The Universal AvoMinor Model 1 was a 22 range moving coil multimeter with a 3-inch scale manufactured by the Automatic Coil Winder and Electrical Equipment Company of England. This precision electrical testing instrument was a new offering available for purchase in 1957 and cost twelve pounds in old money. “AVOMINOR” was a registered trademark belonging to the AVO Company, to represent a line of portable multimeters. At that time, there were two models available, where Model 1 had sensitivity of 400 Ω/V and total resistance of 200,000 Ω, and Model 2 had sensitivity of 4000 Ω/V, and a total resistance of 4 MΩ. They both had the same maximum DC current limit of 500 mA; however, Model 1 was limited to 500 V for AC and DC voltages, whilst Model 2 could measure up to 1000 V for AC and DC voltages.
The Model 1 has dimensions 4 3/4 inches × 3 7/8 inches × 1 7/8 inches, and weight 18 oz. If you see an AvoMinor for sale on eBay, then check the AC and DC voltage ranges, because Model 1 has a maximum limit of 500 V and Model 2 has 1000 V.
I bought this meter on eBay for ten pounds selling as “Spares and Repairs”. It was out of my poor blogger budget but I decided to bite-the-bullet and go for it as the seller had mentioned something about the movement making a full swing as normal. When I received it, I connected the leads, but there was no swinging of any sort :-) so I decided to open it up and have a look. I used a partially depleted “AAA” battery taken out of a remote control to check the meter movement. Applying a current directly to the voice coil terminals should give an indication whether the movement was working. Luckily, it was working so I decided to apply the voltage a little way down through a series shunt coil, however, the movement was not responding. Therefore, it took 30 seconds to diagnose that the series shunt, was open. I took the shunt apart and joined the wires where they had burnt so that I could measure its resistance value. I measured it to be 18 Ω, however the wire was approximately 2 cm shorter and therefore approximately 2 Ω less, therefore I approximated it to be around 20 Ω. I simply got some nichrome wire and wound a new precision resistor, and the meter started working.
I also had to clean the movement due to dust and foreign particles causing the needle to stick; however, it was all elementary, as Sherlock Holmes would say. In the following pages of this multi-page article you can see all the design features of this meter.
This Article Continues...Universal AvoMinor Model 1
AvoMinor eBay Cost Price
Universal AvoMinor Design and Engineering
AvoMinor Movement Repair
AvoMinor Loose Dial Glass
AvoMinor Movement Balancing
AvoMinor Battery Compartment
AvoMinor Leather Case