Wednesday, August 21, 2019

AvoMinor 67004-442

From the company that invented the multi-meter, a small and simpler instrument. The 'AvoMinor' was introduced in 1933, upgraded in '35 with a socket inside the Ohms adjuster knob. (As with every little niche, there is quite a lot of information available online. Especially informative were the pages at Richards Radios.)

When purchased a while ago, the case looked a bit tired and the instrument didn't work.

Luckily the fault was repairable and the case cleaned up nicely. (Again; experience with typewriter cases restoration comes in handy.)

With newly made leads, the fixed instrument is again usable. Even though quite limited in function when compared to modern or even the contemporary regular Avo meters, it is good enough for most of my simple measurement needs.

The back of the instrument screws off, the cardboard battery compartment can then also be taken out. The battery leads were already broken, otherwise these will need un- and re-soldering. A quick inspection showed that fortunately the coil movement was still fine. With another meter, the various parts were inspected and tested.

This is very 'mechanical' and visible electrics, before packaged components were used. With the aid of the explanation and diagram at Richards Radios, the only actual fault was found to be the top-right coil being open-circuit.

This coil could then be taken out by unscrewing its 6BA bolt and nut and unsoldering the leads. 

Then unpacking the coil to find the fault, it turned out that luckily it was a case of lead-in wire having come loose from the coil wire proper. This is often the cause of old coils being open-circuit, rather than breaks somewhere deep inside the wound coil. The thicker lead-in wire was originally merely twisted with the coil wire, now repaired by soldering and again wrapped in paper-tape.

Battery contacts soldered again with the battery-well in place and a regular 1.5V AA battery provided with some padding or sabot then fits well enough inside the compartment.

Re-assembled and functional again, with the original instruction booklet. The serial number of the instrument indicates manufacture in April of 1942. It survived very well, the dial crisp and clean and no damage to the bakelite housing either.

The contact bushes are (of course) an odd Imperial size, they're 1/8" so a little over 3mm diameter. Even though this is an unusual size, a batch of 3mm banana-plugs were sourced. With some modern flexible litz wire and some shoelaces, look-alike reproduction leads were mocked-up. They turned out too large diameter when compared to originals - but they work and don't look too much out of place.

Added some 3mm alligator-clips, and the meter is in use - here showing that this lantern-battery is quite tired, nearly exhausted.

A nice vintage and very usable instrument again. And in reality quite complex enough, for use by a mainly-mechanical user too :)


  1. An instrument that you can use after the apocalypse to determine why everything else doesn't work :D

  2. No need to worry about EMP. Great job on the restoration.