The Universal AVOmeter
model 8 was
the most ubiquitous multimeter in the AVO range which started life back
in 1923, and although it was initially a DC-only instrument many
of its features remained almost unaltered right through to the last
Model 8 of 2008. In particular the mirror scale, the "smiley"
meter
scale, the two switches and two terminals. AVO multimeters are renowned
for their reliability and robustness, the early incorporation of a
mechanically operated cut out linked to the meter movement and the two
switch range selection system have endeared these meters to generations
of electrical and electronic engineers. By 1965, the company had
already created over one million AVOmeters. The firm also produced a
range of smaller multimeters, the AVO
Minor and later the AVO
Multiminor as well as a number of special instruments such as
the Heavy Duty
model. Production of the iconic model 8 meter ceased in 2008. Though
known for their Avometer general purpose multimeters, they made a wide
range of test gear including valve testers, oscillators and light
meters. You will find more information
here about these instruments and other similar models. AVO also produced a range of
smaller multimeters which are described
here.
AVO Model 8 (sometimes referred to as the 8 Mark 1) this
original model with replacement movement. working
It was given to me by Richard Fuller, it belonged to his late Father in
law.
AVO Model 8 MkII 1962 complete with leather case
and leads working
It was purchased new by my late father. I also have another made in
1962 given to me by Ian Griffiths
AVO Model 8 MkIII 1968 working
Given to me by Charles Husband
I now have another made in 1967 which unlike the one above still
has the AVO factory seal
AVO Model 8 MkIV 1971
working Given to me by David Bone. (the top RH terminal is a
replacement)
(The model 9 MkIV is electrically the same but has International
standard markings on the front panel and scale.)
AVO Model 8 MkV 1973
working
Given to me by Ian Griffiths
AVO Model 8 MkVl 1984
working Given to me by Ian Griffiths
All these models have 37.5 uA movements and are housed in
substantial cases. As can be seen the front panel, range selection
switches and meter scale markings are substantially the same. The
instruction plates on the rear of the instruments are different, the
later instruments indicate that they were manufactured by AVO Ltd,
Dover, England and that AVO was a member of the Thorn Group for a
period. There are other significant differences:
(1) The original model has black terminals of the same type as
the Model 7 and model 40 instruments and does not have a 500 Volt DC
range.
(2) The battery compartment on the MkIII is fitted with two fuse
holders one to hold a spare and the other to protect the instrument.
(3) Germanium diodes rather than a copper oxide rectifier is used for
AC measurement.
(4) Several of the purpose wound resistors have been replaced with
conventional metal film types.
(5) Most of the purpose wound resistors have been replaced with
conventional metal film types and the internal construction is very
different in the MkIV instrument.
(6) The early models are housed in black Bakelite cases and the MkIV is
in an olive green thermoplastic case. AVO multimeters are renowned for
their reliability and robustness, the incorporation of a mechanically
operated cut out linked to the meter movement and the two switch range
selection system have endeared these meters to generations of
electrical and electronic engineers.
AVO Model 8 interior
[The penciled number on the meter magnet reads 33301 and that on the
back of the scale plate is 40404]
AVO Model 8 MkII interior (the rectifier is not original)
AVO Model 8 MkIII interiors 1965 and 1968 versions (see bottom of
this page)
AVO Model 8 MkIV interior
AVO Model 8 MkV interior
AVO Model 8 MkVI interior
Not much changed on the inside over the years. Note how the MkIII
instrument has been opened and repaired over the years from 1975 to
1983. When this instrument was given to me by Charles Husband the glass
had become detached from the casing, the pointer was slightly bent and
the batteries of 1990 vintage had leaked and corroded the battery
terminals, these were easy enough to rectify and the meter worked on
all ranges apart from the resistance ones. I eventually found that
there were two problems:
The contacts for the fuse were also corroded and the high resistance
prevented the instrument from being zeroed.
The contacts which were meant to be closed on the resistance ranges
were dirty and causing erratic readings.
This is the AVO8 that
belonged to Richard Fuller's Father in Law. It looks as if the meter
has been repaired as the serial number on the scale has been
obliterated, the genuine AVO leads are replacements and the instruction
manual is that for a MkIII meter. It arrived in this purpose made
cardboard box which has kept it in first class condition..
The original AVO meters date from 1923 but the model 8 with auto cut
out and decimal scale is a direct descendant of the Model
7:
Model 7-1936
Model 40-1940
Model 8-1951
8 Mk II-1956
8 Mk III-1960
8 Mk IV-1969
8 Mk V-1972 [new internal design]
8 Mk V1-1984
8 Mk VII-1998-2008 when production ceased
Inflation!
In 1953 the original Avometer model 8 cost �23.50 (see advert
below)
and in 1970 the Mk4 model cost �34.80 and the leather case �5.50. The
now no longer manufactured Avometer model 8 Mk7 was �585!!! However
you
should be able to purchase a reasonable second hand Mk2 or Mk3, have a
look at: http://www.ebay.co.uk/
What is it worth? You should be able to buy a model 8 MkIII
for �20- �30 The later models may appear in better condition but were
made with the cost of production in mind and you need to be aware
thathe repair of the later models can be more prolematic. Read the
comments at the bottom of the page before
considering the purchase of one of the later models.
Instructions on rear panel These are often very difficult to
read so here you will find Model
8 Mk II - Rear panel text. [kindly supplied an old electronics
technician]
How old is it? Most AVO meters can be dated from the last 3
or 4
digits of the serial number under the right hand end of the scale. The
serial number on later models have a self adhesive sticker on the right
hand side of the case.
These define the month and year of manufacture. For example No.
105043-360
would mean that this one which was bought new by my father was made in
March 1960 and 79168-468 which was given to me by Charles Husband was
made in April 1968.
Batteries Early models of the AVO 8 use the BLR121
Later models use the BLR154 type, both are 15volt and rated at
40mAh.
BLR121 Height: 36mm. Width: 15mm. Length: 26 mm.
BLR154 Height: 34mm. Diam: 15mm.
When Ever Ready stopped manufacture AVO came up with a novel
solution in the form of a miniature voltage converter [type VC1]
Although these are reputed to be as rare as 'hen's teeth' I have one of
these and you can have a look at it HERE
Where do you buy a replacement for the type BLR121 15 volt
Battery?http://www.maplin.co.uk/
then search for battery blr121 which cost cost � 9.99 when I last
looked. http://www.cellpacksolutions.com
is another source for batteries, they will even make up special
batteries to your own specification.
Or if you want to use readily available batteries and have a
soldering iron you can wire two 9 volt PP3 batteries in series with a
5v zener diode, insulate the metal cases and slide them in sideways
between the contacts for the correct battery. The connecting wires can
then be wedged behind the contacts. No guarantee, but it works for
someone who visited this page.
Another suggestion Purchase from your local "Pound Shop" a
card of assorted button cells and select ten of them (they need not be
the same size). Cut a small block of wood 34 mm long and drill an 8mm
hole through it. Cut a slot in one end and glue a thick piece of wire
into it as a contact. Find a small helical spring (mine was from an old
lamp holder) Take a narrow piece of masking tape and tape your ten
cells
into two bundles, insert them into the wood block with the spring in
between. You can stop the cells from popping out when inserting the
block with a thin piece of metal. Alternatively use the plastic casing
from a BLR21 and insert a new set of ten cells kept in place with
adhesive tape. Five 3volt CR2025 or similar lithium button cells can
also be used with a suitable spacer.
Repairs? A small company http://www.algo-sales.co.uk
are repair agents for Megger/Avo. Some of their calibration standards
are of extremely high quality and manufactured by Sullivan and
Cambridge. They also have some spares for old instruments, Avo meters
in particular if you ever need any.
Can I fix it myself?
One problem I have experienced and have been asked about in the past
is that the meter works on everything other than the resistance
range[s]. If the contacts for the fuse or either battery are corroded
the high resistance would prevent the instrument from being zeroed or
even reading at all. It may be that the contacts which are meant to be
closed on the resistance ranges are dirty and causing erratic readings.
To help understand the circuit I simplified the diagram click here
by omitting all the parts that do not come into play on the resistance
ranges. Although the diagram is based upon the Mk3 model the earlier
and later models are substantially the same. The batteries are housed
in the lower part of the case and the connections are made via pillars
with pointed ends which mate with leaf springs on the inside of the
case [the red lines on the diagram].
AVO Model8 battery and high voltage contact leaf springs.
[The four inner ones are for the batteries]
AVO Model8 Mk4 battery compartments and connections to meter.
After checking the contacts which mate with the batteries for
corrosion and the fuse and it's holder [not all models have a fuse] I
suggest you make sure that contacts between the leaf springs and the
pointed pillars are OK. If none of the above work the contacts on the
switches or the zero rheostat may be at fault. A piece of thin card
soaked in switch cleaner moved back and forth between the switch
contacts may solve the problem.
History AVO Ltd. used to be at Avocet House, 92-96
Vauxhall Bridge Road, London, SW1 and was a member of the Metal
Industries Group of companies. Though known for their Avometer general
purpose multimeter, they made a wide range of test gear including valve
testers. The Acton Lane works closed down at about that time - around
1986. The current range of products can bee seen at: http://www.avointl.com/ The firm
now called Megger Ltd is based at: Archcliffe Road, Dover, Kent, CT17
9EN, United Kingdom.
For further information have a look at my pages on the model 7
AVO and Test Meter No1 here and here
Click on image for details of
instruments and accessories
available in 1953
This advertisement is from 1953.
The
cost then was over twice the the average weekly wage of a male manual
worker. The equivalent today [2013] would be over �1000!
Here is a list of the main variations of
this meter
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 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
It is a
small World
John Hancock who worked for Instruments to
Industry Ltd of 8-18 Wyre
street Manchester noted the pencil markings on the rear of the Mark3
scale plate above as probably having been made by himself!
Whilst he repaired a range exotic industrial instruments such as
temperature transmitters, a thing with a temperature probe which
mechanically adjusted a pressure controlling device like the device on
top of a gas bottle. Fed with compressed air it returned a pressure
related to the probes temperature. A pressure gauge calibrated in
degrees C indicated temperature remotely. But of course there were lots
of DMM's Oscilloscopes, Clamp ammeters and AVO's galore. He comments as
follows:
Mainly 8 MKII and tropical variants both of which I found had needed
great efforts to kill.
MkIII's were thin on the ground which makes me surprised it's a MkIII I
think I recognise.
Mk4s were rare and I did not see enough to form an opinion.
Mk5 and up....hmm
Well I used to see lots because they kept
coming
back. The movements were always giving trouble and if it did not kill
itself by the magnet coming loose and trapping the coil, over voltage
would wrap the pointer round the end stop due to the cut out being very
insensitive. Add to that open circuits in the flexible wiring and baked
resistors. Well put it this way, I never wanted to own one. I do
however have a couple of MKII's
By the looks of the MKIII it either had
repairs or was returned for
calibration. The second date is pushing it a bit as it was around then
that I jumped ship and went to work at U.M.I.S.T. But there I serviced
several AVO's that I had seen in my previous job.
The premises were only a few minutes walk apart, so perhaps it came to
you and it's previous owner via U.M.I.S.T. (Charles Husband who
gave it to me lived in Sheffield which is not so far away). The more I
look at the label the more I feel it's my child like J which has
certainly changed over these many years.
