AVO Multimeter survey

[The Philpott]

Thanks John, that's quite a smart looking thing. I'm not sure if the yellow legends on the front are original, or whether they should be ivory. It's basically a tweak of the Model 8 and 9 design, carrying Model 8 (english!) legends but Model 9 type (100/30) scale.

Certain components are potted in resin against humid environments. Mainly discrete components in there at this stage rather than windings of resistance wire so reliability quite good.

Richardsradios is 'the' website reference point for different versions of the avometer.

Meters that are damaged or killed are usually the victim of electrical misuse, a drop from height, or corrosion from old batteries.

The inside of the meter is quite cramped, some repairs can be awkward to carry out. A meter that has no corrosion, a healthy movement and unworn leaf switches is something that should carry on working for a very long time.

Dave

[Superscope]

Welcome to the Forum John,
and in particular, welcome to this corner of Avo inflicted Forum Members,
where the Avometer gets a regular airing.

Dave has pretty much said most of it.

You have a fairly early Model 9sx, from December 1963.
You didn't say if it is in working order or not?

The Model 9sx was the first Model 9 produced, so essentially it is the
defacto Model 9 Mk I.
It was the first to use the 0 - 30 Scale on a full sized Avometer, rather
than the 0 - 25 found on the then current Model 8.

They are not Rare Meters, although the 9sx labelled ones are less common.

They were made between 1961 and 1971, but there is evidence that they
might have still been leaving the Factory in small numbers as late as 1974.

It was later, simply known as the "Test Set Multirange No1" and the Model 9sx
monika was dropped although the Meter was largely the same device.

It was made especially for the British Military, and there hasn't been any
evidence seen so far to suggest if it was ever sold directly to the Civilian
Market when New.

The (Multimeter Type 12889)and 5QP/17447 are the earlier Military designations
before the "Nato Stock Numbers" NSN were introduced.
It later had the NSN 6625-99-105-7050
Originally, it would have come in an Olive Drabe Coloured Soft Vinyl Case.

It was replaced by the Military variant of the Model 8 Mk V in around 1972.

I still use a 9sx regularly.

Ian

[ex 2 Base]

I have one of those "Test Set Multirange No1" with a vinyl case. I fitted it with it with 5 X 3 volt cells from pound shop for the Megohm range, really nice meter and 3KV range with an earth connection. Recently I was given an AVO 7 which doesn't work at all, so I'm hoping you experts can or will to guide me to a successful conclusion. My question is, where on the 7 do I connect my "Test Set Multirange No1" set on the ohm range to check that the movement is in working order.Ted

[Superscope]

Hi Ted,

Might be a good idea to start a new Model 7 Fault finding Thread.
This is not really the right Thread for fault finding help.


Not quite sure of the question, but if you are trying to test the Model
7's Movement by using your TSMR 1 to measure the movement resistance,
there are Two issues.

1) The Leads will need to be reversed as the Negative Terminal on the
TSMR1 will be Positive in Resistance measurement mode.

2) the current supplied by the TSMR1 will be over 50mA going through
a Model 7 set on DC Current 2mA Range. Which will be risky.
It's the 2 mA (0.002) DC current Range you need to get direct access across the
Movement from the outside world.

If you do it, jab the Leads very quickly across the Model 7 Terminals.
You only need to see a blip on the Movement to assertain it is at least not OC.



Ian

[johnsingleton]

Hey Dave.

Thanks for the info mate, I find it quite fascinating really - does that make me weird
My machine works brilliantly and I use it as a demo machine for RNZAF Avionics students that I instruct.
Its a pity that it hasn't got the original case, I have one for a Mk8 - brown leather.

Will check out Richards Radios, thanks for that info too.

Stay safe,

John

[johnsingleton]

Hi Ian.

Thanks for the info mate.

My AVO works brilliantly and I use it as a demo machine for the RNZAF students I teach.
It's a shame that I only have a brown leather case for it - I remember the green ones from my early days in the RAF and even in the '90s when I worked on the T91 radar at Staxton Wold.
I'm beginning to think that the AVO's here were provided as test equipment for the BAC Strikemasters and Hawker Siddeley HS 780 Andovers that the RNZAF used to fly. A guy I work with used to calibrate them many years ago.
****** hell, I feel old now!!!

Thanks again for the info, stay safe.

John

[The Philpott]

The meter looks dark brown rather than black, John- i believe that this is a grade of bakelite supposed to be more resistant to moisture; all part of the pan-climatic specification.

Certainly good to see one in instructional use.

(PS somewhat jealous though that NZ has a Mosquito and we don't!)

D

[johnsingleton]

Originally Posted by The Philpott

The meter looks dark brown rather than black, John- i believe that this is a grade of bakelite supposed to be more resistant to moisture; all part of the pan-climatic specification.

Certainly good to see one in instructional use.

(PS somewhat jealous though that NZ has a Mosquito and we don't!)

D

Ha ha.
Hopefully I'll get to see it before it it heads off to the States

[Superscope]

Hopefully, this is of interest.

An almost Mint example of a Pre War Model 7.

7116-233

So November 1936 and actually, currently the oldest Model 7 in the Survey,
unless anybody has been holding back on their Serial Numbers!


Points of interest are the Temperature Compensator which is a slightly different
design from what I've seen before and it appears that the Battery Bay, may not
have originally had Brass Contact strips.

Difficult to tell, but the Battery Bay is virtually Mint, and there are no Marks in
the Bakelite where the Brass strips would have bolted down.
Looks like just the Brass Screws and Nuts might have been used for the 9v
contacts on their own.

Clearly, this Meter has lead a coverted life and I'm told, it has been in the same
Family since New.

Amazingly, the Case just popped off without any hassle.
Black Crinkle Paint 99.9% still intact.

Build quality is very good.

An absolute Time Capsule of a Model 7.



Ian

[pmmunro]

Congratulations on finding that Model 7 Ian. I would say it's the rarest Model 7 in the survey being made just 3 months after the introduction of the model. I would rank it quite close to a 22-range DC Avometer (Model 6) for rarity and historical importance.

The condition is absolutely wonderful and it makes this meter a very valuable historical record.

How did you mange to find it? It almost possible to believe that it found you! It's very good to know that it did, as it could easily have suffered the fate of so many "Ugly black electrical things" as the Avo-philistinic might think. (I have the late SPCh very much in mind here).

PMM

[The Philpott]

Thanks Ian, Meter No.233!

Is it an effect of the camera or are the screws around the battery compartment dome head? The black crinkle paint is the same as my 1939 meter.

When i removed the battery box from mine i found that the fixing holes in the bakelite had been neatly countersunk- presumably by someone who had not seen the finished article.

Dave

[Superscope]

Hi Peter,

It took several years of searching for to be honest, following up leads ect.
I just happened to see this one advertised, and I knew it was a candidate
for an old one, so I asked for the Serial Number.

I have a particular interest in the Model 7 Avometer.

I guess it found me, but not without effort.

Amazingly, it actually works, although I think the Q knob needs cleaning, as
it reads high intermitantly.
Hence why I opened it.

I think it has been opened at some stage in the past, but I can't see any obvious
repairs.


Ian

[Superscope]

Dave,

Blimey, forensic examination of a Photo, well spotted.
I didn't notice that.

The Screws at the back, you know the ones, the ones that are near
impossible to get out are flat topped.

The Three around the front and side and the one holding the Cover
threaded plate in place are all dome Head.

All the Screws holding the case on, are also Dome Head.
Another point of interest is that the case Screws aren't the most
perpendicular fixing Screws I have seen.


The black Crinkle paint is identical to the Model 4 and 5, and I think this
was used well into the Fourties.


Ian

[Superscope]

A few more photos, for those who are interested.



Ian

[The Philpott]

Properly unmolested- and it's been kept in a house by the looks of it..

I'm not surprised it works....and i'm also not surprised that the Q pot is glitching! if there's a precious metal insert in the end of the prod make sure it hasn't worked loose. (It's only happened to me once...but it WAS on a Model 7)

I was interested to note that the Model 7 doesn't perform particularly well if you, for example, measure a DC current of 500mA (pushed by 6v) on the 1A range. Granted you would be using a high pointer deflection which is inadvisable, but it will under-read by 7% which is somewhat worse than a Model 9, Model D and even Type E that i subjected to the same test. Reverting to the high range on each meter gave good results.

Using AC of 12v to push a similar current, all the meters concerned (apart from the Type E which failed to qualify for AC) actually preferred a high pointer deflection to a low one. They don't tell you this in the manual..

Dave

[Superscope]

I had time over this Weekend to give the 1936 Model 7 a thorough Test.

It is fully functional on Volts all the way up to 1000 Volts AC & DC!
and it's fully functional on AC & DC Current Ranges all the way up to 10 amps!

The only work I did was to clean the Q Knob Switch.

Resistance ranges were a bit of an issue, and I spent a few hours running
round in circles trying to diagnose an unstable reading.

Bearing in mind, the Historic importance of this Meter, I didn't want to attack
it with a soldering Iron unless absolutely necessary.

I'm not sure if a Schematic exists for a Model 7 of this age, but I can't help
but think that some of the resistance values are different from the later model 7's.
It was very diffiicult and not entirely sucessful in comparing this Meter with a
later Schematic.

I have settled on leaving the Meter fully functional except for the 10K Ohms
Range which has a sometimes unstable reading.
I believe this is either a dirty Range Select Switch Terminal, or the 3.6 Ohm/10K Range Switch.

As much as I want this Meter to be 100% functional, I don't want to risk invasive repairs.

It is 95% in complete spec, but it does drift out to 1.5% in a few places on DC.
The AC Ranges are all in spec surprisingly.
This really isn't bad for an 86 Year old Meter which has only had minimal cleaning.

Final points of note Summary:

1) Temperature Compensator is slightly more compact and neater in design.
No idea if it functions as well or worse than later versions.
2) Movement can be removed without first having to remove the compensator
3) Rotary Range Switches are the Model 4/5 style, not the familiar later Model 7/8/40 Style.
4) The ohms Board is perpendicular to the panel, and not placed between the Upper and lower Resistance Boards as in later versions. (See Photo).
5) The upper and Lower resistance Boards have bakelite noticably thinner than on later versions.
6) Protective Movement Rectifier is different to later Horseshoe Movement Rectifiers as found on the later 7 and Model 40's.
7) Battery Bay appears not to have Brass Terminal Strips for the 2 x 4.5 Volt Batteries.


Ian

[The Philpott]

-They did tend to make a lot of changes. I was able to use a Mk2 transformer in a Mk1, the windings were of the same values but several of the taps were in different places!

The low resistance range instability could be dulling of the brass 'R' pot shaft where it bears on it's spring? Doesn't look accessible on this meter without removing the movement unfortunately..

Dave

[factory]

Was given a couple of meters yesterday, including an AVO Model 7, scale is marked for Air Ministry and Model D in the lower right. Serial No. D. 1927-441 1941, instructions on the back is also marked A.M. Type D REF No. 10A/10610.

The cutout needs some attention, but at least the meter movement is working, aluminium case has been damaged around the edge, maybe someone gave up trying to open it (all case screws are missing), expired Duraleak™ leaktube was removed.

David

[Superscope]

Dave,

She seems to be working fine this Morning, so I'm leaning towards a dirty Rotary
Range Switch contact, which may well be self cleaning itself with use.

The accuracy appears to be within the designated +-5% .

Your right, no way to get to the R Control without taking the Movement out.




David,


Your Model D is interesting being built on a Model 7 Front Panel.
Not seen this before.
They can often be found built on Model 40 Panels of course,
athough the D and 40 are at least similar.

But how confussing was that?

Imagine:

Squadron leader shouting "Get an Avo Model 7 and test those Condensers"

Cadet " What the hecks going on?"






Ian

[The Philpott]

Certainly an interesting (and early) Model D. Shows the versatility of the facia- the orifice for a 'divide by 2' button and a K switch is the same, but the former is inserted from inside and the latter from the outside. There (should be) light detents in the facia to allow 'K' to lock into K=1 and K=2 posns.

I converted a rather knackered Model 47A to use the 'K' facility, just to see if it could be done. It does at least make it 'hands free'.

Those aluminium cases can be a pain as they're naturally tapered. They do shift eventually if you wheedle the rubber 'o' ring out and go round rotating a wide blade s/driver or blunted cold chisel.

Dave