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Old 14th Mar 2019, 11:08 pm   #1
The Philpott
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Default Avometer Model 40 Special (7.5mA FSD)

This is a well used and very original looking Model 40 (s/n -18664-747) but that's where the normality stops- the scale plate is compressed (or expanded!?) to 150/600 from the normal 120/480. The ohms range is truncated to suit, thus does not extend to FSD.

Red paint is used to demarcate the 25% over-read allowed for on this meter, and there is also an error- '100' is painted in as '10'.
'20' has been overpainted to a high standard where paint has flaked from the scale plate.

The instruction plate is the stock article. Rotten batteries from the early 1970's were in the battery compartments, with a clever adaptor made to allow fitment of an SP2. The lid of the battery compartment has a '4' digit infilled in white which looks professionally done.

High voltage DC ranges dead but 12v range working (usual o/c winding/s probably) No temperature compensation, the scale plate is marked 20C as expected. As to whether the movement is standard but the swamp has been altered- or whether the movement is a one-off- i am not yet sure!

Very odd.

Dave
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Old 14th Mar 2019, 11:21 pm   #2
Aitor Eneko
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Interesting unit.

For what do you think this Avo was design for?
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Old 15th Mar 2019, 8:42 am   #3
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Not sure Aitor- it doesn't seem to achieve anything that cannot be done with a Model 7. Perhaps there was a shortage of certain parts post-war for one type..?

Dave
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Old 15th Mar 2019, 11:41 am   #4
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Default Re: Avometer Model 40 Special (7.5mA FSD)

The Philpott,
Is there a part number on the scale plate?

Regards
Andy Gilham
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Old 15th Mar 2019, 12:21 pm   #5
micheal
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Hi Dave,
are the ranges 1.5,15,150,600 & 1,500volt ?
sort of a cross between model D and model 40 interesting

Mike
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Old 15th Mar 2019, 6:24 pm   #6
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Andy-
I have peered thro' at the edges of the scale plate and there is no part number.
'Accuracy BS1' is written at the lower edge- should that be BSI...?

Micheal-
The range knobs are standard for a Model 40, all the way up to 120v,480v, and 1,200v.
It has no significant advantages over a Model D, although arguably a bit more flexibility on resistance, with 12, 120, and also 1,200 ohms at centre scale with the 9volt internal source.

It's all a bit unusual, instinct has me thinking it's part of a small batch for a particular customer. It has certainly seen a lot of use.

Dave
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Old 15th Mar 2019, 9:23 pm   #7
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Dave,

Congratulations on a great "find". It's good to know that such an unusual meter has found its way to someone who appreciates it.

It looks to me like a "customer special" varient on the Model 40 with provision for readily monitoring voltages likely to exceed the standard Model 40 range of 480V. This would suggest a use in connection with three-phase supplies with a nominal voltage of 240/415V or 250/440V (AC). The meter was made in 1947, at a time when mains supplies were not fully standardised so voltage up to 440V would not be unusual.

I would suggest that it was not for well-regulated mains such as we now expect from "the Grid" but even local mains generating and distribution system would not normally be up to 125% of the nominal voltage. Possible applications might have been local generating systems, including ship board, or things like transformer testing or protective relay set-up and testing.

Do you know anything of its provenance? Original owner locations such as Stafford (English Electric/GEC) or Hebburn (Reyrolle) would be good clues to possible applications.

In 1947, much of British industry was still struggling to recover from wartime conditions and I would think this applied to ACWEECO as much as any other. If this speculation is valid, it would not seem likely that there would have been much spare design or manufacturing capacity for "specials" except for large, or potentially large, or influential customers. cf. The "Heavy Duty" for the GWR whic wasn't delivered until they had become BR (Western Region). However, there may have been a Prototype Department which would have been responsible for prearing sample specials. The production methods of the time would have fairly readily accomodated the manufacture of a few sample specials.

It's quite possible that you have a unique meter, the only one of its type ever to be made.

PMM
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Old 15th Mar 2019, 9:55 pm   #8
AndyGilham
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Default Re: Avometer Model 40 Special (7.5mA FSD)

The Phiilpott
Regarding the marking of "Accuracy B.S.1", that is correct. It's the intended movement accuracy of the meter (I believe measured on a milliameter). So to either B.S.1 or B.S.S the accuracy was the best, then lesser meters were to B.S.2.
I am sure PMM can elaborate on that more than I.

Andy
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Old 15th Mar 2019, 10:45 pm   #9
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Thanks Andy and Peter. The previous owner's daughter may be able to shed some slight on it's history- I will certainly ask.

There's a pencilled date of 19-7-56 on the rear of the scale-plate, and there are brass inserts to receive the case screws- both of these clues accord with it having been back to the factory for a refurb. (Although i have not found an ACW mark to back this up)

These brass inserts for the casing screws line up very well with the positions typically used in wartime Model D meters, however there are alternative positions drilled and tapped into the bakelite in all 8 positions- so either an error was made or an alternative case/back-box was fitted at some point in time. The back-box has been neatly relieved inside at all four corners, and all the way across the bottom- indicating that the meter facia is effectively oversize. (This discrepancy reminds me of my High resistance Avometer {of similar age} which has an undersize facia by comparison with it's back-box.)

This grinding action has made the box slightly too thin for comfort at the corners. (photos to follow of course)

The high voltage shunts are actually OK (surprising since the high range is a glass tube type) and it's a couple of short trimmer shunts for 120vDC and 480vDC that have failed. I have found this previously on a couple of meters, i wonder if the trim windings are made of a different composition/grade of wire that is prone to fracture/corrosion.

The braided insulation on all the link wires is lime green in colour, not something i have seen before. Tool marks suggest the movement has been removed in the past. The batteries haven't done irreparable damage- as they're not alkaline.

Dave
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Old 16th Mar 2019, 11:12 am   #10
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Default Re: Avometer Model 40 Special (7.5mA FSD)

'S' has been in touch and advises that the owner (her Father) was born during WWII, and met her Mother whilst working for Lucas, Birmingham in the '60s. (The owner's Father, incidentally, was a metal machinist.)
Sadly the owner passed on in 2007.

There is no distinctive odour from inside/outside the meter other than 'early Avo' so no clues there. It could be that it was commissioned by Lucas, ultimately retired due to it's well worn condition, then passed to the employee.

Dave
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Old 16th Mar 2019, 11:17 am   #11
AndyGilham
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Default Re: Avometer Model 40 Special (7.5mA FSD)

If there was a part number on the plate, i would have a starting point. However, I will look through the scale plate and deflection figure drawing I have (probably about 100) on Monday. See what comes up. Hopefully some further detail will be revealed...!

Andy
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Old 16th Mar 2019, 3:41 pm   #12
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Default Re: Avometer Model 40 Special (7.5mA FSD)

The Lucas connection is certainly possible. Although a bit tight, the 15V "overvoltage" range would have worked for nominal 12V motor vehicle systems which were designed to charge at 13.6 to 14.4V.

It might be thought that the Model 'D' would have been ideal for the application, or at least a modification of it. They were widely available on the surplus market during the 1960s, which is probably why they are still quite common. However, in 1947 there may still have been some contractual inhibition to making an Air Ministry specification meter available to a commercial customer. Even if there had been, simply using the normal Rev. MC button rather than the K=1/K=2 switch would have been a significant distinction.

There is the urban legend of ministry surplus Avometers being dumped down a disused mineshaft but I've never seen or heard any firm evidence to support this. The possible benificiary would have been AWEECO who would not have wanted to lose sales to surplus units, but given their chronic inability to keep up with orders until the Dover factory was built, this seems to have been more an anticipated than a real problem.

In support of the possible Lucas connection, there is the Model 12 which was designed for servicing automotive systems when alternators first came into common use. I understand this design was instigated by Lucas and there are Lucas doides in at least some of them. The choice of these diodes may not have had any link to the Lucas involvement in the design.

PMM
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Old 16th Mar 2019, 4:34 pm   #13
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Andy- Let's hope some relevant documents survived purges of the archive, thanks.

Peter- Dynamo regulators were certainly set at a lower voltage than we see in alternators so a 15v scale would have been adequate for them at least. Lucas would be the best (only!) candidate thus far.

The wear to the case where it has been moved across surfaces (handle uppermost) suggests a lengthy stint in an environment laden with abrasive but rather fine dust. If so this factor might also be contributing to the lack of precision in the ball and socket detents of the rotary selectors.

Dave
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Old 16th Mar 2019, 10:57 pm   #14
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Default Re: Avometer Model 40 Special (7.5mA FSD)

1) Serial number
2) Rear view
3) & 4) Two views of the 12vAC winding- sitting on a piggyback board.

Correction- the date 19-7-56 is on the rear of the mirror, pencilled on the rear of the scale plate is 'No.11'

Dave
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Old 17th Mar 2019, 2:24 pm   #15
pmmunro
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Default Re: Avometer Model 40 Special (7.5mA FSD)

Dave,

From the interior photographs, there are a number of things about your meter which look unusual to me.

The first is the multiplier wound on the glass former; this looks to be longer and of a smaller diameter than in most Model 40s. From memory, it's also considerably later in date than mother Model 40s which used this type of multiplier - most seem to be from the early 1940s. The wire used in those I've seen is very thin, less than 50 swg, which could be whu they're so fragile and seldom seen in useable condition today.

Then, all the other shunts and multipliers a slab wound resistance elements. By 1947, most Avometers were using component resistors for several multipliers. Could the use of slab wound multipliers have been for better temperature stability or higher accuracy, or both?

The cut away apertures in the case would suggest that the meter had been incorporated in some control or monitoring desk or along with other instruments in a test set or jig of some kind.

PMM
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Old 17th Mar 2019, 4:55 pm   #16
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

The cutaways are due to me using my 'live testing chassis' Peter, which was homebrewed from a severely bowed Model D case! Going back in time a couple of years, it occurred to me i could make this jig rather than throwing out a 'D' case which was bowed in by 3/8" at the rear. It has come in very useful for testing and tweaking meters without damaging them- and also for posing photographs.

I do recall seeing an identical glass multiplier in a Mod 48A or 47A (Early 1940's as you say)

The V shunts are certainly accurately wound enough to challenge the accuracy and resolution of the DMM's which i am using to check them- it had not occurred to me that a Model 40 would normally have component resistors by 1947 (as i have no other contemporaries to compare it to, only a MkII of around 1970 vintage which is of course full of precision sky blue resistors)

I'll get some more images uploaded, i have just bypassed the o/c V trimmer shunts (shown on the wiring diagram as nominally 500 ohm each) with discrete 1/4w resistors and am achieving expected accuracy on DCV ranges. I am just about to check the rectifier against 240vAC and 13vAC.

Dave
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Old 17th Mar 2019, 6:53 pm   #17
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

1) Early spelling of 'shews' on instruction plate.
2) Case ground away at top corners, view of battery box.
3) Case ground away at bottom corners, and all the way along the bottom.
4) Note extensive use of nickel plated fixings and hex shaped P pillar.
5) Just about visible, one of the previous set of threaded holes for casing screws...Note that the composite tape is applied AFTER all 8 of these orifices were made. (There are 16 holes in evidence in total)

AC and DC voltage ranges within spec- despite magnetic shunt being at it's minimal position. DC current ranges show signs of life, AC current ranges- TBA.

I do like fettling but must admit it's a relief not to have immediate and obvious leaf switch clearance problems in an Avo of this age.

Dave
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Old 19th Mar 2019, 10:40 pm   #18
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

AC current ranges OK (I think i have only ever encountered one of these transformers which had a problem, and that was a Model 8 with a dry joint to the 10A winding)

If for example (on any range) the needle is energised to 2/3 FSD, it takes 6 oscillations in total to settle. If memory serves correct this is more than it should be- i would expect about 3. The movement is in very good balance, so i will see if i can detect any excess slop back and forth in the sprung jewels. (I bet there isn't any)

The screws and contact leaves on the battery box have suffered from electrolyte damage, verdigris being slightly contagious any parts that i have spares for i will likely replace especially if etch-cleaning them in lemon juice shows deep pitting.

As we are in 1947 the self-tappers holding the leaves to the battery box are cad plated steel, so that might be a brass nut and bolt job if they have swelled too much.

The rivets for the leather handle being a little flappy i have drizzled superglue around them as this seems to be able to stabilise them in the bakelite, and the excess glue can be removed mechanically. Where the bakelite surface shine has been abraded through on the case i have treated it with baby oil (previous success with mildly spalled bakelite, first coat takes a few hours to soak in.) Apart from anything else, my theory is that the case should be dry beforehand, and the oil prevents moisture ingress (& distortion) in future.

Dave
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Old Yesterday, 9:56 pm   #19
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

1) Precautionary removal of R knob, expecting to see blackened brass spindle, the usual cause of a noisy pot... revealed that this spindle is silver plated, and shiny, as is the spring leaf that presses against it from alongside. Worth remembering if you have a broken early meter- it may contain silver or nickel plated gems that can upgrade a later meter and reduce maintenance requirements.

2) Batteries from the past. The 1289's have a Halfords label - 34 pence.

3) Homemade D adaptor. A little nylon bush isolates +ue from -ue at the top where the screw is.
I reused this but downsized to a C cell using rubber coolant hose, as i wasn't happy with the isolation at the side of the adaptor.

4) This battery leaf only just hits it's prod. Admittedly this skeleton case is from a Model D, but the dimensions, i believe, are accurate to the Model 40.

5) The other three leaves hit their prods- but only after loosening their mounting screws and biasing them to get ideal alignment. Obviously this misalignment is very difficult to assess in ordinary circumstances since you can't see inside the meter!

Dave
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Old Yesterday, 10:04 pm   #20
The Philpott
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Default Re: Avometer Model 40 Special (7.5mA FSD)

1) Cleaning congealed grease and dirt from behind the selectors. With care this can be done without disturbing the spring and detent ball, as these components are aligned with the indicator arrow moulded into the facia.

2) A good opportunity to see the clearances inside an assembled Avometer of this period.

Dave
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