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United Kingdom
Presenting some of LIFE's ODDITIES and plenty of RANDOM JOTTINGS


Monday, 23 August 2010

1923 - 2009 AVO (Amps-Volts-Ohms) Measuring meter.

Produced from 1923 up until 2009, various models developed for the electrical, telecommunications and wireless industry used by the military and at one time found in nearly every radio workshop worldwide, a standard measuring instrument.

In my working life I have used many different test instruments, my first real experience of the AVO was using a model 7 though I never owned one. The model 7 was more suitable for use on power and telephony circuits as it drew quite considerable current to give a reading compared to the later AVO's, it was never really suitable for use in normal radio receivers as true readings would be distorted by the current required. Sensitivity was 1000 Ohms per Volt compared to the model 8 which was 20,000 Ohms per volt.

 Some examples of AVO dials showing the main differences

                                           AVO 8:

Note the ranges are based on 1-10 and 1-25, the antiparalax mirror is at the bottom of the scale on this version.

                                           AVO 9:

                          This is the military version

In this model the scale is calibrated 1-30 and 1-100 this change was made to allow easy Decibel ratio readings.
The antiparalax mirror now sits between the Ohms and the linear scale calibrations.

The controls of the AVO have remained very similar through the model 8  - model 9 range,  until the latest model 8's

                                    AVO 8 controls:

AVO 9 Controls:

These meters were a main standard measuring tool in many industries, the use of the 3000 Volt feature presented some safety hazards resulting in the provision of a metal cased version provided with earthing facilities and international symbols for the AC and DC switches.

The Metal cased version

Complete with earthing terminal on the top of the cover

(Thanks to Bill Curtis of Kent for this photograph)

Both of these meters had the same basic sensitivity of 37.5uA full scale and were fitted with a shunt to read 50uA full scale, giving a sensitivity and circuit loading of 20,000 Ohms per Volt on the Voltage ranges.

The resistance ranges were powered by two dry cells contained in the case, a single "D" size cell for the lower Ohms ranges and a special multi-layer 15V pack for the higher ranges, the "D" cell is still available today as a standard item, however the 15V is not produced by most main battery suppliers, only a few specialists thus it is rather expensive.  

Hobbyists usually replace this with a pack of modern  lithium 3V cells as used in computers to make up the 15Volts, or use a 12V electric lighter cell, which does work when very new but soon loses the ability to adjust the meter to a full scale deflection when measuring high Ohm values.

AVO 8 Circuit

The AVO 8 in it's last few production years gained a much more modern look to the casing, The rather restricted look of the dial went, the opening taking on a wider look with the MK7 models also considerable changes to the legends on the AC & DC switches which became easier to operate.

I do not own a late model 8 AVO so I am looking for some pictures of one to complete this section, I have located suitable ones on another site 

AVO 8 Mk7 dial

AVO 8 MK7 switches

Thanks to the original photographer for his excellent source for these clear images

(Russell W. Barnes)

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