AVO Multimeter survey

[Phil G4SPZ]

A new one for me, acquired as a source of spare parts: Air Ministry Model D, serial number D-15655-944 (September 1944 I presume) in a very sad state, having apparently suffered being dropped onto the resistance boards hard enough to break the rear spindle off the moving coil, as well as other impact damage. Someone else has already helped themselves to the R potentiometer and one half of the cut-out contact set, so unfortunately this instrument is too far gone to indicate again in its own right.

However, as a donor of parts it has already repaid the 99p that it cost me, as it has an intact scale mirror and a pair of terminals that I needed to complete the restorations of two Model 7 Mk 1s. The shunt and multiplier boards, switches, glass, scale plate and Bakelite case are also fine.

For PMM's benefit, the magnet is the horseshoe pattern.

I have a couple of Avo Minors on the way in the next few days and will report on them separately.

[Ex Greeny]

I have Model 7 MK2, S/No: 5xxxx-A-1054. This meter has a 'MoD Arrow' stamped on the casing, and a date of 8/7/59 on the display scale. I had to dig out corroded batteries, and then I modified it to take modern batteries.

[pmmunro]

Electrono,

The arrangement is known as a "universal shunt" and is used in most conventional Avometers.
As well as assisting the arrangement for measuring a range of currents it provides damping for the movement. (One indication that either the moving coil or the shunt chain is open circuit is that the pointer will have very little damping).

In the case of your universal Minor, the universal shunt has a total resistance of 90 ohms - twice that of the movement together with its swamp. At full scale deflection of 1.67mA through the moving coil there will be 75mV across both the movement and the shunt chain. Therefore half the current value that flows in the moving coil will flow in the shunt chain, i.e. 0.8333mA. 1.6667 + 0.8333 = 2.5mA. So that's how the current divides.

PMM

[pmmunro]

Phil,

I'm rather surprised that you haven't met the Type 'D' before. The newest one I have seen was made about 1947/8 and that is probably when the last was made but, judging by their serial numbers and the fact that they're still quite common, they were probably made in large numbers.

From the ranges, I believe that these meters were designed for the electrical services of aircraft of the late 1930s and 1940s whose system were not very different from contemporary motor vehicles, at least in principle. I have tried for some time to confirm this but have not unearthed the specification (10S10610). If you look on the internet, you may find an Australian site which states that this was the case but I almost certain that he got his information from me and just failed to mention that I had clearly said that this was speculative in default of documentary evidence.

The other question is about Government markings on these meters; I'm sure I've posed it before. Some have a feon (broad arrow) at the top centre of the scale, others a crown with the letter 'A' on the left and 'M' on the right. My understanding is the the feon could apply to any of the armed forces but the AM is obviously for the Air Ministry. Given the utility of this meter for motor vehicles it would have been of great use to the REME and ROAC amongst other Army units, so the question is would they have recognised (or even known about this meter design) and would honour and tradition have allowed them to adopt a meter meant for "Brylcream Boys"?

Amongst the parts which you should save are the cut-out components as these are the earlier type with angled contacts.

Thanks for the information about the magnet. The other thing I would like to know is the form of the large multiplier resistor. In many 'D's its an SRBP board with four wire wound resistors but others have a glass cylinder 1.125" in diameter wound with two sections of extremely fine resistance wire. The wire is therefore very delicate and I haven't seen many intact multipliers of this kind.

PMM.

[pmmunro]

Ex Greeny,

Welcome to the forum and our ongoing Avometer survey/discussions. Thanks for your information. I'm a little confused as your meter appears to have two dates - one from the serial number and another which you state is on the scale plate.

The serial number is usually at the lower right hand side of the scale plate and at this time was (usually very neatly) hand written. The number you give conforms to the standard pattern.

A Model 7 from 1954 should have plain black knurled terminals. By 1959 these had changed to the red and black sockets combined with screw terminals.

PMM.

[Phil G4SPZ]

Originally Posted by pmmunro

I'm rather surprised that you haven't met the Type 'D' before. The newest one I have seen was made about 1947/8 and that is probably when the last was made but, judging by their serial numbers and the fact that they're still quite common, they were probably made in large numbers...

Sorry Peter. By "another new one for me" I actually meant that an additional Avo had arrived, not that the Model D was a new type to me. In fact I have two Model Ds, and I hope the thumbnails plus the following information about them will be helpful.

Originally Posted by pmmunro

The other question is about Government markings on these meters; I'm sure I've posed it before. Some have a feon (broad arrow) at the top centre of the scale, others a crown with the letter 'A' on the left and 'M' on the right. My understanding is the the feon could apply to any of the armed forces but the AM is obviously for the Air Ministry....

I have one of each, Peter, and have previously reported both serial numbers via this thread so please don't duplicate them! My complete, working Model D is 342-D-9/1951 (so a relatively late instrument) and has a block magnet with a multiplier board holding four large resistors - see photo. This has a single broad pheon at top centre of the scale plate, and "10S/10610" engraved in the Bakelite immediately above the window.

The more recent acquisition is D.15655-944, and has the 'AcrownM' device at the left hand side of the scale plate. As for its multiplier board, I can only surmise that it may have been the tubular glass cylinder type, because a previous owner has replaced said part with a miscellany of carbon resistors - see thumbnail.

Originally Posted by pmmunro

Amongst the parts which you should save are the cut-out components as these are the earlier type with angled contacts...

Too late! Already robbed, I'm afraid, but the spindle, cut-out table etc look worth saving.

To add further intrigue, I also have an Air Ministry Model 7, serial 7544-16272 which has the 'AcrownM' logo on the left hand side of the scale plate, together with "REF No 10S/1" below the mirror. This is a 1944 model with the bimetal temperature compensator and horseshoe magnet. It is missing a few common-type parts, which I hope can be extracted from D.15655-944

Finally, thank you very much for the circuit notes! Most grateful.

Kind regards,

[Paul Adams]

From Thisthread.

This Avo has driven me silly, it had numerous faults including that of the shunt chain being o/c, the switches not making proper contact with the srbp board and finally the movement had a rather irritating intermittent fault, whereby if the movement was tapped slightly resulted in no readings.

Fortunately a replacement movement from an 8 mkIV cropped up so I swapped the scale from the old 9 mkIV, which gave fairly accurate readings, however I noticed that the small push rod to the cut-out was missing, thankfully due to the grease inside the bottom of the case, I found it in there.

Refitting the push rod was more troublesome than actually doing all of the repairs to the meter, as it kept on falling out.

I later discovered that the rod fits into a recess in the adjuster screw for the cut-out, this part took me 2 hours to get it back in and finally re-adjusted the cut-out.

To keep the meter in it’s original state, I also replaced the old shunt (R10,11,12 ) from the donor meter (8 mkIV) which cost me a mere £1.20.

Don’t have the serial for the donor meter as this came in bits.

Another one saved and working.

Paul.

[Phil G4SPZ]

Another Model 7 Mk II for the survey, serial number 83215-A-160. This arrived with a loose glass and some debris in the movement, but otherwise untouched inside and appears to have had very little use. Calibration spot-on. After reattaching the glass and a clean-up of the movement and the P pot, and new batteries, it's good to go!

Phil

[Phil G4SPZ]

I have decided to limit my Avo collecting to Model 7 Mk 1s. Another arrived today, serial number 22492-A-251, with a very interesting fault. The acceleration cut-out assembly has dropped off the moving coil former, but someone has helpfully secured the little aluminium assembly to the magnet with thin strips of sellotape! This meter was described as "well used but not tested" but at £5 I'm not complaining, as I can repair it - challenging, but possible, and I've done one before - or it will yield a host of good quality spare parts for other more deserving recipients.

The photos show the separate parts plus how the whole thing should look once reassembled.

[Phil G4SPZ]

Two more Avos to add to the collection here.

Model 7 Mk 1, s/n 7643-10234, June 1943, scruffy but in perfect working order, with the added bonus of an undamaged Ever Ready R1226 1.5V cell still inside.

Model 7 Mk II, s/n 62451-A-256, February 1956 (same age as my sister) in scruffy condition (like my sister again ) with some o/c multipliers, but unusual in having screw-down Mk 1-style terminals instead of the usual red and black caps, complete with a perfect set of Avo test leads and prods.

Phil

[Nicko]

Picked up a "Universal AVO Minor, s/n U.40399-241" for a quid.

Seems OK - meter s/n is the same as the s/n written in pencil (very stylishly) on the inside black cover.

Any information available on these and where can I get a set of leads from?

Also - is there a good was to clean the mottled front of the unit? I can use Paste #5 etc. on the rest, but not sure about the front...

Sockets were very yucky indeed - cleaned them with a cotton bud + Paste #5 (just wanted a light abrasive) then use a cotton bud soaked in Horogrene to clean them properly... Movement seems good and all ranges seem functional.

Ta.

[threeseven]

Blutak is good for removing just dust but an electric toothbrush with soap solution removes stubborn grime.

[pmmunro]

Nicko,

There has been quite a lot of discussion of the Universal Avominor on this forum. Yours is from February 1941 and may well have been for Services use at that date.

You'd probably be best to make up your own leads but you will need 3mm wander plugs for this. If it concerns you these leads will be rated for use at 50V or below by modern standards.

Avominors are not usually too dirty as they have been privately owned, at least in their later lives. The Bluetak method of cleaning sounds quite good, or you could try artists' cleaning putty. On the bigger Avometers I have used a strong solution of dishwashing detergent applied by a stiff brush, almost a paste. This will take a lot of work with a barely wet sponge to remove but is usually effective. On the full size meters, if it is necessary to clean them, it is usually best, and going to be necessary for other reasons, to remove the movement from the front panel. With the movement out, there is no reason not to rinse the front panel carefully with running water, but the transformer must be kept dry. However, the compactness of the Avominor makes the movement awkward to remove. In all cases, it is important to avoid any risk of steel tools knocking the magnet as this may cause loss of flux.

I usually clean sockets with a thinned cotton bud and Brasso, polished with a dry bud. and finally carefully rinsed off with isopropyl alcohol or methylated spirit.

PMM.

[Nicko]

Thanks for that.

Just found that the AC ranges work fine, but nothing on DC. This is odd and implies that the switch is dirty/damaged.

How do you open these things?

Thanks

[Nicko]

Originally Posted by Nicko

Thanks for that.

Just found that the AC ranges work fine, but nothing on DC. This is odd and implies that the switch is dirty/damaged.

...or that there is a dry joint on the DC side of the switch... which there was

All works now...

Still looking for a source of nice 3mm banana plugs - I measured the sockets and they are really probably 1/8th inch...

Nice little meter... that's 6 Avos now, 3 x 8V (07/73,08/79,12/79), 1 x M40 (or maybe M47, 12/42), 1 x MultiMinor MkV (date unknown), 1 x Universal Minor (02/41)

[Phil G4SPZ]

Originally Posted by pmmunro

On the bigger Avometers I have used a strong solution of dishwashing detergent applied by a stiff brush, almost a paste. This will take a lot of work with a barely wet sponge to remove but is usually effective.

I used to use the technique described by Peter, but as he says it's quite time-consuming. More recently I've used Baby Wipes. These are remarkably effective at removing the accumulated grime and years of dust from Avometer front panels, and have the added advantage of being available free - my wife keeps large stocks ready for the frequent visits by our young grandchildren, and she doesn't notice if the odd pack goes missing...

For what it's worth, all makes of Baby Wipes are good but Asda's own brand seem particularly effective and very cheap.

Phil

[Phil G4SPZ]

Yet another Avo Model 7 Mk II has moved in, serial no. 84223-A-460. Complete and intact, but needed all the contacts cleaning and the magnetic shunt adjusting to get accurate results on all ranges.

[Phil G4SPZ]

Hopefully this won't be a duplicate in the survey - Model 7, s/n 7544-16272, written up here.

[russell_w_b]

Freshly acquired Model 7 II. S/N: 6813-A-563-F

This meter has a label on the case: Syon Trading, Leatherhead, Surrey. I seem to recognise this address from the RadCom, years ago. Surplus company?

[Phil G4SPZ]

I'm very pleased with my latest acquisition, a 36-range Universal Avometer, s/n 57-6625. Could this be from May 1937?

Apart from non-original terminals, it seems to be in full working order!

Phil