AVO Multimeter survey

[Paul Adams]

Another serial Number: E. 7987.1943 Air Ministry Avo Minor.

Paul.

[hkruis1]

I have two AVO meters model 8 with the serial numbers:

69043-C158

117036-C161

Best regards,

Henk

[mark_in_manc]

Hi Folks
I have 4 AvoMeters sitting here, all working:

8X MK11 143223-C-762 . This is in a metal case painted in bronze hammerite, has an earthing connection in the centre-top above the scale, just above what looks like a NATO part number (6625 99 943 1532), and looks rather like what I've also seen described as a multi-range test set number 1. Voltage ranges are 2.5-10-25 etc, up to 500-1000v. Having made a 15v battery from a stack of button cells, it's odd that ohms/100 range calibration fluctuates due to contact resistance on 1.5v cell - the two batteries seem to be in series.

40 mk2 3618-B-168-M Rescued from the bin at work - there were more - I left them there at the time (for shame...). Hammered blue metal case.

And 2x 8 Mk7 1128 M930860 1188 and 1012 M521800 0790

cheers

Mark

[Phil G4SPZ]

Just acquired my first 'modern' Avometer kindly donated by a friend, a Model 8 Mk V, serial number 0548661LE 1/85 (I presume this means January 1985) in a black NATO case, stock code 6625-99-650-2822.

All ranges work correctly, and it still had the original 1.5 volt D cell dated 1985 which was rusty but luckily hadn't leaked. The 15 volt BLR121 is still OK.

[Phil G4SPZ]

...and still they keep arriving! A couple more faulty Model 7 Mk 1s join the growing 'to-do' pile:

11449-A-949 with the pointer sticking, and all AC ranges reading low, but a good panel and case, and for £4 what can one expect?

7544-16272 with an Air Ministry logo on the left hand side of the scale plate, a horseshoe magnet and a coiled bimetal temperature compensator; this device doesn't appear on the circuit diagram, does anyone know how it works and the values of the wire resistance elements? This meter has been cannibalised for spares by a previous and not very careful owner. I think I have enough parts from a donor Mk II to rebuild it.

[pmmunro]

Phil,

The bimettalic spiral was a patented feature of the earliest Model 7 meters - up to the change to block magnets around 1947.

There should be a slide wire which the bimetallic spiral slider runs along. It is shown on the circuit digram for these early models. I have a copy which I will scan and send to you but I don't remember the size of paper - it may need copying and reducing. I will try to remember to look it out tonight.

PMM.

[Phil G4SPZ]

Peter,

Thanks very much, I would indeed appreciate a circuit. I think this meter dates from 1944 and the spare parts I have date from the 1950s, so I will need to check whether there were any component value changes in the interim. Hopefully the P and Q pot windings from a Model 7 Mk II will replace those missing from this earlier Mk 1.

Thanks again,

[Phil G4SPZ]

The postman just brought another birthday present.

Model 40, s/n 17665-B-247

[mark_in_manc]

And another (all these pulled from obscurity, belonging to my place of work and lurking in a semi-forgotten cupboard...)

Test set multirange No.1 high sensitivity 7130 370 (so March 1970?)

Looks like the 8 Mk2 in my last post - metal case, hammered bronze, dark brown Bakelite.

Oh - and ignore my battery comment upthread - resulted from a silly mistake on meaning of ./. or x 100 (scale reading, not actual value of resistance) by someone not used to using them.

Now I'm off to find a thread on getting at the zero ohm pot - since this one is really hit or miss - before I say anything else silly.

[Paul Adams]

Hello all,

Postman’s been here too, with this rather strange unidentified AVO, no markings on it to say what type it is, looks very early and I cannot work out the date, would this be 1933 ?

No reset either, instead a fuse.

And the only serial on it is: 15-3398.

Paul.

BTW, Many happy returns Phil.

[Phil G4SPZ]

Paul, I've got one just like that. It is a 36-range Universal Avometer ("Model 5") and, based on the serial number formats quoted earlier in this thread, the first two or three digits represent the month and year of the 1930-1939 decade, hence yours dates from January 1935.

Mine is s/n 27-6238 so dates from February 1937.

[Paul Adams]

Just acquired another Model D, AVO and I wish that I hadn’t as it looks like it didn’t survive the ravages of the Post Office, as this one came in a kit of parts.

The movement is jammed solid, reason being became apparent when I opened up the meter, and horror of horrors as the movement has obviously had some major shock in transit as the movement has come out of its pivots, and if that wasn’t enough the battery compartment is in two pieces as there was bits of bakelite inside the case.

Serial Number is: D.6806-1042 1942

And finally has this been ’got at’ before as there seems to be a 2ba brass screw fitted where the battery spikes are fitted to the front of the meter. (last photo)

Paul.

[pmmunro]

Paul,

It looks very much a case of swings and roundabouts with your two most recent acquisitions; the 36-range Universal looks one of the best I've seen but the 'D' is quite a wreck.

From what I can see from your photographs, I wouldn't put the blame entirely on the Post Office. It looks as though the "getter at" had already done a fair bit of the damage and the rust on the magnet and green spots around the moving coil suggest it has been left in a very damp place. The displacement of the coil may not be as serious as it first appears and it may be possible to ease it back into the sprung jewels without any further harm. I wouldn't hold out too much hope however that the coil is not open circuit or the pivots damaged. It is also possible that one of the pivot plates has been detached from the coil face.

Thanks to you and everyone else for keeping the serial numbers coming.

There have been quite a few from the 1930s and 40s in recent posts which suggests there are still a lot of really old Avometers around. It may be that, until recently, people were much less likely to dump instruments in skips and bins than now.

PMM

[Phil G4SPZ]

Good point Peter about the sprung jewels - I've just suggested to Paul (via PM) that he might need to release the front bearing plate screws, but you're right in that the spring loading on the jewels is relatively light, and would have been the reason that the spindle was able to pop out in the first place.

[noble kiwi]

Just to hand an Avometer Model 8, no suffix so presume it is a MK 1, but would like confirmation in this regard. Plain screw terminals without sockets. Serial No. 34360 - C - 555.

Also a Type HRM, serial No. 2644 - 151.

I am particularly pleased to secure the HRM, as the one other I have has an open circuit movement, whereas this one checks out at approximately 2.5K ohms including the swamp. It appears basically fairly sound but requires general restoration. I will search for specific data including a circuit diagram, but if anyone can assist, or point me in the right direction, this will be very much appreciated.

Kiwi Trevor.

[pmmunro]

Trevor,

Your Model 8 will be an original at that date, about a year before the Mark II was introduced.

I have some information on the HRM which should help, but can you let me know more about it as there are two main versions? The principal differences are in the resistance ranges.

Unfortunately a large proportion of the surviving HRMs seem to have an open circuit moving coil. I have speculated that this is why the Model 7 style inertia cut-out was not used on the Model 8 as extra stress is put on the very fine winding, but I don't know whether this is valid.

I have managed to fit a Model 8 moving coil to an HRM, including the inertia cut-out but it is not entirely satisfactory. I had to reverse the coil former to fit it all together which means connecting it the wrong way round.

I also have an HRM which was fitted with a Model 8 Mark III movement, complete with thermistor, but retaining the original scaleplate. It looks like a factory repair which would have been done around the mid to late 1960s. I assume that they matched the scale shape of the new movement to the original scaleplate. It was the first HRM I found so it took me a little while to convince myself that thermistors were not available in 1948.

At one time London Instruments, a servicing subsiduary of AVO, rewound moving coils which is obviously still possible, if the wire can be found, but would need a delicate technique, possibly involving making suitable jigs.

Your HRM is one of the last if I am correct in thinking that production was discontinued when the Model 8 was introduced in May 1951.

PMM

[noble kiwi]

Kia ora PMM,

Special thanks for the information you have sent me via email. I trust that the constant enquiries you receive add interest to your Avo investigations, rather than prove irksome.

Kind regards, Trevor.

[Paul Adams]

Another acquisition to the collection, an AVO 8 but with a cracked mirror. (request gone into the wanted section)
Cannot quite make out the serial, as it looks like it’s partially rubbed off:
51982-C-105? Could be a zero or an eight.

Paul.

[Phil G4SPZ]

My latest is another Model 7 Mk II, serial number 66907-A-956 which works perfectly, not bad for 99p.

[Brandon_R]

Hello, My AVO 8 has No. 45309.C.356

I received this mint condition AVO meter from an Elmer/retired engineer in my Amateur Radio Club. It is still in a perfect leather case as well. AVO meters here in Sweden were imported by Radio Bolaget, the back plate is in Swedish. Although probably purely out of vanity I will send this to an English company that can still "officially" calibrate with seal and paperwork. I've contacted one in the Mother Land and AM saving up for her journey. Overkill, but then again, If I owned a 57 Corvette I'd probably wax and polish it every other day..

I wish their was an historical/technical book covering the AVO meter, UK engineering at its best, and played such an important role for many generations. As a yank, I wish the company was still in hands that actually appreciated such a historically significant instrument.