Treasures from Mum's loft: A 1948 AVO meter & more

[Slothie]

Now Mums moved to a retirement home, my sister and I have been clearing the house. I have rediscovered treasures I remember as a child but had dropped from my memory, including some that will need work to restore.

First is my Dads AVO High Resistance Multimeter (Type 1), manufactured in June 1948 and probably bought as War Surplus some time later.Its an unusual model in that it uses a 4.5v torch battery rather than the usual (unobtainium) 15v battery. The 1.5v cell for the low range is a C cell rather than the D cell it should be, probably a symptom of Dads frugal ways! Aside from being dirty from being under the stairs for 25+ years, it appears to be undamaged, although I haven't had chance to test it yet. The batteries were in it, dead but miraculously not leaking.

The second picture is of a Ferranti "Air Ministry" 500 microamp panel meter, in its original box (slightly foxed) and marked with the date 1941.

Unpictured I also unearthed my 1978 Commodore PET 2001-8 which will get a full referb before I risk powering it up,, A paper tape punch I interfaced to it (alas the paper reel I had has gone missing, so its unable to be tested), A 60s/70s Olivetti calculator with Nixie tube display I am overjoyed to discover as I thought it had been given away years ago.

No sign alas of Dads R1155 receiver, he must have jettisoned that at some time.

And theres still more to look through!!

[The Philpott]

Very nice Avo, the first of the 20kohm/volt ones. Must be one of the first 600-700 to be made..? I have one, i think mine is July 1948. Looks like i am not the only one whose scale plate red text has faded to 'magna carta' brown.

Those terminals are quite fragile when extended, mine were distorted!

I used a (four) AA holder for the 4.5v battery, with the fourth AA cavity cut away with an old soldering iron.

Dave

[Slothie]

I was thinking last night about the scale plate and if it could be "restored", and decided that I was more likely to ruin it than improve it. Do you think it mould be safe to clean with a damp cloth, or is the paint likely to come off? I'm presuming it was screen printed rather than some kind of transfer.

Unless it exhibits any serious fault I'm inclined to just clean it up externally and leave well alone. This one has a metal back with an earth screw on the left hand side,Is this the magnetic sheild I have heard of?

[The Philpott]

Most of the script was hand-painted at this stage. If the paint has not started to flake, a bit of old cotton t-shirt or cotton bud can be used to shift any loose debris. Some marks may be impossible to remove, and must be regarded as patina! The most aggressive i have been is a cotton bud soaked in a little wd40, followed be drying off with clean cotton bud/s. Anything containing alcohol (this includes petrol) is verboten as it will bring the paint straight off the scale plate. The zero setting of the needle can be altered so that your cleaning can include the whole scale plate. The mirror can be cleaned with a tiny dob of glass cleaner on a cotton bud, followed by polishing with a dry one.

Removing the scale plate requires a lot of care as the needle is then very vulnerable to being damaged; without the scale plate it is almost invisible from certain angles.

If you can read the scale plate, restoration is probably best avoided. It has been known for people to scan and reproduce scale plates, but not many of this model- and it is in two colours--so unlikely it has been done...yet. Similarly you will be unlikely to be able to obtain a second-hand one.

I will post a photo of mine for comparison, the faded bits are just about legible....i have just cleaned it up- not brave enough to proceed further. Keep it out of sunlight like you would a vampire- it doesn't take very long for the greenhouse effect to ruin a scale plate, especially an older one!

The meter is probably the shielded version of the HRM meter...mine does not have the earthing screw you refer to.

Dave

[Dave Moll]

Originally Posted by Slothie

Its an unusual model in that it uses a 4.5v torch battery rather than the usual (unobtainium) 15v battery. The 1.5v cell for the low range is a C cell rather than the D cell it should be, probably a symptom of Dads frugal ways!

The 4½V torch batteries are certainly more widely available (and cheaper) than the BLR121 15V batteries, though the latter are still obtainable from specialist suppliers.

I assume that some sort of conductive spacer has been added along with the "C" cell to make it fit into the space intended for the longer "D" cell.

[Sinewave]

Nice find!

[The Philpott]

Attached is the scale plate on my meter. (No.593-748)

There were several versions of the High Resistance Avometer, features being a higher resistance range, a screened version, and a panclimatic one with selected internal components potted for reliability against humidity.
By my calculations- i think- that this gives potentiality for 8 versions.

The different versions certainly had separate runs of serial numbers.

The question of whether they are scarce is a bit nebulous- depends on whether we see all the versions as a whole, or assess as individuals.

[Sinewave]

Originally Posted by Slothie

I was thinking last night about the scale plate and if it could be "restored", and decided that I was more likely to ruin it than improve it. Do you think it mould be safe to clean with a damp cloth, or is the paint likely to come off? I'm presuming it was screen printed rather than some kind of transfer.

Unless it exhibits any serious fault I'm inclined to just clean it up externally and leave well alone. This one has a metal back with an earth screw on the left hand side,Is this the magnetic sheild I have heard of?

Due to how delicate they are and considering their age, apart from an exterior wipe, I'd leave well alone.

[The Philpott]

Looks like i used a C cell as well, plenty big enough for a stable reading....I didn't actually cut the fourth part of the AA holder (faulty memory) i just put a wire link across it. A bit of spongy plastic (polythene?) keeps it secure.

[Slothie]

Originally Posted by Dave Moll

I assume that some sort of conductive spacer has been added along with the "C" cell to make it fit into the space intended for the longer "D" cell.

The C cell has wires soldered to it, and these go to small brass screws at the top of the battery compartment. It looks like there was once some metal contact fingers that went to the screw posts that must have broken or corroded; I can remember 40 odd years ago dad changing the battery by getting out his soldering iron and swapping the wires over from the old one, so its been like that a long time.

Although this meter isn't particularly valuable, it was what me and Dad used when he taught me electronics, what we used to "see" into our circuits and work out why they didn't work, what we used to fix the electrics in our cars and is connected to so many of the happy times we spent making things together. To me it is a priceless memory of my Dad, who in many ways was also my best friend, and sadly is no longer with us.

Last night I picked it up by the strap to put on a shelf and the strap snapped - fortunately it had been on my bed and dropped back onto that unharmed. I confess I had a chill run through me thinking about the times I carried it by the handle over tarmac and concrete to and from the car when returning from Mums. The strap is some kind of rubbery material that showed no sign of cracking or wear, but it snapped cleanly in two places. Perhaps I can find a replacement or make one from neoprene or similar.

[Techman]

It shouldn't be a 'C' or 'D' type cell, it should be a proper square type EverReady R1662, or equivalent, as shown in my photo below. However, a C or D cell with wires soldered on will work just as well. The 4.5 volt batteries shown can still be obtained as far as I know. It's ok being an AVO collector, but I'm also an AVO battery collector, as you can see

[gingpeakin]

Can we have a picture of the calculator please?

[Dave Moll]

What I have come across (I believe as an official modification) was a Paxolin frame with contacts top and bottom to fit a "D" cell, with wires from it to the pair of screw terminals. Presumably because the square cells went out of production.

[electronicskip]

For me its the Commodore pet!!
I'm green with envy as its one of the Pcs ive always wanted but never found.
We had a couple at school, but only the elite were allowed near them. not snotty 3rd or 4th formers!

[duncanlowe]

Commodore pet takes me back too. First thing I ever programmed. My Dad borrowed it from work over the summer holiday. Even remember him coming home and telling us he had a new pet in the boot of the car. He even took it down to the pub once!

[Dave Moll]

Meanwhile, just to keep you salivating here is a picture of mine:

[Slothie]

Originally Posted by gingpeakin

Can we have a picture of the calculator please?

Sure, as soon as I get it home. I'm going back next week for (hopefully) the last carload. My flat looks like Steptoes yard at the moment!!

[Slothie]

Originally Posted by Dave Moll

Meanwhile, just to keep you salivating here is a picture of mine:

Mine will look like this when I've cleaned it up!
I've also got a 2032 PET with the business keyboard, at the moment unworking.

[The Philpott]

The strap on the Avo would have been leather originally, it will be quite cheap to get a length from a tack shop and cut to length. Approximate dimensions and the size/shape of the holes are recorded on here from when i measured a decent one. A leather one will last for decades- but it's just as well that it's not going back in a loft!

It makes a nice change for the instrument to be retained and cherished (as opposed to a query as to how much it's worth.) The low resistance range is a very useful feature. It could be a friend for life as even if the movement ever broke it is highly likely that a later one from the '50's or '60s could be used.

Dave

[Slothie]

Ah that makes sense, looking at it I can see it is (was?) leather. Fortunately bring in Newbury we are surrounded by the equestrian industry and I expect if I take the 3 pieces to a saddlery as a template they might be able to make me one or at least supply the leather.

I'm glad I found it too, as well as the sentimental value there are some things that analogue meters do better than digital ones, like when a voltage is changes you can see how it changes more readily than a with a digital meter.