Avometer Model 10

Manufactured from about 1977 (this one is dated 10/11/79)

Click on image above to view instructions  which include the circuit diagram

This multimeter appears to have been made mainly for the export market. It incorporates two transistors in conjunction with the 15 volt battery which are used in the meter tripping mechanism, though the meter also has a mechanical mechanism. I am indebted to Kris VanDamme for these pictures.

Avometer Model 70 series

AVO produced the "70 series" range of analogue pocket meters with 2000 Ohms/Volt sensitivity and 3 inch scale lengths at the beginning of the 1970's. 

The following is from a 1972/3 catalogue

Avometer Model 71

This model was soon superseded by the more accurate and virtually identical model 72. The 1A fuse was omitted in the Model 72. See the circuit diagram on the right (open in new window to view)

This one is in my collection

Rank Xerox 600T91177 Multimeter

Avometer Model 72

first introduced in 1972

This one is in my collection

Avometer Model 73A

This one is in my collection

The 73 has AC current ranges with 1.5/3.0/7.5 scaling, and has a number of small neons to give greater protection to the movement.

Avometer Model EM272

Click on image above to view instructions  which include the circuit diagram

Electronic version with six transistor amplifier 3 microamp sensitivity

Post Office Model 12D

This appears to be a variation of the model 72 shown above with the addition of a neon fuse failure indicator and omission of 1000Volt DC range. (images courtesy Gavin Watkins)

Post Office Tester SA9083

This tester was also made by other manufacturers including Robin and Salford Insrtuments. Intended for use by faultsmen for maintenance and testing of telecommunication lines and equipment.

The item on the left ia a printed circuit current shunt.

(images courtesy Gavin Watkins)

Avometer Model 12

meter multi range no15A GPO is identical

Specialised for automotive tests; no transformer; no cut-out but diode protection for movement. Supplied to British General Post Office telephones and labelled 'meter multi range no.15A G.P.O'

DC 3.6, 9, 18, 36 V - sensitivity 5 mA
DC 3.6, 36 A
AC 9, 18, 90, 360 V - sensitivity 11 mA
R 1, 10k ohm (25, 250 ohm center)

Click here for 1965 instruction manual

Click here for 1972 instruction manual

The relevant pages on both of the above have been amended to correct errors on the original schematic diagrams. This is an inside view of a GPO No 15A owned by
Gavin Watkins.

Heavy Duty AVOMETER

1947/8 Early version

Based on Model 7 type design. Early version has  screw terminals and cast aluminium front panel and casing. Was made for use on British Railways.

DC 1, 5, 50, 500 V - sensitivity 1 mA
DC 50mA, 500 mA, 1A, 10 A
AC 1, 5, 50, 500 V - sensitivity ???
AC 50, 500 mA, 1, 10 A
R 500, 50k ohm (12.5, 1250 ohm center)

In 1953/4 a revised more commercial version was introduced which had a moulded Bakelite front panel and casing a form of construction carried through to the models shown below.

There were EIGHT variants of this Heavy Duty instrument only a few of which are included here.

Heavy Duty AVOMETER Mk5

DC 10, 25, 250, 1000 V - sensitivity 1 mA
DC 10, 100 mA, 1, 10 A
AC 10, 25, 250, 1000 V - sensitivity
2 mA on 25+ volts, 5 o
AC 10mA, 100 mA, 1A, 10 A
R 500, 50k ohm (12.5, 1250 ohm center)

(actual meter movement is 600 microamp FSD)

HEAVY DUTY AVOMETER Mk.V (as described in 1960 advertisement)
A multi-range A.C./D.C. electrical measuring instrument of specially robust construction designed for outdoor work and where conditions of rough usage exist. The instrument is housed in a moulded Bakelite case and the movement has been designed to withstand severe shock without damage.

It is a moving-coil meter with a knife-edge pointer and an anti-parallax mirror to facilitate accurate reading. The 3t in. scale is very open and clear. Current consumption is 1mA. at full scale deflection on D.C. volts and 2mA. on A.C. volts.

The following ranges of readings are 'available from one pair of terminals, the range selection being effected by means of' a single rotary switch: -

D.C. VOLTAGE: 1000,250,25, 10 volts.
D.C. CURRENT: 100mA, 1A, 100 mA., 10mA.
A.C. VOLTAGE. 1000,250,25, 10 volts.
A.C. CURRENT: 10A, 1A, 100mA, 10mA.
RESISTANCE: 0-500 ohms (Midscale 12.5 ohms).
0-50,000 ohms (Midscale 1,250 ohms).

Further ranges can be provided by means of extension transformers for A.C. current, shunts for D.C. current, multipliers for A.C. and D.C. voltage, and a Resistance Extension Range Unit.
Supplied complete with leads and interchangeable test prods and clips.

Size: 7.5 X 5.525 X 4 in. Weight: 5.5 lb.

click here for instruction manual

Heavy Duty AVOMETER

This is a variation made to British Railways specification in 1985

  Bakelite front panel; Resistance on bottom of scale plate; banana terminals

DC 1.5, 3, 15, 75, 150, 750 V - sensitivity 1 mA
DC 75mA, 300 mA, 1.5, 15 A
AC 3, 15, 150, 750 V - sensitivity ???
AC 1.5, 15 A
R 500, 50k ohm (12.5, 1250 ohm center)

AVOmeter Models 15 and 16

Models 14 15 and 20 are of similar construction varying only in the ranges covered (model 14 has no AC current ranges) this one dates from 1969

These instruments were primarily introduced for use overseas. They incorporate, not only most of the design features which have made Avometers famous, including the Avo automatic cut-out and Reverse Moving Coil Button, but also many features designed to take advantage of advances in instrumentation result­ ing in compact and versatile instruments which combine accuracy with simplicity of operation.

Metal film resistors ensure stability and the centre magnet 37.5 microamp movement minimises the error due to external magnetic fields. A single switch selects all a.c./d.c. voltage and current ranges and all resistance ranges, whilst two push-buttons select the function required, one push-button covering all a.c. ranges the other all d.c. and ohms ranges. The voltage and current ranges are scaled in basic units of 10 and 3 (12 and 3 for the Model 20).

The AVO 16 has an accuracy of 1.5%, while that of the AVO 15 is 2.5%. They are otherwise identical.

Schematics and photographs:
https://www.radiomuseum.org/r/avo_avometer_16.html
https://www.radiomuseum.org/r/avo_avometer_model_20.htm

A manual covering models 15, 16 and 20 can be found here:
https://www.lost-manuals.com/

AVOmeter Model 20

AVOmeter Model 21 

is similar but has a 6 Amp range instead of 3Amps.

   image provided by Josef Folie

 Electronic Avometer Type EA 113

same size and general appearance as the models 14 through 20

Usually these had blue front panels as shown here but you may come across ones in black casings. Were the latter special in some way?

Back open to reveal battery compartment, fuse holder and spare fuse

The Electronic AVO meter EA113 is a highly regarded and at the time innovative very sensitive and accurate analogue test instrument, incorporating solid state amplifier circuitry - which increased its sensitivity, making possible its use on high impedance circuits without affecting either the operation of the circuit or the reading of the meter.

AVO clamp meter CA-100

made in Japan

0-100 Amps, 0-500V and 0-150V (with buttton depressed)

Images by Richard Souter

This model ATR 1000 is also available in a Heavy Duty version Avometer 1000 HD. The British Telecom SA9083 Linesman's Test Meter is similar. The instrument illustrated here was marketed by Newey and Eyre and branded 'NEWLECAVO' RS Components Ltd also marketed this meter stock number 611-212.
The gap in the handle is to allow the leads through as the three sockets are underneath the top edge of the meter.

Ranges:
DC: 0.3 V, 3V, 10V, 30V, 100V, 300V, 1000V
AC: 10V, 30V, 100V, 300V, 1000V
DC: Current 50µA, 10mA, 100mA, 1A, 6A
AC: Current 10mA, 100mA, 1A, 6A
Resistance: 1-10k ohm, 100k-1M ohm, 1k-10M ohm

AVOmeter Model ATR 1001

The one on the right was made in 1987 and the other marketed by RS components was made some time after 1991

The ATR 1001 is superficially the same as the previous model but you will note that apart from the scales being 0-10 and 0-25 the resistance scale is now between the other two scales and there are no AC current ranges (so only two sockets needed)

These images were supplied by Gavin Watkins

Ranges:
DC: 2.5V, 10V, 25V, 100V, 250V, 1000V
AC: 10V, 25V, 100V, 250V, 1000V
DC: Current 100µA, 1mA, 10mA, 100mA, 1A
Resistance: 1-10k ohm, 100k-1M ohm, continuity beeper

Acknowledgments

Images other than those of the meters in my possession have been gleaned, cleaned up and resized from various sites on the internet.
Others including Paul Thomas and Sean Mckinney have made comments and provided information which has now been included.

The basic details of many of these instruments have been compiled and provided by Andrew Usher who felt that there was a need for an AVO page similar to that for Simpson meters shown here: www.simpson260.com