms (dropped a zero from the middle value).

[pmmunro]

Just a little correction beyond what SPCH has already explained characteristically well - the Model 7 was not originally a "power" engineers meter.

At the time of its introduction in 1936, the Model 7 was an improvement on the "36-range Universal Avometer" in that it drew only 2 mA for FSD against 6mA for the 36 range meter. (Both these FSD currents are halved when the divide by two button is pressed, requiring the scale indication the be divided by two, i.e. the sensitivity is doubled).

The Model 7 was thus better suited to "radio" work than the 36-range meter. The original Avo patent ran out around this time I believe and upstarts like Taylor and Simpson in the USA brought out multimeters with a basic sensitivity of 50 mA for FSD (20 000 ohms per volt). Avo's response was limited to a 20 000 ohm per volt version of the Avominor, advertised as being suitable for television work.

The 36 range Avometer was replaced during 1939 by the Model 40 which inherited the constructional techniques and the automatic cut-out from the Model 7 but retained the 0-120 scale divisions, sensitivity and range pattern of the 36-range meter. At this time Avo did not explicitly state the application of each meter.

It was only in 1947 that the High Resistance Avometer offered 20 000 ohm per volt in a full size meter. The availability of this meter on the civilian market appears to have been limited and it was superseded by the Model 8 in 1951. As rationing receded, the need for export sales had to be addressed and this was a major consideration for Avo for the rest of their existance as a British company, i.e. before most of British industry lost its nerve in all but paying excessive salaries to bean counters in suits whose only decisive action was to close things down and selling out or merging with whoever offered a quick profit before closing down its former rival.

It was to meet the export drive that the Model 9 was introduced, accompanied by others such the Models 10, 14, 15, 16, 20 and EA 113, only the last of which was sold in any quantity in the UK.

The Model 8 Mk V was specifically designed to meet NATO needs as well as the civilian market and the Model 8 Mk VI and Mk 7 are developments of the Mark V. It is probably mostly because of NATO and aerospace manuals specifying its use that the Model 8 is still on sale.

Once the digital multimeter was firmly established, AVO abandoned UK multimeter production (except for the Model 8) and sold first fairlly cheap Japanese made instruments with Avo badges and later rather good German instruments made by a division of ABB but with their trade mark dark green colour scheme changed to yellow with Avo badges.

(At that time German company directors were either too stupid to realise that quality technical goods could not be made economically in Europe or too stupid to employ bean counters to tell them and so they continued to make a profit and retain their industries. The French, Italians and Spanish were little better and even to this day still try to make multimeters and cars and trains and 'planes and power stations, although the are learning, and beginning to ship as much production as far east as they can).

PMM.