Universal AvoMinor Multimeter Checkout

[DMcMahon]

Brought this very recently through the Forum.

Overall in good condition for its age, a small chip broken out from the Bakelite on one of the side panels and the underside instructions is badly worn. Has a nice vintage feel to it.

Instructions from another meter in Photo 1.

From web research it looks like it is a Model 1 possibly the 1952 vintage. I remember using these and slightly later versions when an apprentice in the late 1960s.

A few spot checks on AC and DC voltage and it appears to read reasonably accurately, although have done no accuracy comparisons. DC current measurement still to be done.

The original battery would have been a 1.5V Ever Ready U10 which is longer and fatter than a AA battery, so need to fit spring/metal spacer to make an AA battery fit, so as to be able to test the resistance measurement operation.

David

[The Philpott]

Nice one. The best battery spacer i devised was a brass nut (17mm hex, 10mm thread) with a cut-to-size piece of brass studding staked into the centre. A little vulnerable to corrosion in a damp atmos. if the sprung terminals in the meter are manky though.

It's a 1945 meter you have there, subtly different text on the {brass} instruction plate (and also month/year code on the scale plate!) The front moulding is one that was often used on earlier meters, and lack of nickel plating here and there is normal for the earlier universal avominors. Those (later) wander plugs are usually steel, perfectly ok if they're not corroded though.

Red disc missing on the needle (as they often are!) but not necessarily a sign of abuse; they come loose on their own. This might make the movement very slightly out of (tail) balance and also hard to see at FSD... but doesn't affect accuracy to any noticeable degree.

Current shunts almost guaranteed to be ok unless they have been scorched- i've never come across one that was o/c.

Dave

[DMcMahon]

Thank you Dave for your detailed information.

The meter battery contacts were a little corroded but I have cleaned them up.

I noticed that -845 was at the end of the scale plate serial number but never latched on to the fact that it could a month/year code.

As I said the instruction plate text is badly worn as per attached photo, that is why I attached the good one previously just for instructions reference.

The wander plugs look like steel and are in good condition.

Yes I noticed from looking at other AvoMinors that some had the needle red disk.

David

[The Philpott]

Ah yes, i see- thinking about it now and comparing it to mine, the (replacement) instruction plate looks to be a VERY early one from it's style- late 1930's i reckon.

I'm certain that where the (original) instruction plate has been abraded, it was to erase 'Type H' military designation- Type H was a military issue meter but identical in all respects to a standard Model 1. Probably just tagged to discourage people from pocketing what is a very pocketable unit!

If the rubber feet crumble away another member (TiPwun) did make some 3D printed replacements a while ago- he would still have the file that refers to it no doubt.

Model 1 stayed with this design right up to the end of production ('56-'57 ish) but on some there was an additional winding on a card strip, soldered over the low voltage bobbins. I think this was an effort to improve the AC 5v range.

I think the plastic GEC rectifier is a later upgrade, originally a larger westinghouse copper oxide was used.

It transpires that Model 1 meters were para-dropped to the dutch resistance as part of their covert radio equipment kit.

Dave

[DMcMahon]

Thankyou Dave, I didn't realise that it should have rubber feet. Looking now I see one is missing and the other 3 are worn right down to base level. Will look around, may be able to find or fabricate something suitable, not concerned if replacent feet do not match originals.

I also can see the feet on the various web images.

David

[The Philpott]

-The little button mushroom feet avoid scratching the instruction plate (or the varnish on someone else's table!) Digging out the remains of the stalk is a bit twitchy as you're near quite a weak part of the moulding and they are a tight fit- i used a drill bit twirled in the hand.

I recall having to trim replacements a little- tolerances are close meaning they sometimes foul on the instruction plate.

Great little meters, even the resistance range is rather good.

Dave

[DMcMahon]

Yes see that there is not much clearance to the instruction plate and potential weak point at the corners, certainly will have to be careful removing the remains of the old rubber feet.

Would probably use it most for resistance continuity checks, so much more satisfying seeing a needle deflection compared to a digital readout.

David

[DMcMahon]

Fitted a temporary spring spacer to do functional test of the resistance range.

David

[The Philpott]

That expanded polythene used as a side-spacer is great stuff- not liable to interact with other materials at least in the medium term. (Not too sure about the Duracell though!!)

I have had success filling holes in black bakelite using acrylic paint and araldite mix (but it did take 2 weeks to harden off.) Presumably your repair would need formwork of some sort..

Dave

[DMcMahon]

Thank you Dave, I was wondering what I could use to repair the damage.

I had to Google Formwork.

David

[Andrew Sinclair]

I had some success with araldite, mixed with hammerite smooth finish black paint, to repair a telephone handset.
A friend suggested that i should have mixed in some wood flour (sawdust) to strengthen it. Apparently this stops cracks propagating through the araldite.
If you look at the manufacturer's data sheets for bakelite type materials, it usually has a "filler" added. In the past this was often wood flour or asbestos.
It might be worth experimenting on a few samples before attempting the repair.
The bakelite page in wikipedia gives some info.

[The Philpott]

Hammerite smooth, being solvent based, would certainly be preferable to the paint that i used. Worth checking the broken edge of the meter for any more chunks that are trying to come loose- you know what bakelite is like, sometimes it can tear like cheshire cheese leaving a high spot, rather than breaking cleanly.

Dave