The BLR121, also called B121 is a 15 V dry cell battery used in the AVO range of analogue multimeters.
The AVO 8 uses a commonly available 1.5 V U2 dry cell, and the less commonly available 15 V BLR121. When I was replacing mine, I found a twenty-year-old Ever Ready cell in the battery compartment.
Using without the 15 V battery
The 1.5 V U2 cell is for the “ohms” settings, whilst the 15 V cell is exclusively for the “Ω ×100” setting. The meter works perfectly fine without the 15 V cell, and you can still measure lower resistance values; however, the only setting that will not work is “Ω ×100” for high resistance values.
All the other functions for measuring current and voltage work fine without the 15 V cell.
The positive end of the BLR121 connects to the terminal clip with a red painted bolt.
Rebuilding a BLR121 Battery
There are many cowboy methods of building another 15 V battery. You could connect ten 1.5 V button cells in series. Alternatively, perhaps use two 9 V (PP3) batteries in series with a Zener diode and a resistor to drop the extra three volts.
As a professional who works in avionics, I cannot possibly recommend such methods. My advice is to buy the proper replacement component. There are many online shops selling the equivalent BLR121 battery for almost nothing. At a glance, I found two firms selling an equivalent battery for as little as £5 and these prices are set to drop further.
An electronic measuring instrument is a very delicate and critical piece of equipment, and it is not clever to fit non-standard components. Considering that these batteries last for many years, my recommendation is to buy a proper replacement battery, and keep your AVO 8 in its original condition.
This Article Continues...AVO 8 MK 2
AvoMeter 8 Range
AVO 8 Battery Replacement BLR121 / B121
AvoMeter 8 MK2 Inside View - Animation
AVO 8 MK 2 Circuit Diagram / Schematic
AVO 8 Test Leads, Probes, and Terminals
AVO 8 Specification and Features
AVO 8 Repair Service / Problems
AVO 8 MK 2 Moving Coil Meter
AVO 8 Circuit and Components
AVO 8 History
AVO 8 Back Cover
AVO 8 MK 2 Meter Dial Accuracy