Avometers - dates and marks

[Sinewave]

Hello,

With the Avo mark 1, 2 and 3, I've noticed every so often I'll find a mark 3, with mark 3 written on the rear plate. I'll also find amark 2 with mark 2 written on the plate. But occasionally I'll find a mark 2 or 3 with simply Avo 8 and no 'mark' denotion on the back plate.

Would this indicate that the rear plate is not original and possibly from a mark 1?

[Thyristor]

I guess you are referring to the AVO 8 ?

I've noticed lots of stuff like that, too.

I think its because a meter has been returned for repair, and what I call - 'Dovered'

From what I've seen, the MKI and MKII models, originally had simply 'AVO 8' on the rear plate, with either the VBR, or Douglas Street address.

The MKIIIs are usually denoted as such on the rear, and either Dover (late), or VBR (early)

Very late Dover MKIIIs (1969) don't have '8 MK III' on the front, just a blank, I think this is because they are very late production, using front plates intended for MKIVs.

Anyway, back to the plot.
If a DS, or VBR meter was sent into repair at Dover, I have reason to believe it was retro-fitted with a Dover instruction plate, MK as appropriate.

For instance, my 1963 VBR 8 MKII, has a Dover 8 MKII instruction plate. it also has a MKIII movement; with 1/7/69 pencilled on the rear, presumably by the engineer who repaired it on that date.

Disclaimer: I'm no expert, these are just my observations.

[The Philpott]

As the AVO 8 benefits from a 'cassette' style instruction plate that can be removed/replaced by undoing the top 3 screws, there has, no doubt, been a lot of swapping going on over the years, i would think some of it by the maintenance team and some by the owner/s.

(I think that) the only significant data change in the text from the early to the late plates (ie Mk1,2,3) is the change from copper oxide rectifier to germanium rectifier, so in the case of a badly scratched plate there has always been a good reason to swap for a better, if slightly inappropriate one.

Because of the level of general interchangeability there are a certain amount of odd 'marriages' about, a classic example appeared for sale recently.

The instruction plates certainly do get obliterated if the meter is treated roughly.

These days i only clean them with WD40, as modern petrol -being ethanol doped- does start to dissolve the paint if you rub too much.

[Sinewave]

Thanks all. Yes it's the AVO 8 models I've been noticing this on.

I should add my collection to the survey.

[pmmunro]

At least one cause of this problem has been casual repair in owners' workshops where the technicians involved didn't much care which back-case went with which meter or felt they were justified in canebalising meters they couldn't repair for replacements for broken cases. I certainly met this problem amongst a batch of meters which had been "got at" by someone who clearly had little ability in repairing them and probably had several lying open on or under a bench at the same time with little prospect of many being correctly repaired.

It's frustrating to us now because it's of interest to us to be able to make a historically accurate assessment of their development. It has to be admitted however, that Bakelite cases were easily broken and many were replaced. Replacements were supplied without instruction plates and good practice would have been to transfer the original instruction plate to the new case.

For the record, the Model 8 was introduced in May 1951 and then had no mark number; it's just convenient to now refer to them as Mark I. The original Mark II, 1956, had a more compact magnet assembly and the then new red and black terminals which would accept 4mm plugs. Later Mark IIs had the 500V (DC) range which had previously been missing.

In 1957, the company name changed from ACWEECO to AVO Limited and it became part of the Metal Industries Group in 1959.

The Mk III followed in June 1964 with fuse protection for the resistance ranges, another change of magnet type and many of the multiplier coils replaced by component resistors. Parallel production in London and Dover had begun a few months earlier. By June 1966, advertisements were using the Dover address exclusively.

Around 1970, the Model 8 underwent an extensive internal redesign which I have been told was a first attempt to meet a decree from Jules Thorn to reduce production cost or end manufacture.

A complete redesign to meet both commercial customers and NATO specification, superseded all versions of Models 8 & 9, including panclimatic types, resulted in the Mark V in December 1972. This lasted until c. 1980 when a further internal redesign transformed the Model 8 into the Mark 6, which lost its 3000V ranges because of the cost of high-voltage multiplier resistors and safety considerations. The Mark 7 (1993 - 2008) was slightly modified and had more comprehensive fusing.

PMM