AVO Multimeter survey

[Phil G4SPZ]

An interesting observation on the apparent lack of damping, Ted. Dave is quite right in that the Model D movement does accelerate more quickly than other models. I'd quite forgotten about this, so I had to bring one of my Model Ds briefly out of retirement and give it a bit of exercise, just to remind myself!

The Model D has a 2.5mA FSD movement and lacks the 'permashunt' of the Model 7 (a 1mA FSD moving coil, shunted to 2mA) or the Model 8 (a 37.5uA FSD moving coil, shunted to 50uA). The permashunt adds a degree of damping that is absent in the Model D, which presumably relies solely on eddy current damping within the moving coil former, plus a bit of air resistance acting on the pointer. The damping is noticeably greater on the K=2 setting because the moving coil is shunted by a resistance of 20 ohms.

[pmmunro]

Here are some pictures of the main resistor boards which may help. The resistances wound on the glass tube are just one of several arrangements, and one in which the windings are rarely intact. One of the alternatives is in the form of an SRBP board with four large high stability resistors and some small carbon resistors used to trim the exact value.

PMM

[ex 2 Base]

Many thanks to you Peter, Phil and Dave for the Model D info. I've been using Avo since early 1950's and had never seen a needle swing up so quickly as the model D. Yes Peter its those board detail which I needed. When I click on them one at a time, they come up large and clear, however when I go to print the picture produced is very small, and difficult to read. I'm not very competent using a computer I freely admit, any help/ suggestion will be much appreciated. Ted

[TimLiggins]

AVO 8 III 80197.568 just arrived today

[The Philpott]

s/no given by Tim (and taken in conjunction with others) indicates that in 1968 some 1,000 Model 8's per month were being completed.

[ex 2 Base]

Peter,pmmunro and others, got the image size sorted, daughter arrived and of course they/she new straight away.Ted

[TimLiggins]

Sadly, my newly acquired Model 8 III has a broken hairspring in the meter movement. Presumably this is not a viable repair unless I can find a replacement movement. The instrument appears to be in superb condition otherwise.

Tim

[The Philpott]

Broken is a problem (sure it's broken not just tangled..?)

An old school watchmaker would have the steady hand and skills (but it would be expensive and you would have to provide the replacement spring) so i think the movement is a goner.

If all the tiny leaf switches inside look good, and all the shunts show correct resistance, it is well worth fitting a movement from another meter. (tatty meters for sale often have a good movement and can be used as a parts donor)

As Pete has said before, beware of using the 'old' scale plate on a 'new' movement as there are different scale shapes and the number at the bottom must match. (it is often a 5 or a 6)

Much better to keep the movement and scale plate together, although to the purist this gives the meter a new serial number (Hasn't stopped me in the past!)

Above all do not be disheartened since in my experience most Model 8's on general sale will have a gremlin of some sort.

[The Philpott]

Additional- if in fact the spring has come de-soldered at one end it is viable --but fiddly-- to try and re-solder it.

I think Peter Vis' website has a few comments on this.

[Sinewave]

Originally Posted by TimLiggins

Sadly, my newly acquired Model 8 III has a broken hairspring in the meter movement. Presumably this is not a viable repair unless I can find a replacement movement. The instrument appears to be in superb condition otherwise.

Can you take a photo of this please? There may be something you can do if it's the cut out spring which has hooked onto something else.

[TimLiggins]

Thanks for the replys. On closer examination the spring isn't broken, just tangled slightly. I'm struggling a bit to try and sort it since I'm waiting for cataract surgery. Now where did I put that magnifying glass?

Tim

[TimLiggins]

I've come to the conclusion that someone else has had go at this and has irepairably distorted the spring. Tried to take photos but without success. Given the condition of the rest of the meter I shall look out for a spares donor. Is the meter movement unique to the mark 3 or interchangeable with 1 and 2s?

Tim

[Sinewave]

I'm told the meter movement scales are 'calibrated' to the device itself, they were printed to suit the characteristics of the meter.

Situations like yours is why if I'm buying something from that auction site, I'll ask if it is working or not and bid accordingly.

Similarly a lot of sellers there don't even know what it is they're selling.

[The Philpott]

I believe that an earlier Model 8 movement complete with scaleplate will suffice, the typeset/font is slightly different on the scaleplates, but still good and easily legible.

I don't believe a movement transplant will cause you calibration problems as such, a minor tweak on the magnetic shunt might be required to compensate for ageing if the donor meter has been sitting gathering dust for decades.

[philthespark]

Hi All, sorry I've not been here recently, bit busy at work. Well here,s the pictures, and one of a rather weird piece of AVO kit, I've only ever seen one before. The 3 large AVO's are a model7, model 40, and the other I haven,t worked out yet.

[philthespark]

the rest

[Phil G4SPZ]

The weird piece of AVO kit is an "AVO Test Bridge" which measures resistance and capacitance. I have one, and the circuit if you need it.

[philthespark]

Originally Posted by Phil G4SPZ

The weird piece of AVO kit is an "AVO Test Bridge" which measures resistance and capacitance. I have one, and the circuit if you need it.

ah many thanks, someone told me it was for measuring power factor, I don't think I'll use it, it's just for looking at.
To be honest in my day job I use a Fluke multimeter, it's a bit more compact, but I still like the old AVO's, I'm trying to get hold of a reasonably priced model 8, the last type they made, I did have one, but when I was out of work with health problems it was one of the things that had to go.

[philthespark]

Originally Posted by The Philpott

Continued-

1- more detail of instruction plate
2- instruction plate
3- improved image of front
4- scale plate detail. labelled 'Type 3' which is probably the scale shape.

There was a lot of (high resistance) verdigris and dezincification inside the wander plug ports, and one test lead had actually verdigris-ed through completely. This, together with the 25v bobbin being o/c, may have led to the meter being deemed useless and thus being converted into summat else.

New 3d printed feet are good, as is sometimes the case a thin sliver had to cut from one side of the rear ones, to avoid them catching on the instruction plate as it is opened. (A problem that could arise with the original feet.)

If anybody ever has the wish and technology to make a few Model 1A scale plates from worn out Model 1 plates, by all means PM me and get a better image from me on email, (but send me a couple of examples when they're done, eh?!)

Another meter saved, anyhow!

Dave Philpott

May I say how nice it is to see your restoration, I may get around to doing mine soon. It upsets me to see this current steampunk craze for butchering old meters and turning them into lamps, a meter that has survived over 50 years as a meter, will probably end up being thrown in a skip within a year of being converted to a lamp! Why oh why can't people just clean them up and admire them for what they are, after all they don't convert old books into lamps do they!

[The Philpott]

Thanks Phil. Fortunately the two components that marked that Avominor out as a 1A hadn't been touched so survival of the concept was assured even if the remaining parts had been ruined. The craze for 'the industrial look' has taken it's toll but as is the nature of fashion i hope it will fade one day.

(There was a conversion of the universal avominor back in the day, to a 'protometer' which was a conventional damp meter, retaining the movement and (modified) scale and blocking up most of the terminal ports.)

It's surprising how quickly those wander plug terminals fill up with dust and fluff, which then has the annoying habit of dulling the brass. Looks like you'll have to get in there with a cocktail stick and air pressure on yours!

Dave Philpott