Avometer Model 7 Mk2 Batteries

The AVO Model 7 Mk2 requires three batteries for measuring resistance. The 1-mega-ohm range requires a pair of vintage 4.5 V batteries (Eveready Type 1289) to make 9 V. This battery also goes by the name 3LR12, and MN1203, and it is 62 mm × 22 mm × 67 mm in size. The 10,000-ohms and 100,000-ohms ranges require a standard and readily available "D" size 1.5 V dry cell.

It occurred to me to check the circuit diagram to determine why this meter required a pair of 4.5 V batteries, and as it turned out there was absolutely no reason for it, because they simply connect in series to make a single 9 V power source. Therefore, I decided to make a modification and install a 9 V PP3 battery connector. The connector wires simply solder onto their relevant counterparts from the outside of the battery compartment box. This modification also requires drilling a very small hole to route the connector wires outside of the battery box.

Usually this meter would have some type of battery holder for the "D" size battery; however, it was missing, so I decided to make a modification and install a battery holder for an "AA" size battery. I decided to use a smaller size battery because it is much more convenient to replace. An extremely high quality Sony LR6 can provide many years of service.

The battery box houses the batteries, and usually remains fixed to the aluminium back cover; however, I decided to remove it for cleaning purposes because it had some battery acid.

In the following pages of this multi-page article, I show how I performed a small modification so that I could use the standard and readily available PP3 battery.

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