Oiriginal Avometer  (Model 1)   

introduced 1923 The meter shown here belongs to the great grandson of Donald McAdie who gave it to his son in 1943. A letter addressed Dear Bill indicates that “As the Auto Coil Wiring Coy had presented me with a model 40 which has over 40 ranges and gives alternating as well as direct current tests I am sending the old original one to you as you may find it useful if you have any testing to do. It is in perfect working order …..”
 

DC 12, 120, 600 volts - sensitivity 12 mA
DC 120 mA, 1.2, 12 A
R 10k ohm (225 ohm center)

The P and R adjusters are used to compensate for the Potential (voltage) and the internal Resistance of the internal cells used on the resistance measurement ranges. Instructions here

It has been suggested that the movements may have been supplied by Elliott Brothers. (I have two Elliott meters which can be seen here and here

DC Avometer   '13-range'

(Model 2)

Introduced 1927. 

An internal fuse is wired in series with meter.

DC 0.12, 1.2, 12, 120, 1200 volts - sensitivity 6 mA
DC 12, 120 mA, 1.2, 12 A
R 1k, 10k ohm (12.5, 125 ohm center)                                 
100k, 1M ohm (1.25, 12.5k ohm center) with external power
        (In fact = 7.5, 75 volt ranges, with no adjustment)

The front panel now manufactured from a one piece Bakelite moulding and the casing made of aluminium.

Early models such as this were made in the factory at Rochester Row London SW1

This is an example of an intermediate model, it has the same front panel as the original 13 range model shown above but is fitted with a replaceable screw in fuse on the front panel.

This rear view shows the battery compartment introduced with the model 2 and used for all models up to the model 7. the battery shown here is smaller than the Siemens type T or equivalent for which it was designed.

 

DC Avometer   '13-range'

(Model 2)

This one described here may have been made in 1934 

This one is a later version which has a smooth front panel and has a replaceable screw in fuse on the front panel.  A later model 6 introduced around 1934/5 has a divide by 2 button effectively doubling the number of ranges. and has terminals for the test leads.

Interior, note that the horse shoe magnet is aligned with the front panel. In the model 6 and later models using horse shoe magnets the magnet is at right angles to the panel (see images below)

The meter used an inbuilt 1.5 volt rectangular cell for the two lower resistance ranges (Siemens type T or equivalent). This was accommodated in a wooden recess on the underside of the instrument (as shown above)

Later model 6 with divide by two button (click on image) 

Universal Avometer   '20-range' 

(Model 3)

introduced 1932. This one described here was made in November 1932

    Added AC capability using transformer and copper oxide meter rectifier rectifier.

DC 0.12, 1.2, 12, 120, 1200 volts - sensitivity 6 mA
DC 12, 120 mA, 1.2, 12 A
AC 1.2, 12, 120, 1200 volts
   sensitivity: 6 mA on 120, 1200 volts, 120 mA on 1.2, 12 volts
AC 120 mA, 1.2, 12 A
R    1k, 10k ohm (12.5, 125 ohm center)
        100k, 1 M ohm (1.25, 12.5k ohm center) with external power     
         (In fact = 7.5, 75 volt ranges, with no adjustment)

Universal Avometer   '34-range' 

(Model 4)

Introduced 1933

A 1934 trade manual advertised this model at 12 guineas (£12.60)

The 13 range DC only model was also available at the time

Added 'divide by 2' button - sensitivity doubled when pressed. Deleted 1.2 V AC range (probably was inaccurate)

DC 0.12, 1.2, 12, 120, 1200 volts - sensitivity 6/3 mA
DC 12, 120 mA, 1.2, 12 A
AC 12, 120, 1200 volts
  sensitivity: 6/3 mA on 120, 1200 volts, 120/60 mA on 12 volts
AC 120 mA, 1.2, 12 A
R     1k, 10 k ohm (12.5, 125 ohm center)
        100k, 1M ohm (1.25, 12.5k ohm center) with external power  
        (In fact = 7.5, 75 volt ranges, with no adjustment)

(images from Dave Philpott)

The meter used an inbuilt 1.5 volt rectangular cell for the two lower resistance ranges (Siemens type T or equivalent). This was accommodated in a wooden recess on the underside of the instrument. An external voltage was needed for the higher ranges.

Universal Avometer   '36-range' 

(Model 5)

introduced 1934.

Added 480 V AC range (240 V when button pressed)

DC 0.12, 1.2, 12, 120, 1200 volts - sensitivity 6/3 mA
DC 12, 120 mA, 1.2, 12 A
AC 12, 120, 480, 1200 volts
 sensitivity: 6/3 mA on 120+ volts, 120/60 mA on 12 volts
AC 120 mA, 1.2, 12 A
R    1k, 10k ohm (12.5, 125 ohm center) with 1.5 volt cell
100k, 1M ohm (1.25, 12.5k ohm center) with external power
(In fact = 7.5, 75 volt ranges, with no adjustment)

Universal Avometer   '36-range' 

(Model 5)

Note the changed orientation of the meter magnet

Note changed instruction plate

DC Avometer   '22-range'

(Model 6)

Introduced about 1934/5. This one described here may have been made in 1936

Added 'external battery adjust' (Q knob on Models 7) not incorporated into Universal

DC 0.12, 1.2, 12, 120, 1200 volts - sensitivity 6/3 mA
DC 12, 120 mA, 1.2, 12 A
R    1k, 10k ohm (12.5, 125 ohm center)
       100k, 1M ohm (1.25, 12.5k ohm center) with external power.

This image is from an AVO 1935 catalogue when it was priced at 8 guineas (£8.40)

Universal Avometer  '46-range' 

Model 7 (early)

Introduced 1936. The first one shown here was made in 1944 and the second in 1945.
Note the differences in the front panel mouldings. The second includes "MODEL 7 UNIVERSAL" in a sans serif font rather than the more usual "UNIVERSAL"  

Note also that the zero adjuster on the earlier model is made of brass and the later ones are black Bakelite. This seems to be true for all instruments made after 1944 or thereabouts.

Versions of this meter were made for the British Army and labelled with the part number  ZD00021. those for the Air Ministry were known as Testmeter Type F with leads labelled with the stores reference 10S/1 or 5QP/1

Universal Avometer  '46-range' 

Model 7 (later)

Introduced 1936,this one described here was made in 1948 but this model was still being produced at the end of 1951

The Model 7 was the first with with auto cut out instead of a fuse and decimal scale (rather than duodecimal as in previous models) and has the basic design on which the model 40 and model 8 were based. It has higher sensitivity and was the first with dB scale and block magnet assembly for the meter movement. Earlier models would have horseshoe magnet on the meter (see the model 40 below)

Bakelite or aluminium case with battery cover designed to hold the test prods.

DC 1, 10, 100, 400, 1000 volts - sensitivity 2/1 mA
DC 2, 10, 100 mA, 1, 10 A
AC 10, 100, 400, 1000 volts
 sensitivity: 2/1 mA on 100+ volts, 20/10 mA on 10 volt
AC 10, 100 mA, 1, 10 A
R    1k, 10k, 100k, 1 M ohm (50, 500, 5k ohm center) Q knob required on highest range
C 0.01 - 20 uF (1 uF center)
Power 2 W into 5k ohms (100 volts)    
[Earliest models had 4 W into 4k ohms (127 volts)] Click here for Instructions

AVO used a standard movement design for most products, but with small variations. This were referred to as the Movement 45 (1949/1950), which was superseded by Movement 46 (1965).

Internal 9V battery 2x4.5V Siemens type P3 (cycle lamp type) for high resistance range and as previously a 1.5 volt cell Siemens type T or equivalent for lower ranges. Other equivalents are Exide B23 and Ever Ready R1662 which have these dimensions: 1.125 x 3.3125 x 1.125 inch / 29 x 84 x29mm

The rectangular battery  became obsolete many years ago and most owners resorted to soldering a U2 or equivalent cell in its place.

This is a standard model 7 instrument supplied for the British army dating from 1951

Universal Avometer

Model 7 Mark II

Introduced about 1952 and last manufactured 1986.

This one  described here was made in 1955

New movement design but no external changes initially. At some
point during the 1950's, added power factor jacks for P.F.
unit and red and black banana terminals in 1956. Last manufactured 1986.

Model 7 Universal AvoMeter MKII (from 1960 advert)

A multi-range A.C./D.C. Measuring Instrument providing 50 ranges of readings on a 5-in. hand-calibrated scale fitted with anti parallax mirror.

Range selection is effected by means of two rotary, switches, which are electrically interlocked. Full scale deflection on voltage ranges is obtained with a consumption of I mA or 2 mA according to whether the -:- 2 press button is used or not. The total resistance of the meter is 500,000 ohms.

The instrument is self-contained, compact and portable, simple to operate and almost impossible to damage electrically. It is protected by an automatic cut-out against damage through severe overload.

CURRENT: A.C. and D.C. 0 to 10 amps.
VOLTAGE: A.C. and D:C. 0 to 1,000 volts.
RESISTANCE: Up to 40 megohms.
AUDIO-FREQUENCY POWER: 0-2 watts.
CAPACITY: 0.01 to 20 mFds.
DECIBELS: -25dB to + 16dB.

POWER FACTOR AND POWER can be measured in A.C. circuits  by means of an external accessory (the Universal AvoMeter Power Factor and Wattage Unit). Two models of this Unit are available, one for use with the Model 7 Avo Meter and the other with the Model 40 AvoMeter, but in either case the Avo Meter must be a model fitted with sockets marked "P.F." for connection to the Unit. All recently produced Avo Meters have been fitted with these sockets.

(click on image)

Universal Avometer

Braille Model 7 Mark II

Introduced in 1951, the Braille model 7 meter was modified for use by visually-impaired servicemen and consisted of a more-or-less standard meter with a 'feeler mechanism' pointer over the scale, swung from the movement. You will note that the Braille range markers are on the front face of the meter rather than on the knobs. There are Braille markings moulded above the arc of the scale window for each of the ranges. The switch positions have similarly-marked Braille mouldings and a milled groove on each of the range selector switches so that the positions can be found by touch.

This idea was also implemented on the Model 8 which was introduced in 1951 though the model 7 and 40 meters continued to have engraved knobs until the end of production.

The markings on the braille scale are said to allow users to interpret readings to within 1% of FSD.

See picture below of a Braille model 8 Mk3

This Radio Ham QSL card suggests that AVO may have rovided operators with these cards as a way of advertising their products. (images from Aitor de Elejabeitia)

Universal Avometer

Model 40

 Introduced 1939. This one described here was made in 1946 

Universal Avometer model 40 re-done after Model 7: same as Model 7 except ranges and sensitivity. No dB scale. Until about 1948 meters would have had a horseshoe magnet such as this one. Several components including the rectifier have been replaced in this instrument. The scaling is similar to that on the model 5 Pre-War 36 range instrument which included a 480 volt AC voltage range, more suitable for power engineers who would want the facility to measure 3 phase mains voltages.

DC 0.12, 1.2, 12, 120, 480, 1200 volts - sensitivity 6/3 mA
DC 12, 120 mA, 1.2, 12 A
AC 12, 120, 480, 1200 volts
  sensitivity: 6/3 mA on 120+ volts, 60/30 mA on 12 volts
AC 12, 120 mA, 1.2, 12 A
R    1k, 10k, 100k ohm (20, 200, 2k ohm center)
Q knob required on highest range

Housed in aluminium case. 

Special version of Model 40 (7.5mA FSD)

made in 1947

The scale plate is compressed (or expanded!?) to 150/600 from the normal 120/480. The ohms range is truncated to suit, thus does not extend to FSD.

Red paint is used to demarcate the 25% over-read allowed for on this meter, and there is also an error- '100' is painted in as '10'.

The range knobs are standard for a Model 40, all the way up to 120v,480v, and 1,200v.
It has no significant advantages over a Model D, although arguably a bit more flexibility on resistance, with 12, 120, and also 1,200 ohms at centre scale with the 9 volt internal source.

It assumed that this was part of a small batch for a particular customer.

This unusual meter belongs to Dave Philpott who  has sbmitted this picture (open the image in a new window to view more closely)

Universal Avometer Model D

AM Testmeter Type D with leads.  10S/10610
Type D connecting leads NO.4  10S/10612
Type D  Battery  5]/2000
Type 15 Fuse Testmeter, type D 10s/10611 (for early models only)

This is an early (1939) version  of the one shown below, the construction including a fuse is similar to that of the 36 range model 5

Universal Avometer Model D

(AM part number 10S/10610)

1940-47 This one described here was made in 1945

Military version (Air Ministry). Modified Model 40; Switch instead of 'divide by 2' button. No Q potentiometer and only two resistance ranges.

DC 0.3, 3, 30, 300, 1500 volts - sensitivity 5/2.5 mA
DC 30, 300 mA, 3, 30
AC 15, 150, 600, 1500 volts
  sensitivity: 5/2.5 mA on 150+ volts, 50/25 mA on 15 volts
AC 15, 150 mA, 1.5, 15 A
R    1k, 10 k ohm (25, 250 ohm center)

This meter only requires a single 1.5 Volt cell unlike the model 7 detailed above.

The rectangular battery  became obsolete many years ago and most owners resorted to soldering a U2 or equivalent cell in its place.

Universal Avometer Admiralty Pattern Model 47A.

Note that on this model 40 the front panel moulding includes "MODEL 40 UNIVERSAL" rather than the more usual "UNIVERSAL" and that a sans serif font has been used.

Universal Avometer Admiralty Pattern Model 48A.

The model 47A is a Military version (Admiralty) modified version of the model 40

DC 0.12, 1.2, 12, 120, 480, 1200 volts - sensitivity 6/3 mA
DC 12, 120 mA, 1.2, 12 A
AC 12, 120, 480, 1200 volts
sensitivity: 6/3 mA on 120+ volts, 60/30 mA on 12 volts
AC 12, 120 mA, 1.2, 12 A
R    1k, 10k, 100k ohm (20, 200, 2k ohm center)
Q knob required on highest range

The Model 48A is a Military version (Admiralty) of the
AVO  Industrial Test Set incorporating a modified Model 40 multimeter.

This is an admiralty pattern 48A (described here) made in 1944 but housing a much later model 40 Mk.II meter

The model 48A is a 47A meter unit together with additional shunt, transformer and multiplier to enable it to additionally read:

DC 3600 V
DC 120, 480 A
AC 60,120 A

DC 0.12, 1.2, 12, 120, 480, 1200 V
DC 0.012, 0.12, 1.2, 12 A
AC 12, 120, 480, 1200 V  (25-100Hz)
AC 0.012, 0.12, 1.2, 12 A
OHMS: 1k, 10k, 100k

it requires an external 9 - 25V supply for the 100k ohms range, uses the internal 1.5V rectangular cell for other ranges (Admiralty part number 14120)

Avo Industrial Test Sets

Universal Avometer

Model 40 Mark II 

Introduced 1952 This one described here was made in 1975

In 1980 this instrument would have cost £124.60 exc VAT

New movement design but no external changes initially. At some point during the early 1950's power factor jacks for P.F. unit were added and the banana terminals in 1956. This instrument has a germanium rectifier, which must have been a later change. Last manufactured 1986. Housed in aluminium case. 

Model 40 UNIVERSAL AVOMETER Mk.II (from 1960 advert)

A self-contained multi-range A.C./D.C. instrument providing 40 ranges of current, voltage and resistance. Higher ranges are obtainable with the aid of external shunts, transformers or multipliers.

Range selection is accomplished by means of A.C. and D.C. switch knobs, and in addition a -:- 2 press button halves the value of any current or voltage range. Full scale deflection on voltage ranges is obtained with a consumption of 3mA. or 6mA. according to whether the press button is used or not. Total resistance of the meter is 200,000 ohms.

The instrument is similar in design and appearance to the Model 7 AvoMeter, and is fitted with an automatic overload cut-out.
CURRENT: A.C. and D.C. 0 to 12 amps.
VOLTAGE: A.C. and D.C. 0 to 1,200 volts.
RESISTANCE: Up to I megohm.

POWER FACTOR AND POWER can be measured in A.C. circuits by means of an external accessory (the Universal AvoMeter Power Factor and Wattage Unit). Two models of this Unit are available, one for use with the Model 7 Avo Meter and the other with the Model 40 AvoMeter, but in either case the Avo Meter must be a model fitted with sockets marked "P.F." for connection to the Unit. All recently produced Avo Meters have been fitted with these sockets.

Battery compartment with cell carrier (see above) in position.

Front panel mouldings for most models changed in the late 1940's, the knobs were deeper and the panel markings were white filled and incised rather than raised as previously.      

The same moulding was often used for the front panel and the model number and Mk was often but not always indicated as shown here.

High-Sensitivity HR Avometer model 1

Introduced 1948? is described here

High-Sensitivity HR Avometer model 2

Note the diffferent ohms scale and buutton labels.

First with 20,000 ohms per volt; no transformer or AC current ranges on this model. High-voltage terminals added. No Q knob or 'divide by 2' button. Permanently attached shunt chain taking 1/4 of meter current

DC 2.5, 10, 25, 100, 250, 1000, 2500 volts - sensitivity 50 uA
DC 0.05, 0.25, 1, 2.5, 10, 25, 100, 250, 1000 mA
AC 10, 25, 100, 250, 1000, 2500 volts - sensitivity 1 mA
(Non-linear scale for 10 V)
R    resistance 'Low ohms' (about 3.7 ohms center, reversed some models lacked this)
50k, 5 M ohm (500, 50k ohm center)

This instrument was available in two versions model 1 and model 2, the latter had an extended resistance range  and  could measure up to 20 M ohm. Instruments in screened cases were also available.

Universal Avometer Model 8

(sometimes called   'Mark I')

 Introduced 1951. I don't know when this one described here was made as the movement is a replacement and undated. Versions of this meter were made for the British Army and labelled with the part number  ZD00579

Development of High-Resistance model, added back transformer and normal switch design.

Meter reverse button added. Interior re-design to incorporate commercially available variable resistors on resistance ranges. Bakelite case with battery cover designed to hold the test prods.

DC 2.5, 10, 25, 100, 250, 1000, 2500 volts - sensitivity 50 uA
DC 0.05, 0.25, 1, 10, 100 mA, 1, 10 A
AC 2.5, 10, 25, 100, 250, 1000, 2500 volts
       sensitivity:  1 mA on 100+ volts, 4 mA on 25, 10 mA on 10, 40 mA on 2.5
AC 100 mA, 1, 2.5, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)
Also LR (= 0.325 volts, for use with low range of Resistance Range Extension unit) and Ins. (2 M ohm center, with external power)

The model 8 was designed to accommodate the more readily available cylindrical U2 cell (size D) and because of the higher resistance ranges a small 15 Volt battery. Early models of the AVO 8 use the B121. This battery was superseded by the B154 type (squarer cross section) and in 1970/71 AVO started issuing instruments with an adapter to accommodate the smaller dimensions. The B121 and B154 were made with ten biscuit type layer cells. Modern equivalents are constructed from round button cells (hence the R in the type number). The meter battery contacts seem to have been changed in the late 1960's from the flat type shown here to pointed spring contacts more suitable for the BLR types.

BLR121 Height: 36mm. Width: 15mm. Length: 26 mm.
BLR154 Height: 34mm. Diam: 15mm.
The BLR121 and BLR154 are both rated at 40mAh.

These can still be obtained but prove to be very expensive. I have successfully refilled the plastic casing with ten new cells obtained from a "Pound Shop"

This one  is the same as the model 8 Mk1 except that it is fitted with an extended metal push button with rubber boot to reverse the moving coil.

This type of pushbutton was used on some miitary versions but not all and some if not all have a steel outer casing with an earthing terminal. The one on the right is labelled 10AF/96

These instruments preceded the Test Set No.1 (see below) which has ranges in multiples of 1 and 3 like the model 9 rather than 1 and 2.5 of the model 8

Later versions of the model 8 with this tyoe of push button were being made and labelled with 10S/16499 in the late 1960s.

Universal Avometer

Model 8 Mark II

Introduced 1956. This one described here was made in 1960

Added red and black banana terminals.

Added 500 volt DC range early in production.

The rectifier has been replaced in this instrument.

With the change to the familiar red and black terminals new leads and prods were made and the battery cover was a simple piece of metal with two keyholes and a piece of expanded rubber to keep the 1.5 Volt cell in place.

The following are both PDF files and will open in a separate window

Click to download Instruction booklet 

Click to download backplate instructions (the Mk1 and Mk2 had the same instructions)

Instruction plates for Mk1 Mk2 & Mk3 can be found here

Model 8 UNIVERSAL AVOMETER Mk. II (from 1960 advert)
This instrument has been produced in response to a demand for a high sensitivity version of the world-famous Model 7 AvoMeter. It follows the standard design, retaining the traditional simplicity of operation and compact portability. It has a sensitivity of 20,000 ohms per volt on all D.C. ranges and 1,000 ohms per volt on A.C. ranges from 100 V upwards.

In addition to the many well-known features such as the automatic overload protection device, dual-knob range selection, etc., it has a push-button for reversing the polarity of the movement to obviate the inconvenience of changing over the leads when encountering opposite potentials in respect to a common reference point.

CURRENT: A.C. and D.C. 0 to 10 amps.
VOLTAGE: A.C. and D.C. 0 to 2,500 volts. RESISTANCE: 0 to 20 megohms (with internal batteries);,O to 200 megohms (with external D.C. supply).
DECIBELS: -15 db to + 15 dB.
ACCURACY
Where applicable, all AvoMeters and AvoMinors meet the standards of accuracy laid down in Section 6 of British Standard Specification 89/1954.

Shrouded plug in type leads are now supplied with all AvoMeters.

This is the inside of a model 8 Mk2 showing the four inner leaf springs which make connections between the battery compartment and the meter movement and the two outer ones which make connections to the HV terminals.

A similar arrangement is used on the model 7 and model 40 meters.

(click on image)

Universal Avometer

Model 8X (Panclimatic)

A panclimatic version of the model 8 was introduced in the early 1960s. Electrically and mechanically the same as the model 8 it was a tropicalised version incorporating encapsulated components and using materials resistant to mould growth. Under bright light as can be seen on the image here the moulding material for the casing and knobs has a brownish tinge.

The model 8X was also supplied in a steel screening case referred to as the model 8SX.

The military version shown here is engraved with the NATO part number 6625-99-943-1532

Later versions were given the title Test Set No.1
(see description below)

Universal Avometer

Model 9 Mark II

Introduced in about 1964

The model 9 was intended for the export market and is similar to the Model 8 MarkII apart from the international symbols and 3-10 ranges instead of 2.5-10 and the positioning of the anti parallax mirror.

No dB scale; resistance on bottom of scale plate instead of top; no LR or Ins. position

DC 3, 10, 30, 100, 300, 600, 1000, 3000 volts - sensitivity 50 uA
DC 0.05, 0.3, 1, 10, 100 mA, 1, 10
AC 3, 10, 30, 100, 300, 1000, 3000 volts - sensitivity:
1 mA on 100+ volts, 3 mA on 30, 10 mA on 10, 30 mA on 3
AC 10, 100 mA, 1, 10 A
R 2k, 200k, 20 Mohm (20, 2k, 200k ohm center)

Click here for Instruction manual

The rear of the front panel of a Model 8 MkII (the switching arrangement for models 7,8 and 40 etc is much the same)

Battery compartment found on Model 8 Mk2 and 9 Mk2 instruments.

Insert 1.5V cell with positive towards the centre, the red dot indicates how the 15V one should be inserted.

Test set No.1

(NATO part number 6625-99-105-7050)

military version of Model 8/9 Mark II

This one described here was made in 1971

Note that Test set No1 was made for several years and was based upon the current commercial model at the time.
(see model 8 Mk5 below)

Test set No.1 (a military version of Model 8/9 Mark 2) Essentially the same as Model 8 Mark 2 apart from 3-10 ranges instead of 2.5-10; no dB scale; resistance on bottom of scale plate instead of top; no LR or Ins. positions. At some point added fuse on low ohms ranges.

This model is housed in a protective steel case.
No dB scale; resistance on bottom of scale plate instead of top, no LR or Ins. positions

DC 3, 10, 30, 100, 300, 600, 1000, 3000 volts - sensitivity 50 uA
DC 0.05, 0.3, 1, 10, 100 mA, 1, 10 A
AC 3, 10, 30, 100, 300, 1000, 3000 volts -
     sensitivity: 1 mA on 100+ volts, 3 mA on 30, 10 mA on 10, 30 mA on 3
AC 10, 100 mA, 1, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)

Later versions of 'Test Set No. 1' were essentially identical to the Mk 5 (to 1986)
and Mk 6 (from 1986 to end of Avo use).

Click here for Instruction manual

(click on image for schematic)

Battery Compartment. You can see that one fuse is a spare and unconnected. The negative contact of mine is a bit green and you will see that I have split open the case of the original battery to refill with new button cells.

Universal Avometer

Model 8 Mark III

Introduced about 1964. This one is described here was made in 1968

This is a 1965 version:

Replaced copper-oxide with germanium rectifiers to improve AC sensitivity and frequency response (typically 20K Hz at 2.5V). These were not incorporated in Model 9 until the Mark 4 version. Fuse added to low ohms ranges. Early models of the Mk3 do not incorporate the additional protection features described below and shown on the circuit diagram.

DC 2.5, 10, 25, 100, 250, 500, 1000, 2500 volts - sensitivity 50 uA
DC 0.05, 0.25, 1, 10, 100 mA, 1, 10 A
AC 2.5, 10, 25, 100, 250, 1000, 2500 volts -
sensitivity: 1 mA on 10+ volts, 10 mA on 2.5
AC 100 mA, 1, 2.5, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)
Also LR (= 0.325 volts, for use with low range of Resistance Range Extension unit) and Ins. (2 M ohm center, with external power)

 A miltary version of the 8Mk 3 (AM stores number 10S/16411) with extended Rev MC button was also made like that shown on the earlier model above.

Looking at the diagram on the right the meter has thermal compensation plus additional  protection diodes around the movement and in circuit with the ohms range. The ohms range was vulnerable to putting it across AC so the diodes back to back would protect the meter and draw the current through the fuse. Also a diode across the movement deals with AC on a DC setting so protects the movement, so it sees 1/2 the waveform thus tripping the mechanical movement otherwise the movement would just see AC and not move therefore not cut out and ultimately be destroyed. Previous models it seems would not be protected if on the DC setting but were connected to a high AC supply voltage.

Universal Avometer

Braille Model 8 Mark lll

This is a later and more sensitive version of the Braille model7 shown above.

Have a look here for a description and  additional pictures of this model:

http://www.pa4tim.nl/?p=1833

Universal Avometer

Model 8 Mark IV

Introduced 1969. This one described here was made in 1971

In 1980 this instrument would have cost £98.50 exc VAT

New internal construction using PC boards. Wired connections to battery compartments.Reliability problems with this model (no military versions). 2.5 A range replaced with 10 mA and resistance moved to bottom of scale plate to match Model 9. AC sensitivity increased again

DC 2.5, 10, 25, 100, 250, 500, 1000, 2500 volts - sensitivity 50 uA
DC 0.05, 0.25, 1, 10, 100 mA, 1, 10 A
AC 2.5, 10, 25, 100, 250, 1000, 2500 volts
sensitivity: 0.5 mA on 25+ volts, 1 on 10, 10 on 2.5
AC 10, 100 mA, 1, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)
Also LR (= 0.325 volts, for use with low range of Resistance Range Extension unit) and Ins. (2 M ohm center with external power

Housed in plastic case. Mirror placed mid scale rather than at bottom as on all previous versions. This was the last model to have leather rather than plastic carrying strap.

Avometer Model 8 Mk 4 (from 1970 catalogue}

This multi-range instrument has been designed to meet the requirements of radio, television and electronic engineers requiring an accurate, sensitive yet robust instrument. The Avometer Model 8 Mk 4 incorporates all the traditional design features of its predecessors, so highly valued for Simplicity, together with such additional features as: increased sensitivity in the lower a.c. voltage ranges, fused ohms circuit to provide increased protection against inadvertent overload in the lower resistance ranges, improved temperature coefficient enabling measurements up to 500 amps d.c. to be made with the aid of a range of shunts, and improved frequency response. It also incorporates the familiar Avo automatic cut-out mechanism and a decibel scale for audio frequency tests. 

SPECIFICATION
AC 10mA to 10A f.s.d. in 4 ranges.
DC 50uA to 10A f.s.d. in 7 ranges.
AC 2.5V to 2,500V f.s.d. in 7 ranges.
DC 2.5V to 2,500V f.s.d. in 8 ranges.
Resistance: 0 to 20M ohm(First indication 0.5ohm).
Decibels: -15dB to + 15dB.
Accuracy: AC Voltage and Current ±2.25% of f.s.d.
DC Voltage ±2% of indication. d.c. Current ±1% off.s.d.

Sensitivity: AC voltage ranges 2,000 ohms/V(10V upwards).
                 DC voltage ranges 20,000ohms/V (all ranges).
Size: 204 x 185 x 115 mm.
Weight: 2.95 kg. (including leads).

(click on image)

click here for instruction
booklet

This is the first model to have wired connection between the battery compartment and the meter. The spare fuse is housed in a hole on the right. Access by removing the back cover.

This meter was equipped with an electronic
voltage converter which was supplied from the 1.5Volt cell. This item is normally supplied with a flexible printed connector though in my case the previous owner had soldered wires to the 1.5 Volt cell to power the unit. See here for more information.

Introduced 1969. This one described here has no date

Avometer Model 9 Mk 4 (from 1970 catalogue)

The Avometer Model 9 Mk. 4 is a high sensitivity instrument for the measurement of voltage, current and resistance. It is designed primarily for the electronics engineer and is similar in specification to the Avometer Model 8 Mk. 4. The Model 9 incorporates all the traditional design features of the Model 8 including the Avo automatic cut-out mechanism and reverse moving coil facility.

The Model 9 is, however, scaled in basic units of 10 and 3 and all range switches, controls and terminals are identified by graphical symbols, which in general follow the requirements of the International Electrotechnical Commission.

The protective devices incorporated in the Model 8 Mk. 4 are all retained and an insulation resistance range now enables measurements up to 600A to be made using a range of external shunts. The decibel scale is, however, not included on this instrument.

SPECIFICATION
Similar to the Test set No 1 (see above)

A.C. 10mA to 10A f.s.d. in 4 ranges.
D.C, 50uA to 10A f.s.d. in 7 ranges.
A.C. 3V to 3000V f.s.d. in 7 ranges.
D.C. 3V to 3000V f.s.d. in 8 ranges.
Resistance: 0 to 20M a (first indication 0.5 ohm
Accuracy: A.C. Voltage and Current ±2.25% of f.s.d.
D.C. Voltage ±2% of indication. D.C. Current ±1% of f.s.d.
Sensitivity: A.C. Voltage Ranges 2000 ohms/volt (10V upwards).
D.C. Voltage Ranges 20,000 ohms/volt (all ranges).

Size: 204 x 185 x 115mm.
Weight: 2.85kg.
Price each £34.80

The case and battery compartment on the later model 8Mk4 and model 9Mk4 are shown here. Similar to the design adopted for the subsequent models.

Avometer Model

8 Mark V

Introduced 1972. This one described here was made in 1973

Test Set No1 Mk3 (6625-99-2822) is the military version of this instrument

New internal and external design including movement, the new plastic front-panel no longer includes the word "Universal" Plastic casing redesigned. Leather carrying handle abandoned for a plastic one. Instrument much lighter than all previous models.

dB scale and LR range deleted. (But LR could still be measured with
Resistance Range unit.) Adopted Model 9 ranges and added 600 volts AC range.

DC 3, 10, 30, 100, 300, 600, 1000, 3000 volts - sensitivity 50 uA
DC 0.05, 0.3, 1, 10, 100 mA, 1, 10 A
AC 3, 10, 30, 100, 300, 600, 1000, 3000 volts -
sensitivity: 0.5 mA on 30+ volts, 1 on 10, 10 on 3
AC 10, 100 mA, 1, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)
Also Ins. (2 M ohm center, with external power)

AVO METER MULTI RANGE MMR14C/2
The Model 14C Avometer is a Model 8 Mk5 made to a British Post Office specification. It is essentially the same apart from front panel markings and a different instruction plate on the back.

A military version 'Test Set No. 1' was essentially identical to the Mk 5 (to 1986) except for the front panel markings shown here:

The battery compartment on the Mk4 and subsequent models was accessed from the back rather than the top. The ribbed plastic housing is used to surround the cylindrical 15 Volt battery and keep it in place. The spare fuse is in the centre.

Avometer Model

8 Mark 6 

Introduced early1980's  This one described here was made in 1984

New internal design using rigid PC boards (less rugged) instead of flexible wiring. Deleted high-voltage terminals.

DC 3, 10, 30, 100, 300, 600, 1000 volts - sensitivity 50 uA
DC 0.05, 0.3, 1, 10, 100 mA, 1, 10 A
AC 3, 10, 30, 100, 300, 600, 1000 volts -
sensitivity: 0.5 mA on 30+ volts, 1 on 10, 10 on 3
AC 10, 100 mA, 1, 10 A
R 2k, 200k, 20 M ohm (20, 2k, 200k ohm center)
Also Ins. (2 M ohm center, with external power)

The military version 'Test Set No. 1' was essentially identical to the Mk 6

The ribbed plastic housing is used to surround a cylindrical 15 Volt battery and keep it in place.

Avometer Model

8 Mark 7   

Introduced 1992

Only change from the Mark 6 was the addition of a 10 A fuse in series with all ranges.
Last manufactured 2008.

Click here to view the AVO 8 Mk7 USER GUIDE

Click here to view the AVO 8 Mk7 DATA SHEET

Precision Avometer

Introduced 1963/4 This one in described here was made in 1967

RANGES

DC  1.5V to 1500V and 1.5mA to 15A
AC   3V to 1500V and 3mA to 15A
Sensitivity 100 Ohms/Volt on DC and AC except for 3V AC range which is 50mA fullscale

Designed to provide an accurate and stable local reference or transfer standard for industrial and laboratory engineers. It meets the requirements of Section 6 of BSS 89/1954 for precision grade instruments ie:
D.C. Voltage ranges 0.3% of f.s.d.
D.C. Current ranges 0.5% of f.s.d
The accuracy on a.c. voltage and current ranges is 0.75% of FSD (form factor of 1.11) This accuracy is maintained for audio frequency tests up to 1 kHz on ranges up to 3OOV.
Temperature correction:
true value= 1+.0003(t-20) where t is ambient temperature in degrees Celsius

Range selection is accomplished by means of two controls mechanically interlocked with the third central control to select voltage or current as required. Two 7 inch scales calibrated 0 to 150 and 0 to 30 cover both voltage and current measurements. A test certificate is provided for each instrument housed in the lid. (not shown)

Precision Avometer Mk2

made in June 1970

Pictures of Mk2 version supplied by Adrian Almond

The Welwyn blue high stability precisiion resistors were introduced by AVO in about 1968 to replace the wire wound types used on previous models throughout their range.

Notes

Just to give you all an idea of variants of the Avometer. There are some interesting variants here that may have only ever had small production runs. Thanks to Andy Gilham (Megger/AVO) for this list of official variants. Please be aware that the number of build variants actually runs into 100s, with different component values, lacquer changes, number of windings on the transformers....etc.



Avometer Model 1
Avometer Model 2
Avometer Model 3
Avometer Model 4
Avometer Model 5
Avometer Model 6

Avometer Model 7
Avometer Model 7x
Avometer Model 7sx
Avometer Model 7 (Braille)
Avometer Model 7 (London University Special)
Avometer Model 7 Mk 2
Avometer Model 7x Mk 2

Avometer Model 8
Avometer Model 8R
Avometer Model 8x
Avometer Model 8s
Avometer Model 8sx
Avometer Model 8sv
Avometer Model 8sv (Special)
Avometer Model 8 (Braille)
Avometer Model 8 Mk 2
Avometer Model 8x Mk 2
Avometer Model 8sx Mk 2
Avometer Model 8 Mk 3
Avometer Model 8 Mk 3 (Braille)
Avometer Model 8x Mk 3
Avometer Model 8sx Mk 3
Avometer Model 8 Mk 4
Avometer Model 8x Mk 4
Avometer Model 8 Mk 5
Avometer Model 8 Mk 6
Avometer Model 8 Mk 7

Avometer Model 9
Avometer Model 9sx
Avometer Model 9x
Avometer Model 9s
Avometer Model 9 Mk 2
Avometer Model 9sx Mk 3
Avometer Model 9 Mk 4

Avometer Model 40
Avometer Model 40 (Braille)
Avometer Model 40x
Avometer Model 40sx
Avometer Model 40x Mk 2
Avometer Model 47A
Avometer Model 47As
Avometer Model 48A

Avometer Model Type D

Avometer Model HR1
Avometer Model HR2
Avometer Model HR3

Avometer Model HS1
Avometer Model HS2

Avometer Model Multirange Test Set No.1
Avometer Model Multirange Test Set No.2
Avometer Model Multirange Test Set No.3

OY Universal Test Meter Unit No. 107

Each model is assumed to have been discontinued when the next of that model was introduced, except that the DC Avometer was produced in parallel with the Universal until the Model 40 replaced both in 1939, just after the start of the war. Also, some of the military versions persisted longer due to specifications.

The Model 8/9 was not intended to be a replacement for the Models 7 or 40 outside of electronic work. Aside from the different ranges and sensitivity, the 8/9 were less accurate, at least until the Mark 5 which represented a significant improvement (it was still slightly worse on high resistances due to not having a Q knob, and on all resistances due to ranging by factors of 100 rather than 10). The 8/9 also took considerably more voltage on DC current ranges; due to the universal shunt being the same as used for voltage, the burden on current ranges other than the lowest had to be at least 4 times that of the movement, i.e. 0.5 V before the Mark 5 and later 0.4 V.

The above list indicates only ranges found on the front panel. Additional usable ranges as follows:

The lowest DC current range could also be used as a millivolt range. This was 100 mV on the Model 8 Mark 5+, 125 mV on the earlier Model 8/9's, and 100 mV on the Model 7            
On the DC AVOmeter, Universal AVOmeter, and Model 40, the reverse could be done and the lowest DC voltage range (120 mV) be used as a current range (6 mA).

Higher resistances could be measured with external power, and zeroed using the Q knob. On the Model 7, the 100 and 400 volt ranges could be used as the '10 and 40 megohm' ranges (50k and 200k center). The adjustment range of the Q knob was wide enough to use 240v mains on the former. The Model 40 was similar, having a stated '1 megohm' range using the 120 volt range (again 240v mains could be used).

Click on image for details of instruments and accessories available in 1953

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, Dave Philpott, Hans Caluwaerts and Sean Mckinney have made comments and provided information which has now been included.

The basic details of the 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