AVO model 8 MK2 user manual
Contents
1. To test resistance set the right hand switch at the range required the leads being connected across the unknown component Resistance is read directly on the Q 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 NOTE 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 Q 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 INSULATION RESISTANCE MEASUREMENT Two courses are open the first merely calling for a battery or other source of d c voltage in the order of 130V to 160V The left hand switch should be set at RESISTANCE with the right hand switch at INS and the meter leads should be connected to the b2. dia x23 such as Ever Ready or overseas Berec U 2 15V battery 1 37 14 such as Ever Ready 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 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 instrument is the provision of an automatic cut out which gives a very high degree of overload protection to the whole of the instrument The incorporation of 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 reset when the instrument
3. AO H TISgV L The Model 8 Universal AvoMeter Mk II WORKING INSTRUCTIONS General Description The meter is supplied complete with a pair of special rubber covered leads which are intended for attachment to the AvoMeter by means of its captive terminals The remote ends of the leads are fitted with spring clips which may be interchanged with the AVO Long Reach Safety Clips Mk 2 supplied with the instrument AVO Prodclips have been introduced to enable connections for test purposes to be made at what are normally inaccessible points on a chassis Examination will show that they are completely insulated with the exception of the jaws at one end which can be opened by compressing the stem into the body of the clip Rigid con nections to wiring can thus be made by this insulated device in complicated wiring systems where other types of larger clip could not be attached or if fixed might cause short circuits All tests except those on the 2 500V ranges make use of the pair of terminals at the base of the instrument The meter is extremely simple to use range selection in general being accomplished by means of 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 of the knife edge pointer to be made with great precision Limits of Accuracy Generally sp
4. REPLACEMENT OF INTERNAL BATTERY AND CELL 22 54 s gt be 24 9 MOVEMENT REVERSE CONTROL va 22 a 55 5 10 OvERLOAD PROTECTION w re 6 32 25 oe 10 OF INSTRUMENT 39 2 57 od 23 33 52 p li CURRENT MEASUREMENT 48 es ag 28 sg ee p 8 Da 11 VOLTAGE MEASUREMENT va 2 ig 2 at ll RESISTANCE MEASUREMENT 52 pi 25 p Br zi 12 INSULATION RESISTANCE MEASUREMENT bs 2 5 13 Low RESISTANCE MEASUREMENT 22 57 pe 13 DECIBELS 13 ACCESSORIES 14 D C VoLTAGE MULTIPLIER RESISTANCE RANGE EXTENSION UNIT TRANSFORMERS CONCLUSION i 23 27 ri si 48 15 CIrCUIT DIAGRAM OF THE MODEL 8 a z 2 5 2 16 COPYRIGHT No information or diagrams in whole or in part may be copied or reproduced without the prior permission in writing of Avo Limited 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 of its type in the world for in no other instrument can one find such a unique combi nation of 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 Televis
5. Post Office and Telegraph Undertakings 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 of 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 ku a FHE MODEL 8 AVOMETER MK H CONTENTS PAGE FOREWORD is vi F sa s sa 2 INTRODUCTION g oe 25 des de 5 TABLE RANGES 6 GENERAL DESCRIPTION oe 7 Limits OF ACCURACY 7 DESIGN AND CONSTRUCTION 2 23 8 RANGE CONTROLS z p 24 ss 8 THE MOVEMENT nc 9 SCALING 9
6. 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 operation 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 reset using direct pressure and without twisting the button the instrument lying face upwards WARNING Special care must be taken when using the instrument to service television receivers or other apparatus employing capacitors of large capacity for the inclusion of such components in a circuit may mean that very heavy peak currents may flow when the apparatus is switched on Such surges produce peaky wave form and although these peaks are of only a few milli seconds duration they may never the less puncture the instrument rectifier It is impossible to guard against this cause of damage by means of any form of cut out mechanism but instruments manufactured since July 1954 have been fi
7. 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 of voltage successive ranges are built up on the ratios of 2 1 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 a c and d c ranges 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 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 ohms 100 followed by ohms x 100 using in each case the adjuster to match the range In addition 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 Alcomax blocks associated with mild steel pole pieces Two phosphor bronze hair springs are fitted for the purp
8. 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 multi plied by the value of the resistance in thousands of ohms will give the developed voltage RESISTANCE MEASUREMENT 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 of the usual series circuit and successive ranges are on 100 1 ratio which permits of 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 deflection of the meter Before carrying 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 of the battery within the limits of adjustment The method of adjustment is described later Owing to the natu
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10. Model 8 Universal AVOMETER MK Il WORKING INSTRUCTION ANYG ur AVOCET HOUSE 92 96 VAUXHALL BRIDGE ROAD LONDON S W 1 ENGLAND MODEL 8 UNIVERSAL AVOMETER INSTRUCTIONS FOR USE ANYO urn AVOCET HOUSE 92 96 VAUXHALL BRIDGE ROAD LONDON S W 1 Telegrams Avocet London S W 1 ENGLAND Telephone VICtoria 3404 12 lines nm Q ax 8 FOREWORD For more than a quarter of a century we have been engaged in the design and manufacture of AVO Electrical Measuring Instruments Throughout that time we have consistently pioneered the design of modern multi range instruments and have kept abreast of and catered for the requirements of the epoch making develop ments in the fields of radio and electronics The success of our steadfast policy of maintaining high standards of performance in instruments of 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 of instances where the satisfac tion obtained from the performance of one of our instruments has led to the auto matic choice of other instruments from the AVO range This process having continued over long period of years has resulted in virtual standardisation on our products by numerous Public Bodies The Services Railway Systems and
11. attery The pointer should be brought to zero on the ohms scale by means of the adjuster marked ZERO Q 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 MEASUREMENT 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 of 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 difference of one decibel is about the minimum difference which can be appreciated when comparing two intensities For convenience the scale is marked in decibels 13 both positive and negative from reference point The difference in level between negative value on the dB scale and a positive one is the sum of the two i e the differ ence 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 rat
12. cts being arranged to make before break on adjacent ranges a feature which provides a factor of 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 transformer are also introduced Range Controls The left hand knob provides all the d c current and voltage ranges except 2 500V and the right hand knob the a c ranges except 2 500V 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 readings call for the left hand switch to be set for 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 RESISTANCE and the right hand one to the desired range 8 If the switches 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 small pointer indication however may result if d c current is passed through range but no harm can be done to the meter provided it is not at
13. eaking 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 of current successive ranges have been closely chosen to obviate the need for taking readings on very small deflections The instrument will produce its highest accuracy when used face upwards 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 126 of full scale value over effective range A C Voltage Up to 250V 2 25 of full scale value over effective range 25 2000 c s A C Current 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 of 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 manu facture of its various compo
14. gative 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 500V d c We do not recommend in such cases connecting the multiplier to the 2 500V d c terminal and pressing the moving coil reverse button notwithstanding the fact that the meter is at the earthy end of the circuit 25kV D C Multiplier A 25kV d c Multiplier is available for use in series with the meter set to its 10 d c range readings being made direct in kV on the 0 25 scale It is most 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 NOTE 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 s
15. io of 21 I this corresponds to a reduction of 8 in the indication 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 In the same way 12 should be added for an increase of 4 times on the range or 8 12 20 dB for an increase of 21 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 of an output transformer and that a reading of 9 dB is indicated corresponding to 12 5 on this range If now the output increases to say 40 volts necessitating change to the 100V range the pointer will indicate 7 on the dB scale The 4 I increase in the voltage range calls for an addition of 12 to the dB indica tion so that its true value represents 19 dB The increase over the original reading is 19 9 10 dB ACCESSORIES D C Voltage Multipliers IOkV D C Multiplier A 10 d c Multiplier has been developed mainly to enable tests to be carried out in television circuits The multiplier should be connccted 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 ne
16. ion Engineer that this new instrument has been primarily produced The Model 8 AvoMeter Mk II has the high d c voltage sensitivity of the High Resistance Avo Meter Models I and 2 but in addition provision is made for the measurement of 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 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 reasonable amount of care and attention not forgetting the removal of exhausted batteries this instrument will give lasting satisfaction Qrun eusa xa yua sutuo 7 O 5 QZ 000 0 paureruoo jjas 4 lt 0007 surqo 000 007 0 LGreas pitu 000000 07 0 8 002 0 51593 211 AST AST 01 j A OL W1I05T A ST SC A 001 vu 001 A 001 01 A OST VI OST VU 001 A 005 ST A 00011 VI A 00011 Ol A OOST V OI A 005 yamp 8 Ju rmo 5
17. ll 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 ImA 1 000 ohms per volt The 25V 10V and 2 5V a c ranges consume 4 10 and 11 40mA respectively at full scale deflection The meter maintains high degree of accuracy for audio frequency tests up to 10 on ranges to 250V Whilst discussing the problem of 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 resistance between the scource and the point at which measurement is to be made it is possible for the actual voltage to be higher normally than when the meter is connected All current consuming voltmeters however sensitive draw current to varying degrees from the circuit under test thus causing higher volts drop in the resistances mentioned and thereby causing the voltage to fall at the point of measurement Owing to the high sensitivity of 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 of where it might be taken into account is in the measurement of e h t voltage on a television set or the tapping on a potential divider where the resistances are so high as to be comparable with the resistance of the meter on the range in use It is generally possible
18. n set to 100 It is necessary to connect the meter up to the secondary of 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 for 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 amp O E H MIN SYLYNOAV 8 TAIGON HHL JO KVYDVIG INONDO 5 HOLIMS 27 2 NONNO2 DV p g HOLIAS ID Q HOL1IAS OS V SSS NI N AOOS AO v Vuool vor wsz AOL AOOO 125 155 nose ase AST os ose vwo vi OV fw ase asz nose O O Q Q Q O O Q Q Q Q O 2 2 Q O 50 O 32NV1SIS33 2 o 4NO LND 2 BIN SN SN SH 5 353 1 Di s siloogosi 649 OSL 3 00000000000000000
19. nents 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 of 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 of 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 of measurement but the user should recognise the possibility of 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 of generous and robust design the conta
20. o 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 the order of 4V on all d c ranges except the 50 microamp range which has a drop of 125 milli volts In the case of a c it is less than 1V on all ranges Standard meter leads have a resistance of 0 02 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 MEASUREMENTS When measuring voltage it is necessary to set the appropriate range of or and connect the leads across the source of voltage to be measured If the expected magnitude of 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 if 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 if possible 2 On d c ranges the meter consumes only 50 microamps at fu
21. ose of conveying current to the moving coil and to provide control ling 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 of 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 of the a c ranges Replacement of Internal Battery and Cell Inside the cover under the carrying strap is mounted 15 battery and a 12 cell These batteries should be examined from time to time to ensure that the 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 When replacing batteries the 12 cell and the 15 battery must be inserted with the poles to match the markings of polarity inside the battery box Replacements 1 5 cell 13
22. re 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 of the indication around deflections corresponding to 10 and 90 of full scale deflection Resistance test should never be carried out on components which are already carrying current On 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 voltage is applied quite apart from its magnitude Such cases include electrolytic capacitors and rectifiers When measuring the leakage resistance of an electrolytic capacitor the negative lead from the meter should be connected to the positive terminal of the capacitor and the ohms x 100 range employed 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 range adjust to zero by means of the knob marked ZERO 4 On the Q 100 range adjust to zero by means of the knob marked ZEROQ 100 5 On the Q x 100 range adjust to zero by means of the knob marked ZERO 100
23. tted with a surge suppression rectifier SAI across the main rectifier to give the maximum protection which can be devised OPERATION OF INSTRUMENT 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 Prodclips or clips as required should be connected to the lower pair of meter terminals in all cases except when measuring voltages over 1 000V see next paragraph 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 not however switch off by rotating either of the knobs to a blank position 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 transferred 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 MEASUREMENT T
24. witching to facilitate tests The device should be connected to the lower terminals on the meter For high resistance the meter is set to the 50 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 ZERO 100 knob The Unit switch should then be rotated to TEST and the reading on the ohms scale noted Its value is that shown multi plied by 1 000 corresponding to a range of 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 Q 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 21 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 Cell 13 dia x 23 such as Ever Ready or overseas U 2 Four 30V batteries 1 4 x 8 x2 such as Ever Ready B 123 Transformers Transformers as used on the Model 7 AvoMeter are equally applicable for use with the Model 8 whe
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