AVO 8 MK 2

AVO 8

The AVO 8 Mk 2, also known as AvoMeter 8 Mk 2 is a Universal multimeter manufactured by the AVO Company of England. This particular version is MK 2 (MK II), and part of my collection. Currently I have AVO 7, AVO 7 MK2, AVO 8 MK 3, AVO 8 MK 4, and finally AVO 8 MK 5 in my collection. A multimeter is an electronic measuring instrument capable of measuring current (Amps), voltage (Volts), and resistance (Ohms), and used in the field of electrical and electronic engineering. The initial letters of these units combine to represent the name AVO.

AVO Meters are very robust, and you would be correct in thinking that the army or the military used them, because in December 1943, a special version of this meter went into active service in the British army.


Accuracy / Resolution

Range Controls

The accuracy you obtain from your meter depends on how well you have calibrated it, and whether you understand how to use the mirror strip on the dial for eliminating parallax error.

This meter can measure DC currents from 50 µA to 10 A with an accuracy of ±1 % of full-scale value.

It can measure DC voltages from 2.5 V to 1000 V with an accuracy of ±2 % between full-scale and half-scale, and ±1 % below half-scale deflection.

It meter has three range settings to measure resistance values up to 20 MΩ.

It measures AC voltages from 2.5 V to 1000 V, and AC currents from 100 mA to 10 A, both with an accuracy of ±2.25 % of full-scale reading.

eBay Sale Price

When I first created this article in 2012, this meter was selling on eBay for the price of ten pounds. There was only one other article on Google as well. Today the price can go much higher because everyone seems to want one.

On eBay, you could expect to purchase a Model 8 MK II (or MK 2) for as little as twenty pounds. Obviously, if it is in an excellent condition with the leather storage case, manual and original test leads, then it might go for much more. You can read more about what to expect in the AVO Meter for Sale on eBay article. If you like this meter, then there is a whole series of them to collect.

When looking for a meter to purchase on eBay many people focus on the cosmetic condition and the accessories available with the meter, however, as a professional, you should consider much more. If you are very new to the world of meters then the two main things to consider are sensitivity and accuracy.

Sensitivity determines how much the meter loads the circuit under test. For basic electrical work, commonly known as “power engineering” where voltages and currents are high, a sensitivity of 166 Ω/V is usually considered sufficient and some of the older AVOs such as the Model 40 and earlier AVO 7s are good enough for this.

For general electronics work, you need a sensitivity of at least 10,000 Ω/V, and preferably 20,000 Ω/V when measuring DC voltages. The AVO 7 MK 2 has a sensitivity of 500 Ω/V, however the AVO 8 series is much more sensitive at 20,000 Ω/V. Therefore, if you are studying electronics, then anything from the AVO 8 series will be good enough.

Accuracy is also an important factor to consider when purchasing a meter. Accuracy is the amount the measured value deviates from the absolute value and given as a percentage. A high quality meter will have an accuracy of 1%. This means that if you were measuring 100 V, then the meter will display either 99 V or 101 V. Due to the emergence of digital meters, 1.5 % to 2 % is average, and 3 % is poor performance. On the AVO 8, the accuracy is 2 % when measuring DC voltages and 1 % for DC current. For AC voltage and current, the accuracy is 2.25 %.

A damaged meter movement usually has poor accuracy, and this becomes another pitfall when purchasing second-hand meters. Very often, a seller has no idea what accuracy is, or how to measure it, and therefore if you do not know how to repair a meter, then it is wiser to buy a slightly expensive one that the seller has tested.

Problems

It is difficult to tell what has gone wrong in a meter just from a photograph on the eBay listing. This meter has many functions and sellers generally do not have the time to test all of them.

Some typical problems you could expect to see are depleted batteries, and therefore chances are that your meter will require a battery replacement. This is not a huge problem as you can buy replacement batteries easily, however one of them is a BLR121, which might set you back a fiver (£5.00).

Another typical problem is the front glass being loose. It was originally stuck with glue at the factory, and it often becomes dry. The securing clips also become loose. However, it is easy to fix this problem. I have an article that shows how to fix it. Just refer to the AVO 8 Repair Service article for more information.

Very occasionally, you might find one with a burnt moving coil mechanism. This is very rare though because this meter has an overload protection system to prevent that type of damage.

Spares

Original spare parts are very difficult to find for this meter. You might be able to replace the resistors and capacitors with modern equivalents. In many cases, they work even better than the originals, but some of the other components will be impossible to replace. If you need to replace burnt shunts, then nichrome wire of the correct thickness and specification is also very difficult to find.

The moving coil meter will be impossible to find, the rotary switch is also very difficult to source. Occasionally there might be someone on eBay selling spares but it may be very expensive due to its rarity. Sometimes it is better to buy a broken meter and use that as a donor for your other meter.


AVO 8 User Manual

I have drawn the AVO 8 Circuit Diagram just for fun. It might be useful for repair purposes.

The operating instruction manual, AVO 8 User Manual, contains the circuit diagram, which might prove useful for anyone seeking an AVO 8 repair manual.

This article continues, and in the following pages, you can see the insides, the battery, the moving coil mechanism, and much more.

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