AVO Multimeter survey

[The Philpott]

Avominor Type HR Instruction leaflet;

As the leaflet is even more scarce than the meter, (and this one is starting to deteriorate) i thought it was a good idea to shew it to interested parties:

[Phil G4SPZ]

Thanks for posting that, Dave. It's no wonder the HR DC Avominor is rare today, as it wasn't an awfully versatile instrument. OK as a voltmeter, but only 350uA maximum current, and no self-contained battery for even the lowest resistance range limit its usefulness, and I don't suppose many were sold. I dread to think how fine the wire is on the moving coil! I hope your example is working.

[The Philpott]

Phil, surprisingly the movement is working. They always came with a box, which to a certain extent might have kept them in good fettle regards corrosion and debris.

The high current range is 250uA, (blame my fuzzy camera work -if anyone wants a better copy i will have to have another go.)

The leads supplied as standard are quite a robust grade of rubber, and are still in one piece. The croc clips which fitted over the end are long gone though. Use of the resistance range is to be discouraged at this late stage as a break in the fragile little wirewound pot renders the meter inaccurate, and if replaced with a component resistor accuracy is restored but the damping remains quite poor.

Looking back in this thread i saw our friend in Italy (Palmerino?) has (or had) a Model 2 with a German instruction plate on the rear- a rare example of an early exported AVO purchased through a Berlin supplier. Hartmann & Braun can't have been too pleased at our 'doorstop' competing with their 'engineering brick'.

[Palmerino]

Yes, the mod.2 from Berlin is still in my possession. Dusted and admired from time to time.
I should finish the restoration because the ohmmeter does not well (maybe a burned resistor). In effect it is a rare piece, perhaps the only survivor of World War II. As for the rarity of mod.1, I saw only one Australian on Ebay about 10 years ago

Ciao Palmerino

[The Philpott]

Agree, probably a burned resistor/winding or small switch contacts out of adjustment. Your model 2 is superb. My only meters with German origin/connection are 1938/1940 Avominors containing Siemens Halske resistors. The stock of 20Megohm resistors at ACWEECO had run out by November 1940.

I wonder if any meters were commandeered later on and given a Waffenamt eagle stamp. Fantasy no doubt as no military contract.... but what if.. A real collector's item!

I have now mended a slightly odd Avominor that appears to have been rejected and thrown into the bin at the factory, only to be retrieved later, maybe by an employee. Details follow.

[noble kiwi]

It is the history and "fantasy" which make this thread so compelling. Please hurry up. LOL

[The Philpott]

I can't claim a connection to either the Weimar Republic or the Wehrmacht with this one i'm afraid, but it does stand out as a bit different..

Universal Avominor U15/1135-1143

On the face of it, a 1,500v Avominor from November 1943, the main moulding, scaleplate, movement, components, insulation and instruction plate all contemporary with this date.

The lump missing out of the moulding seems atypical damage, although the lump missing from the OHMS ZERO knob could easily have been caused at assembly if the socket was peened over carelessly, or the jig maladjusted. The meter came with glass, but even under close inspection there is not the slightest trace of glass flakes inside to indicate that a broken glass was removed earlier in it's life.

The voltage shunts were found to be electrically stable- but two of them are wrong, and there is no obvious sign of any of them having been disturbed since manufacture.

The 5v shunt measured 2.35kohm and the second shunt from bottom on the right (marked with a red blob of paint indicating 30kohm) is a 60kohm. These errors gave the meter a nominal FSD of 6v,26v,176v,576v,1576v, instead of the prescribed series of 5v,25v,100v,500v,1500v.

I am now wondering if this meter, at the point where it was awaiting glass and movement, failed quality control and was bashed on the corner of the bench (or skip) to mark it as scrap. Did someone then retrieve it and attempt to build it up to working order..? I notice it lacks scratched initials in the internal corners of the casing which one sometimes sees, lacks pencil marks to the rear of the scale plate, and lacks blue crayon marks on the magnet. The rear phenolic cover lacks any scratched initials.

As you can see i have made it conform to original spec. with component resistors, although one of the 200k shunts at the top was o/c so has been bypassed to give effective FSD of 1,000v on the high range. (Not that i would offer the meter any more than 50 or so volts at any time, it's just good to have a reverse polarity indication that won't damage the movement.)

The movement itself is quite good, the needle being ever so slightly bent looking though, and a couple of little brown marks on the scale plate.

I expected to just use this meter for parts, but it seems to be telling tales so will be kept in one piece..

Dave Philpott

[Palmerino]

Hi everybody.

Serial number is strange. If I read U 15/1135-1143 is correct? My war time
Avometers have this number: U 50109-942
U 54356-143
U 62244-1243
U 63155-144
The complete list of my collection is in the my post 27th May 2016.

Palmerino

[The Philpott]

Yes Palmerino, the U15 prefix indicates it is a special series which lacks a 250v range but has a 1,500v range instead. So it's meter number 1135 of it's series. It's not too rare as in the period from Nov 1943 to Aug 1945, 6,500 more of them had been made.

The meters without the 1,500v range had a different series of numbers, going up to at least meter no. 132,000 in the year 1955.

[Palmerino]

Yes Dave but my Avominor U 50109-942 is also a special series 1500V.

[The Philpott]

Oh i see! These things have been encountered before: I have a model 1A that has s/no 132232-1155* and another that has 3367-257....so it seems that they sometimes allocated the Model 1A meters to the Model 1 number series*.

The Type E has two series of numbers running alongside each other, although i think in this case it may have been down to two different factories making them..?

[Palmerino]

I'll have to check but it seems to me that my Avominor unlike yours has the exsternal resistances of the 1500V range.However I have often found the Avometers whith the open resistances of the high ranges. Have you ever tried to take them apart? It is quite easy by heating the center with an appropiate iron and at the same time levering with a screwdriver underneath. unrollit is noticed that the resistance is printed on a thin strip of paper. I thought of replacing it with a modern resistance interior so as not to show anything outside.

[The Philpott]

Yes, i took a damaged one apart last year, i think they patented the design where the the wire is wound round the paper, then the paper round the bobbin -like a fly-trap paper. I have managed to remove a bobbin in one piece but i find the glue that holds the bobbin in is often stronger than the bobbin itself, even with heat. I had not thought of hiding new resistors inside a bobbin but it sounds like it could work well, thanks.

[The Philpott]

I have just come across an idiosyncrasy of the Model 7, on Page 12 of the Instructions.

It boils down to this:

'If at any time it becomes necessary to re-set the pointer to Zero......this must be done while the instrument is set to a DC range, the meter not being connected to any external circuit. The reason for this procedure is that in order to compensate for small inevitable errors on AC, the pointer is slightly displaced from zero when set to AC.
To produce this deflection, a minute current is drawn from the 1.5 volt cell used for resistance measurements.....'

I haven't observed any discrepancy in zeroing on my Model 7 no matter what the range setting, but the instructions do ring slight alarm bells bearing in mind the leakage behaviour of certain alkaline cells when subjected to an extended small current drain.

[Phil G4SPZ]

I have tried very hard to observe this AC offset. It is produced by the 200k resistor, and the degree of pointer movement is perceptible, but only just. To prove the point, I measured a current drain of around 6uA from the 1.5 volt cell when the Model 7 is switched to AC. Calculation suggests that this should be around 7.5uA, but its purpose is to provide the copper oxide bridge rectifier with a degree of forward bias so that small AC voltages can be detected correctly.

6uA flowing through a 2mA FSD movement would be expected to move the pointer by three-thousandths of its travel, or 0.3 volt on the 100 volt scale. This corresponds to roughly one-third of one small division. That's about what I see in practice.

We all turn our Avometer switches to AC and DC when not in use, don't we? I can confirm that no current is drawn from the battery in these conditions. Even if left switched to AC, a drain of 6uA from a battery of (say) 1,000mAh capacity would eventually discharge the battery... in 19 years. Still, as you say Dave, if some cells don't like small trickle discharges it's wise to switch your Avos away from AC ranges when not in use.

[The Philpott]

Interesting. Certainly a very small drain, even when compared to the quiescent current of the VC1 voltage converter accessory for the later Model 8 versions. If i fitted one of those i would probably provide it with a switch.

I must admit i don't always revert the selectors to the AC & DC positions when i have finished using, i usually just select the required position next time i go to use it. Not best practice, but down to a little bit of OCD regarding wear to moving parts.

[Phil G4SPZ]

I'm fairly sure that the Avo Model 7 is unique amongst multimeters in using the internal battery for anything other than the resistance ranges.

[The Philpott]

And mine (769-2929) is more unique than most- not always in a good way!

There is no current taken from the cell when on any AC setting and no needle movement either, but this is no great surprise as it is rather jerry-built in the ways i detailed when i got it working originally. The movement is however a beaut and acts as though it is brand new.

It relates 240v AC perfectly, and (perhaps significantly) is 2% low when measuring 14v AC.

When the divide by 2 facility is used, the requirement to add 0.5 of a division before dividing the reading by 2, is adhered to exactly. (Using the press button on 1000v range produces gross reading of 475, resolved to 480 by adding half a division)

[Phil G4SPZ]

Is that a fairly early instrument, from 1939? I wonder if the 200k resistor is absent? Perhaps it was a later addition. My earliest Model 7 is an Air Ministry model from 1944, and that's the one I used to measure the cell current on AC.

[The Philpott]

Yep,all indications put it at June 1939, with two 'interventions' in the 'fifties noted in the interior. Capacitor still remains to be woken up slowly and checked when time allows.