AVO Multimeter survey

[Pelham 18]

Hi all, I've got a few additions to my Avo illness, sorry collection.
Avo 12 1953 1266
Av0 8 mk111 168508 h 564
Avo test bridge 687 1244
2 x dc multi minors 14347.44 and 7378.14 these two are probably only for spares as both have stuck meters, are a mess inside and have broken cases.
Can anyone enlighten me as why the four 7's I have are all different internally, the dates are not that far apart so I don't think it is new designs or updates?
Andy

[pmmunro]

Andy,

I take it that the four Model 7 meters you are referring to are these :

7740 4351 July 1940
7545 22153 May 1945
758 1616 Possibly May 1938
37896 A1252 December 1952

The reason I've put "possibly" against 758 1616 is that some photographic identification would be useful in confirming this date. If the serial number, and hence the date is correct, you have a very rare Model 7. Although the Model 7 was introduce in August 1936, instruments from before 1940 seem to be extremely rare. I don't have an explanation for this but some hypothetical information which might go part-way to explaining it.

By July 1940, production would be on a war-time basis with materials restricted by the demands of other industries. The most obvious change from an earlier meter will probably be the high voltage multipliers which could be in the form of resistance wire on a SRBP slab, a thin glass cylinder wound with very fine wire or four high stability resistors also on a SRBP slab. The big capacitor at the top left-hand corner (viewed from the back of the front panel) also changed in type.

Up to some time in 1947, all these versions of the Model 7 would have used a horse-shoe magnet movement with bimetallic rheostat temperature compensation. Your 1952 meter should have an Alnico block movement. You would have thought the change in movement design would be a sufficiently significant modification to justify a change in Mark number, but in fact that change came in came in the 1950s and marked much more minor design changes.

PMM

[The Philpott]

Points of note that i have found on the elderly WWII era meters (unlikely to be worth a thread of it's own)

1. The glass is often secured with mustard coloured or orange coloured putty, which sticks well to the glass but not so well to bakelite. It does have the advantage that it is easy to remove.

2. Some will have wooden battery boxes, either varnished or unvarnished. This is OK as far as it goes, but there are a few which were made to the wrong dimensions, with the result that the fixing screws are too close to the edge and will have split the wood. There is also a finite limit on how many times the battery compartment lid can be removed, as it is secured by woodscrews.

3. The movement tail weight is prone to corrosion and consequent loss of weight, especially if there is contamination inside the meter from burnt shunt/s. The material the weight is made of seems to be less noble than the metals it is attached to. (sacrificial anode)

4. The P and R knobs are sometimes loose in the front panel, with consequent loss of adequate function of the potentiometer beneath. There is a bit of variance in the diameter so slightly fatter ones may be found on a scrap meter if you are fortunate. One has to be careful when fitting a fatter P knob as it is not unknown for the P aperture in the front panel to be out of square, with the result that the fatter P knob could seize in the panel at a certain point in it's rotation.

5. The brass pin which secures the P knob is not always drilled and threaded to accept the battery contact spike AT THE CORRECT ANGLE. If the spike describes a circle when P is rotated, this is your problem. Do not try to straighten the spike, as something will break- simply change the pin.

6. If the back-box is bakelite check it is not warped. I have only encountered one but it was severe enough to scrap/repurpose for another project. It was a 1945 item.

7. If the scale plate is brass it is possible for verdigris to form between the brass and the ivory paint and completely spall the paint off. It is thought by some to be initiated by the meter having been sat on a sunny windowledge for too long, and too often. Aluminium scale plates don't seem to be so vulnerable to this, although isolated bubbles (full of abrasive white oxide!) might be seen under the paint in extreme cases.

8. Some scale plates passed quality control even though the intermediate calibration lines on the scale were painted in using the wrong centre-point. At certain deflections this can give an erroneous impression that the needle is bent- and then looks straight again when it returns to zero. It doesn't affect functionality too much in the real world, it just looks
odd. I have one where the ohms lines are perfect and the amps/volts scale is 'drunk'.

9. There is an almost total lack of nickel plating on the fixings, for obvious reasons. If any of the un-plated little casing screws show corrosion it is worth changing them for newer ones as if they break off you have a little problem.

10. Curiously, the main terminal knobs on the front panel came in more than one length- if you have one that is too long for your meter it can mean that the C-fork termination on the AVO lead will not be securely attached. Bad show!

None of this matters if you just want to look at them- but you want to play with them and get them working well of course!

Dave Philpott

[Pelham 18]

Hi Pmm and Dave,
In reply to Dave's question, I like to play, somewhat amateurish, at fixing them.
The reason for my question of the four 7's was frustration on my part, for when trying to check a possible fault with a known working meter, the differences in layout make things a tad more difficult (I did say amateur)
7545-22153 is at this moment under the knife (my first Avo bought 15 years ago) and I would really like to get working on all ranges.
I have tried to post some pictures, but for some reason the program will not let me. I don't think 758-1616 is a rarity as the meter adjustment screw is Bakelite as is the case. I wish it was wooden cased!
Just picked up an Avo heavy duty, it has 10186-859A crossed out and 7404-557A in red lettering next to it, a new meter after two years I take it.
Thanks for your help guys, I certainly need it, and I will post the pictures as soon as I am able to.

PS. forgot to mention the 1952 7 does have an alnico magnet, and 7740-4351 has no mica capacitors. It only has the 1mfd cap.
Andy

[AC/HL]

Hi Andy, this should help: https://www.vintage-radio.net/forum/...ad.php?t=77650

[The Philpott]

Hi Andy,

If you mean the Z (zero) screw is bakelite, don't overlook the possibility that the original could have broken then been changed for one from a different era. I have certainly used one from a Model 8 in a Model D, the dimensions were the same but the design at the rear a little different.

As a general guide i have a 1942 meter with a metal Z screw and a 1943 meter with a bakelite Z screw. I think though that picking from the parts bin depended on what was available on that particular day.

Some meters have aluminium back-boxes rather than bakelite ones, sometimes with addition to the high voltage warning script on the rear: 'The metal case is not earthed'.

I only started messing with them 2yrs ago, and agree there is a good deal of frustration to go with the satisfaction of getting all ranges working as they should. The model D has been apart 3 times now, but bizarrely the original 300/750v shunt (ie a glass tube with ultra-thin varnished winding) is still showing continuity. Almost all of these have gone open circuit by now and been replaced with the later 'welwyn four-pack'.

[Pelham 18]

Here are the pictures as requested, the first three are of 758-1616 and the fourth and fifth of 7740-4351.
Thanks to all those that forwarded the help I needed, and apologies for calling the movement, the meter, and picture three on its side.
Andy

[The Philpott]

They certainly look quite unmolested, rather good.

I found out very quickly that the woven insulation on the link wires inside goes mushy if you get any solvent/s on it. Other than that niggle it seems to be still 'fit for purpose' as far as i can tell.

[Pmark34]

Hi latest in the collection which I think is a Model 5 , no 84-2866 (not sure what year that makes it )A little tatty & missing battery cover off the back.not had chance to test it at all. will up date when I have done so.

[pmmunro]

Pmark,

Almost certainly a 36-range Universal Avometer (August 1934). After only 6 months production, this model replaced the 34-range Universal meter in June 1934, but the instruction plates do not always correspond. The 36-range version has a 480V (AC) range.

PMM

[The Philpott]

A lot of the fuse cartridges on these early meters will have blown or become o/c many years ago; if yours shows continuity (and is unmolested) it is worth removing it and testing the resistance as accurately as you can. I think i am correct in saying that a quirk of these meters is that the accuracy of the meter depends on the resistance of the fuse, but not a lot of archive survives from those days so it is useful to ascertain this sort of info.

[The Philpott]

... A few of these oldies are turning up lately, a good time to mention that if they are teamed up with the older rubber insulated test leads, it is not unknown for (the leads) to emit sulphurous fumes into the interior of the box or case which then seeps into the instrument and completely blackens any silver contacts. Seems more prevalent where kept in warm conditions. Silver sulphide comes off well with some card soaked in ISO. A cardboard strip cut from the tube inside a toilet roll solved this problem for me- just about abrasive enough when wetted, and a suitable thickness.

[Superscope]

Hi Chaps,

Glad this Thread is still very much active.

I too, have one of the Models 47A with the S printed immediately above the Identity Stamp.
The Identity Stamp clearly says "Model 47A" but the rear says Model 40.

It is still in 100% working order, and well within Spec.
It is in a Standard AVO Leather Case of the period and I am told the case is original to the
Meter and it never had a Wooden Box.

I acquired the Meter from the Son of the Navel Officer who's Meter it was during the War.

Serial No 403192 - 243

I'm not sure how this differs from a standard Model 40, it looks identical. At least my one does.

So this is a Substitute Model 47A and I guessing that might be what the "S" actually means!

As I understand it, War Time shortages meant AVO were not always in a position to fulfil the Admiralty Orders with genuine 47A's, so probably raided other completed Meters, not just robbing odd Parts from the Parts Bin like they were famous for!

The Document highlighted earlier, clearly mentions that Model 40's were sometime supplied instead of 47A's and details how they were marked.

The Document, for those interested, is an extract from "Bulletin" which was an in house publication by the Admiralty Signal Establishment, issued internally to disseminate useful Technical information.

The full document can be downloaded from "http://www.rnmuseumradarandcommunications2006.org.uk" but it is in Nine Parts and confusingly labelled as issues!
Several different actual issues are also available if you can fight your way around the Site.

I have the Full Issue in question in a single pdf, but I can't put my finger on it at the moment, but if there is interest, I'll try to find it and post it. It is the December 1944 issue.

Sorry for the long boring post!

Ian

[The Philpott]

Thanks for that Ian, not at all boring.

They are indeed a Model 40 in thin disguise, if i swapped the identity button from the front panel, scraped the S off and overpainted the handpainted pattern number on the scale plate it would effectively become a 40. The accuracy of the 40 and it's derivatives is impressive to me, particularly on DCV.

The switches rotate on mine as if they are brand new, it's either been re-done or it hasn't had much use.

[Phil G4SPZ]

Originally Posted by pmmunro

Almost certainly a 36-range Universal Avometer (August 1934)

Despite being so old, these meters still have their uses. I have used mine several times to measure very low resistances, and of course a half-decent amount of current (100mA) is injected into the component under test. Keep these instruments away from delicate transistors! I discovered that the static resistance between the running rails on a preserved steam railway was 1.4 ohms, for example. With care, resistance down to 0.1 ohm can be measured accurately.

[The Philpott]

Agree. Having 20 or 25 ohms at centre scale on such a large display has come in very handy.

I actually got a universal avominor down to res. of 100millohm using the factory accessory, it was accurate but a heck of a faff! As James May once said- 'There are procedures to follow. I like procedures'

[The Philpott]

HR AVominor information: i can confirm that an instrument has surfaced bearing the serial number 548-1140. This is of interest since it is quite late and gets us closer to confirming how many were actually made. The owner (custodian!?) has generously provided this data.

Overall production therefore in excess of 547 but feasibly could be 600 to 700 if the rate of production was consistent up to, say, April 1941.

Any Siemens resistors inside this meter will most definitely be old stock!

[The Philpott]

See attached pics of what may be quite an early Model 7. Restoring it thus far has been a challenge, details follow!

[The Philpott]

If the s/n 769-2929 means what i think it means: 'Model 7, June 1939, instrument 2929' then there are possible pointers within as to why pre-1940 model 7's are rare:

-The original soldering on this example (notably to the shunt boards) is.....very poor.
- The bimetal temperature compensation coil is a noble effort (and they were probably proud of it since the data plate claims coverage from 40F to 90F) but it is about as primitive and fragile looking as it could possibly be.
-This same bimetal coil almost completely blocks access to one of the movement mounting screws, discouraging removal to some extent.
- The varnish used on the shunt windings has entrained air bubbles and small bits of debris- as if the treacly dregs were being used from the container.
-The back box sits hard up against one of the board solder joints and also up against the transformer, and this is probably the reason the transformer snapped the insulating bar it is screwed down onto.
- One of the screws supposed to attach the large brass stiffening bracket to the shunt board was never installed.
- The man-sized condenser might well have been a source of problems..?

Despite all these observations it was only one loose solder joint away from showing signs of life; the movement is very good indeed and the magnet does not seem to have aged at all. There are refurb. marks on the rear of the scale plate from 1951 and 1954, and on one of these occasions the movement was certainly removed; tool marks indicate this.

I condemned the leaf stack on the AC switch as the large points used to select DC were eroded to stumps and mechanical adjustment was no longer possible. Fitting a later model 7 leaf stack sorted this but fraught with adjustment of the close clearances involved. The picture shows the new red or blue insulation applied to the wires where possible. The little rod spacer used to operate the upper contacts has a horrible habit of jumping out just when you don't want it to; eventually a little cable tie was used to compress the stack.

One of the transformer wires managed to shear as i was messing about and it was marginal as to whether i could reconnect it.

The low ohms and mid ohms ranges started to generate accurate readings once the other DC (single) leaf points had been cleaned, but poor continuity behind the Q knob was generating problems. On tightening the nut on the rear of 'Q' i discovered a truth-

'Do not overtighten the nut on the rear of 'Q' otherwise 'Q' will break.'

The threaded section on the rear of Q is hollow (yes, really) with an arrow shaped silver insert which makes contact with the leaf. This really is micro-engineering. By this point i had (uncharacteristically) acquired a zen level of calm and simply drilled a hole in the shaft and glued a brass threaded rod in. I may need to solder a wire between the glued components to get continuity. If it doesn't work i will dispense with the 1meg range altogether and simply bypass it to get the low and mid ranges to work properly.

The cut-out points have been quite heavily scorched and have left a ghost of this event on the inside of the back-box insulation sheet. They are however still working.

I really do suspect that due to heavy use and marginal quality control (shadow factory possibly?) that many of these early power-engineer meters would have been thrown away or returned to the factory with fatal damage, compared to disposal rates of other avometers of the period. Well it's a theory.

[The Philpott]

Model 7 Q shaft- exploded view (quite literally, unfortunately.)

The replacement thread is already in place c/w a nut, the thread now needs to be reduced to the correct length, and to a point. It's only brass, but better than nothing.

Note that the arrow prod which sat inside the hollow (externally threaded) section of the shaft appeared to be retained by the nut crushing the tube slightly.

The original prod is, i imagine, either gold-silver, silver, or nickel silver. The parallel part of the prod is 50 thou across and is completely untarnished. If you drop it it's gone forever! I find a double bed c/w white duvet cover is a very good retainer of errant and invisible components- It does take some explaining though..

I notice that any problems with the Q assembly can compromise all 3 resistance ranges. I wonder if this contributed to the D, 47A and 48A military contracts dispensing with Q.