AVO Avometer 8 Service manual

*

H F a 2 < t 7

THE AUTOMATIC COIL WINDER & ELECTRICAL EQ UI PMENT CO., LTD.,

AVOCET HOUS E, 92-96 VAUXHALL BRIDGE RO AD, LO N D O N , S.W.I .

THE

MODEL 8 UNIVERSAL

AVOMETER

INSTRUCTIONS FOR USE

T HE AUTOMATI C C OI L W INDER & ELEC TRICAL E Q UIPMENT CO ., LTD.

A VOCET HOUSE, 9 2-96 VA U X HALL BR IDGE ROAD, LO NDO N, S.W .l.

Tele ph o ne : Vi ctor i a 3404 (9 lines). Tele g ra m s : Au t o wind a, Sow es t, London .

For more than a quarter of a century we have been engaged in the design and manu
facture of “ AVO ” Electrical Measuring Instruments. Throughout that time we have
consistently pioneered the design o f modern multi-range instruments and have kept
abreast of, and catered for, the requirements o f the epoch-making developments in the
fields o f radio and electronics.

The success of our steadfast policy of maintaining high standards of performance in
instruments o f such wide versatility, and making such instruments available at reasonable
cost, is reflected in the great respect and genuine goodwill which AVO ” products enjoy
in every part of the world.

It has been gratifying to note the very large number o f instances where the satisfaction
obtained from the performance of one o f our instruments has led to the automatic choice
of other instruments from the “ AVO ” range. This process, having continued over a

long period o f years, has resulted in virtual standardisation on our products by numerous
Public Bodies, The Services, Railway Systems, and Post Office and Telegraph Under
takings throughout the world.

Our designers have thereby been encouraged to ensure that new instruments or

accessories for inclusion in the “ AVO ” range fit in with existing “ AVO ” apparatus
and serve to extend the usefulness of instruments already in being. Thus, the user who

standardises on “ AVO ” products will seldom find himself short of essential measuring
equipment, for, by means o f suitable accessories, his existing equipment can often be
adapted to most unusual demands.

It is with pleasure that we acknowledge that the unique position attained by “ AVO ”

is due in no small measure to the co-operation of so many users who stimulate our

Research and Development staffs from time to time with suggestions, criticisms, and even
requests for the production of entirely new instruments or accessories. It is our desire
to encourage and preserve this relationship between those who use “ AVO ” instruments
and those who are responsible for their design and manufacture, and correspondence is
therefore welcomed, whilst suggestions will receive prompt and sympathetic consideration.

Ϊ MO DEL 8 AVOMETER

THE

3

CONTENTS

Page

F or ew or d .. .. .. .. .. .. .. . · .. · · · · · 2

Intr odu cti on .. .. .. .. .. .. .. . · .. · · · · · · 5

T able of R anges .. .. . . .. .. .. .. ·. · · · · · · 6

G enera l D escr iption

L imits of A c cur acy

D esign a nd C onst ruct ion .. .. .. .. .. . . .. . . .. 8

R an ge C ont rols .. .. .. .. .. .. .. .. . · · · · · 8

T he M ove ment .. .. .. .. .. . . .. . · · · · · · · 9

Sca ling .. .. .. . . . . .. .. .. .. · · · · · · · · 9

R eplac ement of Inter nal Batte ry an d C ell .. .. . . .. . . . . .. 9

M ove ment R everse C o ntr ol .. .. . . .. .. . . .. . . . ■ 10

O v erload Pr otectio n .. . . .. .. .. .. .. 10

O p eration of Inst rume nt .. . . .. . . .. .. .. .. · · · · 11

C urre nt M ea surement .. .. .. .. .. .. .. .. · · · · 11

Vo ltage M easurem ent . . .. .. .. . . .. .. .. .. .. · · 11

R esistance M easur ement . . .. . . . . .. .. .. .. . . ·· 12

Insul ation R esistance M easu rement .. . . .. .. .. .. .. .. 13

L o w R esistance M eas urement .. . . .. .. .. .. .. . . .. 13

D ecibels .. .. . . .. .. .. .. .. .. .. ■· ·· ·· 13

A ccessories . . .. .. .. .. . . .. .. .. .. · · 14

D. C. Volt age M u ltipl ier.

R esistance R a nge E xte nsion U nit .

T rans former s.

C oncl usion .. . . .. .. . . . . .. .. .. .. .. . . 15

C irc uit D iagram of th e M odel 8 U niversa l A v oM eter . . .. .. .. . . 16

COPYRIGHT

No information or dia g r a ms in whol e or in part may be cop ied or reprodu ce d w it h o u t th e prior

perm ission in writin g of The Aut om atic Coil Wind e r & E l ectri c al E q u ip ment C o., Ltd.

4

INTRODUCTION

Since its conception in 1923, the AvoMeter has maintained a distinct lead upon all

its competitors, and can today quite rightly be termed the most popular instrument o f its
type in the world, for in no other instrument can one find such a unique combination o f
ranges and comprehensive automatic overload protection, in addition to a high degree
of accuracy, reliability and simplicity of use.

Much time and thought is continually devoted by our design department to the

improvement of our products and it is for the Electronic Radio and Television Engineer
that this new instrument has been primarily produced. The Model 8 AvoMeter has the
high D.C. voltage sensitivity of the High Resistance AvoMeter Models 1 and 2, but in
addition, provision is made for the measurement of A.C. current. A further useful
feature which has been incorporated is a push button change-over switch which enables
the direction of the current through the moving coil to be reversed, thus obviating the
necessity of changing leads when working with D.C. voltages and currents which may
be either positive or negative in respect to a basic test position. The excellent qualities
of previous models including the “ AVO ” automatic cut-out have been retained, and
we have great confidence that given a reasonable amount of care and attention, not
forgetting the removal o f exhausted batteries, this instrument will give lasting satisfaction.

5

p

TABLE OF RANGES

D. C . Vo lta ge

D .C . C u rr e n t

2,500 V.

1,000 V.

250 V. 100 mA .

100 V.

25 V.

10 V.

2.5 V.

250a A. 10 v.

50μΑ. 2.5 V.

0—200 M e go h m s—with exte rnal voltage.

0 -2 0 „ )

0—200,000 O hm s - Self-contained.
0—2,000 O hm s )
0—2.5 Oh m s with External Unit.

A.C . V o lta ge

10 A.

1 A.

2,500 V.

1,000 V.

250 V.

10 mA .

1 m A . 25 V.

Resista nce.

100 v .

A.C . C u rr e n t

10 A.

2.5 A.

1 A.

100 mA.

> 6 φ £ «s

T he

Model 8 Universal AvoMeter

WORKING INSTRUCTIONS

General Description

The meter is supplied complete with a pair o f special rubber-covered leads, the hook

ends of which are intended for attachment to the AvoMeter by means o f its non-loss

terminals, A similar pair o f leads fitted with plugs which fit the sockets in the top
of the terminals can be supplied as an optional extra. The remote ends of the leads
are fitted with spring clips, which may be interchanged with either the push-on type
prod or the “ AVO ” Long Reach Safety Clip, both of which are located upon the
battery box cover o f the instrument.

The “ AVO ” Long Reach Safety Clip (patent applied for) has been introduced to
enable connections for test purposes to be made at what are normally inaccessible points
in a chassis. Examination of the safety clip will show that it is completely insulated
with the exception o f the jaws at one end, which can be opened by compressing the stem
into the body o f the clip. Rigid connections to wiring can thus be made by this insulated
device in complicated wiring systems where other types of larger clip could not be fixed,
or if fixed might cause short circuits.

All tests, except those on the 2,5000V. ranges, make use of the pair of terminals at the

base o f the instrument.

The meter is extremely simple to use, range selection in general being accomplished
by means o f two switch knobs.

A clearly marked 5" scale has uniformly divided graduations to match 100 and 250
scale markings, and in addition there is an ohms scale and one for decibels. An anti
parallax mirror permits readings o f the knife edge pointer to be made with great precision.

Limits of Accuracy

Generally speaking, the highest percentage accuracy on current and voltage ranges is
obtainable at the upper end of the scale, but on resistance ranges it is better towards the
centre of the scale. In the case of voltage measurements, which are more frequently

taken than those o f current, successive ranges have been closely chosen to obviate the

need for taking readings on very small deflections.

7

The instrument will produce its highest accuracy when used face upwards, in which

position it has been calibrated.

D.C. Voltage. 2% of indication between full-scale and half-scale deflection.
Below half-scale deflection, 1% of the full-scale value.

D.C. Current. 1% o f full-scale value over effective range.

A.C. Current and Voltage. (25-2000c/s.) 2.25% of full-scale value over effective

range.

The definition of “ effective range ” set down in the British Standard Specification

89/1954 is as follows, when related to the AvoMeter :—
D.C.—from 0.1 of scale-range to full-scale value.
A.C.—from 0.25 of scale-range to full-scale value.

It will be noted that with the exception of the D.C. voltage ranges, the instrument
meets the requirements laid down in Section 6 o f the British Standard Specification
89/1954 for 5" (127mm.) scale-length Industrial Portable Instruments. In practice,
the Model 8 is well within the above limits, due to the great care taken in the manufacture
of its various components, and to the fine initial calibration.

Inasmuch as rectifier moving coil instruments give readings on “ A.C. ” proportional
to the mean and not the R.M.S. value o f the wave form with which they are presented,

they depend for their accuracy not only upon their initial calibration, but also upon the
maintenance of a sinusoidal wave form. Since the form factor (R.M.S. value divided
by mean value) o f a sine wave is 1-11, this has been taken into account in calibrating the

meter, which does, therefore, indicate R.M.S. values on the assumption that the normal

sine wave will be encountered. Generally speaking, considerable wave form distortion
can occur without appreciably affecting the form factor and resulting accuracy o f

measurement, but the user should recognise the possibility o f some error when using

distorted wave forms, squarish wave shapes producing high readings, and peaky ones,

low readings.

Design and Construction

The instrument consists of a moulded panel on the inside of which are mounted the
whole of the switching apparatus, resistances, shunts, transformer, rectifier, etc., together
with the movement. The panel fits into a robust moulded case, the joint being rendered
completely dust proof, whilst a carrying strap is provided to facilitate portability. The
main switching is accomplished automatically by means of two knobs which indicate
on the engraved panel, the range in use. These switches are o f generous and robust
design, the contacts being arranged to “ make ” before “ break ” on adjacent ranges ;
a feature which provides a factor o f safety in use.

When the instrument is set for operation on D.C., the moving coil is associated with
a universal shunt and series multipliers, whilst on A.C., a full-wave rectifier and trans
former are also introduced.

Range Controls

The left-hand knob provides all the D.C. current and voltage ranges (except 2,500 V.)
and the right-hand knob the A.C. ranges (except 2,500 V.) and also the resistance ranges.
These knobs are electrically interlocked so that D.C. readings can only be made after
the right-hand switch has been set to D.C., and the left-hand switch to the range selected.
A.C. readings call for the left-hand switch to be set for A.C. (it must not be left at

RESISTANCE) and the right-hand switch at the range required. Resistance tests
require the left-hand switch to be set to RES ISTANCE and the right-hand one to the
desired range.

8

If the switches are inadvertently left to actual ranges simultaneously, there is no
circuit through the meter, and it is thereby safeguarded against accidental damage or
misleading readings.

It is possible to determine whether a source is A.C. or D.C., since A.C. will not produce
pointer indication when the meter is set for D.C. measurement. A small pointer
indication, however, may result if D.C. current is passed through an A.C. range, but
no harm can be done to the meter provided it is not at the same time grossly overloaded.

The main ranges are engraved on the panel around the switches, and arrow heads on
the knobs indicate the actual range selected. In the case o f voltage, successive ranges
are built up on the ratios o f 2-5 : 1 and 4:1, but in the case of current a wide coverage

has been chosen instead and the 10 : 1 ratio in general is followed. The 2,500V. ranges
A.C. and D.C. are available by means of the two special terminals so marked.

Extremely wide coverage in resistance has been achieved by having a fundamental
range as marked on the scale, together with ranges of X 100 and -i- 100 to supplement
it. Before carrying out resistance tests, the meter should be adjusted for the state of the
batteries. It is merely necessary to join the leads together and adjust to zero in the
following sequence : Ohms X 1 ; Ohms -i- 100, followed by Ohms X 100, using in
each case the adjuster to match the range.

In addition, a 200 megohm range marked “ INS ” is available, using an external D.C.

voltage source.

The Movement

The moving coil consists of an aluminium former wound with copper wire and

supplemented with Constantan in order to reduce temperature error. It is pivoted on
hardened and highly polished steel pivots between conical spring-loaded jewels, and
swings in a gap energised by two powerfully magnetised and aged alnico blocks associated
with mild steel pole pieces. Two phosphor bronz hair springs are fitted for the purpose
of conveying current to the moving coil, and to provide controlling torque. A knife
edge type of pointer is fitted enabling very fine readings to be taken, whilst the whole
movement is perfectly balanced and reasonably damped so that the pointer quickly
comes to rest.

Scaling

The scale plate has three main sets of markings, each o f approximately 5" length, the

outermost being for resistance measurement and is marked 0—200,000 ohms. The
second is for current and voltage (both A.C. and D.C.) and is marked 0— 100, with
divisions approximately 1^ mm. apart. The third scale, calibrated 0—250, has 50
divisions, and is so used for current and voltage measurements. In addition there is a
decibel scale marked from — 15 Db. to + 15 Db., which can be used with any o f the
A.C. ranges.

Replacement of Internal Battery and Cell

Inside the cover, under the carrying strap is mounted a 15V. battery and a l^V. cell.

These batteries should be examined from time to time to ensure that their electrolyte

is not leaking and damaging the instrument. This condition will generally occur only

when the cells are nearly exhausted. If it is known that the meter is going to stand

unused for several months, it is preferable that these batteries should be removed to

prevent possible damage.

9

When replacing batteries, the l^V. cell and the 15V. battery must be inserted with

the poles to match the markings of polarity inside the battery box.
Replacements : 1*5 V. cell, If " dia. X 2 §" , such as Ever Ready (or overseas, Berek) U.2.

15 V. battery, 1^-" X §" X \ \ ' \ such as Ever Ready B.121.

Movement Reverse Control

It sometimes happens that D.C. voltages may be required both positive and negative
to a reference point, or the direction of flow may be reversed. In order to simplify the
matter of lead alteration, a movement reverse press button (REV. M.C.) is provided.

It should be noted that the polarity marked on the terminals is for normal use and does

not apply when the button is pressed.
Overload Protection

Apart from the ability to do its job, one of the most attractive features of the instru
ment is the provision o f an automatic cut-out which gives a very high degree o f overload
protection to the whole of the instrument. The incorporation o f this device will be
found to be of particular value when conducting experimental work, for it imparts to
the user the feeling of mental ease and confidence. When conducting experimental
work with conventional moving coil meters, these can be easily ruined by inadvertently
applied overloads, whereas the AvoMeter is so well protected that it can withstand
considerable mishandling.

If an overload is applied to the meter, the cut-out knob springs from its normal
position in the panel, thus breaking the main circuit, and this knob has only to be
depressed to render the instrument again ready for use. It is important to note that the
cut-out should never be re-set when the instrument is connected to an external circuit,
whilst the fault which caused the overload should be rectified before the meter is
reconnected.

The mechanism is brought into play by the moving coil coming into contact with a

trigger just beyond its full scale position. There is, in addition, a second release at the

zero end, so that the cut-out is tripped if the meter is overloaded in reverse.

Although the overload mechanism gives almost complete protection to the meter, it
cannot be guaranteed to fulfil completely its function in the very worst cases of misuse,
such as the mains being connected across the meter when set to a current range. It
should be noted that mechanical shock to the instrument will sometimes trip the cut-out
mechanism. The cut-out should be re-set, using direct pressure and without twisting
the button, the instrument lying face upwards.

WARNING

Sp e cial care must be ta ke n when using the instrum e n t to servi c e television r e ce i v er s

or oth e r a p p a ra t u s e mplo y in g conden ser s of l a rg e capa city, fo r the inclusion of such com

pon en ts in a circuit may m ean that very heavy peak currents may flow when th e ap par a t us

is switc h e d on. Such su rg e s pr odu c e a peaky wave f orm , an d alt hou gh these pe ak s are
of only a few mi l l iseconds dur a ti o n , the y ma y, neve r - the-le s s , p u n c ture the instru m en t
rectifier. It is impo s sib l e to gu a rd a g a inst this cau se of da mag e by me a n s on any for m of
cut-ou t mech a n i sm, but in str u m e n ts man u f a c t u r e d sinc e Ju l y 1954 have been fitt ed with a
surge su ppr esio n recti fie r (SAI) acr o s s the mai n rectifi e r , to give the max i mu m pr o t e ctio n
which can b e d evise d.

10

OP ER A T IO N OF IN S T R U M E N T

The meter is intended for use horizontally. Should it happen by any chance that the

pointer is not on zero, it may be so set by means of the screw head on the panel.

The leads fitted with prods or clips, as required, should be connected to the lower

pair of meter terminals in all cases except when measuring voltages over 1,000 V. (see

later).

When measuring current or voltage, ensure that the instrument is set to match the
type of source to be measured (either A.C. or D.C.) and then choose a suitable range
before connecting up to the circuit under test. When in reasonable doubt, always switch
to the highest range and work downwards, there being no necessity to disconnect the

leads as the switch position is changed. Do no t , how e ver, swit ch off by rotating either

of the knobs to a bla n k po s i t i o n . If the voltage should exceed 1,000V., the instrument

should be set to measure 1,000V. as described above, but the negative lead should be
transfered to the appropriate 2,500V. terminal.

The instrument is flash tested at 6,000V. A.C., but should the meter be used with
accessories on circuits in excess of 2,500V., it should be kept at the low potential end
of the circuit (near earth potential). If this procedure cannot be adopted other suitable
safeguards must be applied.

CURRENT MEA SU RE M EN T

To measure current, the instrument should be set to a suitable A.C. or D.C. range,

and then connected in series with the apparatus to be tested.

Generally speaking, the power absorbed in the instrument is negligible, but in cases
of low voltage heavy current circuits, the inclusion of a meter may reduce the current
appreciably below the value which would otherwise prevail. The potential drop at the

meter terminals is in the order o f £V. on all D.C. ranges, except the 50 microamp range
which has a drop o f 125 milli-volts. In the case of A.C., it is less than JV. on all ranges.

Standard meter leads have a resistance of 0 Ό2 ohms per pair.

In certain cases, care should be taken to ensure that the circuit is “ dead ” before

breaking into it to make current measurements.

VOLTAGE M E AS U R E M EN T S

When measuring voltage, it is necessary to set the appropriate range o f “ A . C . ” or
“ D . C .” and connect the leads across the source of voltage to be measured. If the
expected magnitude o f the voltage is within the range of the meter, but its actual value is

unknown, set the instrument to its highest range, connect up and i f below 1,000V. rotate
the appropriate selector switch, decreasing the ranges step by step, until the most suitable
range has been selected. Great care must be exercised when making connections to a
live circuit, and the procedure should be entirely avoided i f possible.

On D.C. ranges, the meter consumes only 50 microamps at full scale deflection, this

sensitivity corresponding to 20,000 ohms per volt. In the case of A.C. ranges from

100V. upwards, full scale deflection is obtained with a consumption of 1mA. (1,000 ohms

per volt). The 25V., 10V., and 2-5V. A.C. ranges consume 4, 10 and 40mA. respectively

11

at full scale deflection. The meter maintains a high degree of accuracy for audio

frequency tests up to 10 Kc/s on ranges up to 250V. A.C.

Whilst discussing the problem o f measuring voltage, it would be well to draw attention
to the fact that in certain circuits where the current is limited because of the presence
of a resistance between the source and the point at which a measurement is to be made,
it is possible for the actual voltage to be higher normally than when the meter is con
nected. All current consuming voltmeters, however sensitive, draw current to varying
degrees from the circuit under test, thus causing a higher volts drop in the resistances
mentioned, and thereby causing the voltage to fall at the point o f measurement.

Owing to the high sensitivity o f the Model 8 on its D.C. ranges, this effect is unlikely
to be of importance except in a very few instances. A practical example o f where it
might be taken into account is in the measurement of EHT voltage on a television set
or the tapping on a potential divider, where the resistances are so high as to be com
parable with the resistance of the meter on the range in use. It is generally possible to
use a meter on a higher range than absolutely necessary, and in such a case the higher
meter resistance causes less disturbance than would otherwise be the case. At the same
time adequate pointer deflection for reasonable accuracy should be attained.

When it is essential to obtain an accurate indication of the voltage developed across a
high resistance it is sometimes preferable to insert the meter in series with it, and to
measure the current flowing. The reading given upon the meter, in milliamps, multiplied
by the value of the resistance in thousands of ohms, will give the developed voltage.

RESIS TAN CE ME A SU R EM E NT

There are three self-contained ranges covering from 0-5 ohms to 20 megohms, and
provision is also made for both upward and downward extension of these limits. The
self-contained ranges make use o f the usual series circuit, and successive ranges are on

100 : 1 ratio, which permits o f very wide coverage with three ranges.

On resistance ranges, the meter must not merely start from its normal instrument
zero, but must have, in addition, a resistance zero corresponding to the full scale deflec
tion of the meter. Before earring out tests for resistance, a check, and if necessary
adjustment, should be carried out to ensure that when the leads are joined together, the
meter actually indicates zero ohms, irrespective of the condition o f the battery (within
the limits of adjustment). The method of adjustment is described later.

Owing to the nature of the scale, it is not easy to define the accuracy, but it should be

within 3% of the reading about centre scale, increasing up to about 10% o f the indication

around deflections corresponding to 10% and 90% of full scale deflection.

Resistance tests should never be carried out on components which are already
carrying current.

On the three ranges which utilise the internal source of voltage, it should be remem

bered that a positive potential appears at the negative terminal of the instrument when
set for resistance tests. This fact may be important because the resistance of some
components varies according to the direction of the current through them, and readings,
therefore, depend upon the direction in which the test volage is applied, quite apart
from its magnitude. Such cases include electrolytic condensers and rectifiers.

When measuring the leakage resistance of an electrolytic condenser, the negative lead

from the meter should be connected to the positive terminal o f the condenser, and the

ohms X 100 range employed.

12

Before making resistance tests the pointer should be adjusted to zero in the following

sequence :—

1. Set left-hand switch at “ RESISTANCE

2. Join leads together.

3. On the SI range, adjust to zero by means of the knob marked “ ZERO SI

4. On the “ SI -f- 100 ” range, adjust to zero by means o f the knob marked “ ZERO

SI 100 ”

5. On the “ SI X 100 ” range, adjust to zero by means of the knob marked “ ZERO

SI X 100”.

To test a resistance, set the right-hand switch at the range required, the leads being

connected across the unknown component.

Resistance is read directly on the “ SI ” range, but indications should be divided or

multiplied by 100 on the other two ranges.

If on joining the leads together it is impossible to obtain zero ohms setting, or if

furthermore the pointer position will not remain constant, but falls steadily, the internal

battery or cell concerned should be replaced. It is important that a discharged unit
should not be left in the instrument, since the electrolyte might seep through and cause
damage to the meter.

NOT E. It can so happen that a 15 volt battery may age in such a manner that although

it indicates a potential of 15 volts, its internal resistance has increased so much that
some loss of accuracy can occur on the high resistance range ( Λ X 100). If the battery
has been in use for some time, or if errors are suspected on the high resistance range, it is
worth while removing the battery and checking its short circuit current on the 100mA

D.C. range. If the battery fails to give a reading greater than 5mA. it should be

discarded.

INSULATI ON RESISTANCE ME ASU REM E NT

Two courses are open, the first merely calling for a battery or other source of D.C.
voltage in the order o f 130V. to 160V. The left-hand switch should be set at “ RE
SISTANCE ” with the right-hand switch at “ INS ” and the meter leads should be
connected to the battery. The pointer should be brought to zero on the ohms scale
by means of the adjuster marked “ ZERO SI X 100 ”. To test, connect the unknown
resistance in series with the meter and its value will be that shown on the ohms scale
multiplied by 1,000. Resistances up to 200 megohms can, therefore, be read on this
range.

The alternative method makes use of the “ Model 8 Resistance Range Extension
Unit,” described later.

LOW RESISTANCE MEASURE ME N T

The meter setting marked L.R. is for use with the Model 8 Resistance Range Extension

Unit. The method of use is described in the section upon accessories.

DECIBELS

The decibel scale can be used with any o f the A.C. current or voltage ranges. It has
a logarithmic scale shape and is useful in so far that it gives a measurement closely
related to the impression of aural intensity in sound reproduction apparatus. A differ
ence of one decibel is about the minimum difference which can be appreciated when
comparing two intensities. For convenience, the scale is marked in decibels both

13

positive and negative from a reference point. The difference in level between a negative
value on the Db. scale and a positive one is the sum of the two, i.e. the difference between
— 5 Db and + 6 Db. is 5 + 6 = 11 Db.

It will be appreciated that when changing from one current or voltage range to the
next higher, the pointer indication will fall, although input is kept constant. For a
current or voltage range ratio o f 2 \ : 1 this corresponds to a reduction o f 8 in the indi

cation on the Db scale. It follows, therefore, that 8 should be added to the reading
every time an increase of 2\ times takes place on the range. Tn the same way, 12 should

be added for an increase o f 4 times on the range, or 8 + 12 = 20 Db. for an increase of

2\ X 4 10 times in the range ratio.

The following might serve as an example : Suppose that the meter is connected on the
25V. A.C. range across the primary o f an output transformer and that a reading o f

+ 9 Db is indicated (corresponding to 12-5V. on this range). If now the output

increases to say 40 volts, necessitating a change to the 100V. A.C. range, the pointer

will indicate + 7 on the Db scale.

The 4 : 1 increase in the voltage range calls for an addition of 12 to the Db indication,
so that its true value represents + 19 Db. The increase over the original reading is

19 — 9 = 10 Db.

ACCESSORIES

D.C. Voltage Multipliers

10k V D.C . Multip l i e r

A 10 kV. D.C. Multiplier has been developed mainly to enable tests to be carried
out in television circuits. The multiplier should be connected in series with the meter
on its 2,500V. D.C. range, in which state maximum consumption on measurement

cannot exceed 50 microamps, and may be considerably less. It is recommended that

the meter is kept as near earth potential as possible, and the multiplier used at the high
potential end, e.g., when measuring an E.H.T. voltage where the negative line is Earthy,
the multiplier should be connected between the point of positive potential and the

positive terminal of the meter, the negative lead being connected to the terminal marked
2,500 V. D.C.— . We do not recommend, in such cases, connecting the multiplier to
the 2,500 V. D.C.— terminal and pressing the moving coil reverse button, notwith

standing the fact that the meter is at the Earthy end of the circuit.

25kV D.C . Mu l t iplier

A 25kV D.C. Multiplier is available for use in series with the meter set to its 10V D.C.
range, readings being made direct in kV on the 0-25 scale. It is m ost important to
ensure that the meter is kept in the earthy end of the circuit and the multiplier connected
to either the positive or negative terminal whichever is at high potential. This method
of connection to get forward pointer indication with the meter earthy is recommended
as we do not think it desirable to use the moving coil reverse button when measuring
high voltage.

In general we recommend that neither the meter, multiplier nor leads are handled
whilst high voltage tests are in progress, and a special lead is provided with the multiplier
for connection to the high potential point.

NO TE, The 2,500V D.C. range is not employed when using this multiplier.

14

Resistance Range Extension Unit

This device enables the meter to be used for both high and low resistance measure

ments. It is complete with batteries (except in some instances) and switching to facilitate

tests. The device should be connected to the lower terminals on the meter.

For high resistance the meter is set to the “ SI x 100 ” position, the Unit switch at

the “ SET ” position and the unknown resistance should then be connected to the

High ” terminals. Adjustment to full scale deflection should be performed by means

of the “ ZER O S I X 100 ” knob. The Unit switch should then be rotated to “ TEST ”,

and the reading on the ohms scale noted. Its value is that shown multiplied by 1,000,
corresponding to a range o f 200 megohms.

On the low range the Unit switch should be placed at “ SET ” , the unknown resistance

connected to the “Low” terminals and adjustment to full scale deflection carried out

by means of the “ ZERO SI 99 knob. The Unit switch should then be moved to position
marked “ TEST ” and the pointer deflection on a uniformly divided scale noted. Full
scale deflection corresponds to 2 \ ohms.

In order to avoid discharging the batteries, immediately tests have been completed
the test leads and resistance should be removed from the unit, its switch set to the “ low
set ” position, and the unit disconnected from the meter.

Replacements : 1*5 V. Cell—l f "d i a. X 2f ", such as Ever Ready U.2, or Berek U.2 (out

side the United Kingdom).

Four 30 V. batteries—1^-" X f" X 2-^-", such as Ever Ready B.123

Transformers

Transformers as used on the Model 7 AvoMeter are equally applicable for use with
the Model 8, when set to 100 mA. A.C. It is necessary to connect the meter up to the
secondary o f the transformer before current is passed through the primary, and care
should be taken that the cut-out is in position. If this course is not followed, quite a
considerable voltage will appear at the secondary terminals, if current passes through the
primary. Transformers of 50 amp., 100 amp., 200 amp., and 400 amp., are available.

CONCLUSION

Due to the high operational standards maintained throughout our organisation, and
the close limits within which we work, breakdowns are comparatively rare, and can often
be traced to transit damage or careless handling, for which the Company cannot be held
responsible. Should you at any time have to return your instrument to the Company
for repair, pack it carefully and enclose a note informing our engineers of the faults
which you have found.

E. & O.E.

15

r

Ο C. + i A C. COMMON D. C.-J A C SWITCH RANGES

CIRCUIT DIAGRAM OF THE MODEL 8 AVOMETER