## Thereby forming what is known as a ring type shunt the

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total "ring" resistance value being 375 ohms.

The resistance values of the 75 ohm shunt resistors are determined by multiplying the total "ring" resistance by the full scale current of the meter, dividing the result by each range value, in turn, from the common terminal, and subtracting the sum of the preceding values from each newly-determined value.

When the "ring" value of 375 ohms is multiplied by the full scale sensitivity value of 0.001 ampere, 0.375 is the result, into which each range value is divided, in turn, for determining the required shunt values. For example, the shunt value for the 1.250 ampere range is determined by dividing 1.250 into 0.375, giving a value of 0.3 ohm for that range.

For the 500 milliampere range, 0.500 ampere is divided into 0.375, giving a value of 0.75 ohm for the 500 milliampere range; but since there already is a value 0.3 ohm for the preceding range, it is necessary to subtract 0.3 ohm from 0.75 ohm, leaving a value of 0.45 ohm for the second section of the shunt.

For the 250 milliampere range, 0.250 ampere is divided into 0.375, giving a value of 1.5 ohms for the 250 milliampere range; but since there already is a value of 0.75 ohm for the two preceding ranges, it is necessary to subtract 0.75 ohm from 1.5 ohms, leaving a value of 0.75 ohm for the third section of the shunt. The shunt sections for the other ranges are determined in a similar manner, and can be checked by Ohm's law.

For example, the shunt value of 0.3 ohm for the 1.250 ampere (1,250-milliampere) range is in parallel with the remaining 374.7 ohms of the "ring" circuit, which when multiplied by the meter current of 0.001 ampere, produces a potential drop of 0.3747 volt. With 0.001 ampere going through the meter, the remaining value of 1.249 amperes will be going through the 0.3 ohm shunt, producing potential drop of 0.3 times 1.249 or 0.3747 volt. Since the potential drop across both parallel paths is identical by Ohm's law, it is concluded that the calculations are correct. The other ranges may be similarly checked by Ohm's law.

2. D.C. Potential Measurements.—When the meter is being used for potential measurements, enough resistance must be connected with it to limit the current to within the full scale sensitivity value of the meter.

The value of the multiplier resistor for the 5-volt range is established by subtracting the meter resistance value of 300 ohms from the 1,000 ohms-per-volt value of 5,000 ohms leaving a multiplier resistance value of 5,000-300 or 4,700 ohms.

For the higher ranges the multiplier resistance values are calculated on this basis of 1,000 ohms per volt.

3. Resistance Measurements.—For resistance measurements, the meter is used primarily as a voltmeter, with the current passing through the meter calibrated on an "Ohms" scale instead of being calibrated on a "Volts" scale-. In the multi-range ohmmeter circuits of this tester, however, shunts are used to enable the different sensitivities required for each range, and to this extent, the ohmmeter circuits resemble current measuring circuits in which shunts are usually required.

It will be observed from diagram fig. 20, that for the lowest or 2,000 ohm range, the 33 ohm resistor is a shunt resistor, while the 297 ohm and the 2,723 ohm resistors act as multipliers to the meter with its 700/4,300-ohm shunting, resistor made up of a fixed 700 ohm resistor and a variable 3,600 ohm rheostat for accommodating battery potential variations.

For the 20,000 ohm range, the 33 ohm and the 297 ohm resistors, totaling 330 ohms, act as a shunting resistor, with the

51 ohm and 2,723 ohm resistors functioning as multipliers. For the 200,000 ohm range, the 33 ohm, 297 ohm and 2,723 ohm resistors act as a shunting resistor, and a 3,269 ohm resistor acts as a multiplier resistor.

4. A.C. Potential Measurements.—The a.c. potential measuring functions differ from the d.c. potential in that the meter is connected to the output terminals of a full-wave instrument rectifier and a capacitor is substituted for the 4,700 ohm multiplier resistor, the capacitor being connected in series with the rectifier input circuit. Each of the multiplier resistors above the 5-volt range is by-passed with a calibration capacitor. The elements involved in the a.c. potential measuring functions are indicated in wiring diagram.

5. Capacity Measurements.—When the meter is used for capacity measurements, the resistance value of the meter and of the shunt and multiplier resistors associated with the measuring circuit constitutes one leg of an impedance triangle. See fig. 21. The reactance of a capacitor of unknown value, which may be connected into the measuring circuits for the purpose of determining its value, constitutes another leg of the same impedance triangle.

It is obvious that the resistance value of the meter and of its associated shunt and multiplier resistors is a constant value for any particular capacity-measuring range, regardless of the capacitive value of any capacitor which may be connected to that range, and that the capacitive reactance, in every case, is determined by the capacitive value of the capacitor which may be subjected to the measurement; therefore, the capacitive leg of the triangle is the variable element.

It is further obvious that the meter current is related directly to the hypotenuse of the impedance triangle and will not, therefore, have a linear relationship to capacitive values. For example, assume an impedance triangle in which a full-scale meter current corresponds to a certain hypotenuse length, and in which the reactance leg corresponds to a capacitive value of

FIG. 21—Arrangement of impedance triangle in capacity measurements!

5.0 microfarads; if we remove the 5.0 mfd. capacitor and put in its place a 2.5 mfd. capacitor, the length of the reactive leg of the triangle will be doubled, but the length of the hypotenuse will not be doubled, and, therefore, the meter current will not be reduced to one-half of its former full scale value. In other words, a linear or evenly-divided scale cannot be used on the basis of fixed resistance values for the meter and its associated shunt and multiplier resistors.

From what has just been explained, it is natural to ask a question as to how capacitive measurements are enabled on an

z=Vr2+x2c

evenly divided scale in this tester. The answer lies in the fact that, although the meter, shunt and multiplier resistance values constitute a fixed resistive value for each capacity measuring range, a variable resistive value is introduced by the full wave instrument rectifier employed, and shunts and multipliers are employed of such values as will enable the variable element of the rectifier resistance to approximately counterbalance the variable reactive element introduced by the different capacitive values which may be encountered for measurement.

In other words, the divisions of a meter scale would be crowded on the upper end of the sCale for capacitive measurements if the rectifier were linear in its characteristics, and the non-linear characteristics of the rectifier would cause the divisions of the meter scale to be crowded on the lower end of the scale, if no capacitive variable elements are introduced into the circuit; but when both variable elements are introduced into the circuit in approximately equal and opposite proportions, the meter scale divisions can be equally separated across the whole scale, or, what amounts to the same thing, the regular evenly-divided scales can be utilized for capacitive measurements.

For the measurement of electrostatic (paper) capacitive values, comparatively high a.c. potentials are used, but it is necessary to use comparatively low a.c. potential values for the measurement of electrolytic capacitive values, so as not to puncture the electrolytic film around the electrodes. Actually the a.c. potential applied to electrolytic capacitors in the 0/1.25/2.5/12.5 mfd. ranges is about 9 volts. The capacity-measuring circuits are shown in the wiring diagram.

Supreme Model 585 Diagnometer.—This instrument shown in fig. 22 with the connection diagrams of the tube testing circuit in fig. 23 has the following service facilities. It actually consists of 14 instruments in one compact assembly, for complete circuit and tube checking on all radios, P.A. amplifiers and television sets.

The instrument is a complete point to point set tester, or the "Free Reference" system of analysis direct from tube sockets may be chosen.

The meters provide for the following ranges:

1. Six d.c. potential ranges of 0-7/35/140/350/700/1,400 volts.

2. Six a.c. potential measuring ranges of 0-7/35/140/350/ 700/1,400 volts.

3. Seven d.c. current measuring ranges of 0-1/7/35/140/350/ 700/1,400 m.a.

A d.c. scale 0-14 amp. is provided for checking drain of auto radios and 6 volt mobile sound systems. There are six output meter ranges, ohms 0-200/2,000/20,000/200,000. The first division on the 200 ohm scale is 0.25 ohm. Can be read to 0.05 ohm. Megohmeter 0-2/20.

The 20 megohm range operating at 450 volts is an excellent electrostatic and main filter electrolytic condenser breakdown tester.

Decibels—10 to+6/0 to+16/+10 to+26/+20 to+36/+30 to +46 direct reading on the 500 ohm line; zero level 0.006 watts Electrostatic capacity meter 0-.07/0.35/1.4/3.5/7.0/14.0 Mfd.

Electrolytic capacity meter 0-3.5/7.0/14.0 Mfd. Direct meter leakage test for main filter electrolytics on colored "Good-Bad" scale.

Also a sensitive full size neon test for electrolytic condensers.

All meter services and ranges are selected by indicating rotary switches. New "Free Reference" tube for all old and new radio, P.A. and television tubes, except thyratrons and kinescopes.

FIG. 22—Front view showing arrangement of instruments and switching devices in Supreme Model 585 diagnometer.

With this diagnometer it is possible to test all multi-purpose tubes section by section, as well as for overall performance, there are 48 possible basic combinations of load and voltage to insure proper and accurate tests of every conceivable type of tube.

^MUtttlNG NETER CIRCUIT SELECTOR

FIG. 23—Internal connection of Supreme Model 585 diagnometer.

ELECTROLYTIC CAPACITOR

' ELECTRO- SELECTOS «PACTO! ELECTRO™™ low"

O.e. LEAHAGE

^MUtttlNG NETER CIRCUIT SELECTOR

FIG. 23—Internal connection of Supreme Model 585 diagnometer.

Supreme Model 501 Tube Tester.—The panel view of th^s tester is shown in fig. 24 and illustrates the various controls. The connection diagram is shown in fig. 25. This new improved circuit tests all old and new tubes for radio, public address systems, and television, except thyratrons and kinescopes. It tests all multi-purpose tubes section by section, as well as for overall performance.

FIG. 24—Front view of Supreme Model 501 tube tester.

All quality tests are made at full rated load for highest accuracy. Six sockets test all types and combinations of tubes, as both ends of the filament or heater are free, through switches, for instant connection to any pair of tube terminals including the top cap.

501 tube tester.

Supreme Model 551 Analyzer.—The panel view and connection diagram of this instrument are shown in figs. 23 and 24 respectively.

This set analyzer will handle all service and circuit problems of all types of radios, P.A. amplifiers and television sets. It is also as useful in servicing industrial vacuum tubes and photocell devices.

It has five sockets for the various types of tubes, and a sensitive 4 in. square meter, with easily readable scale. The various ranges and services are quickly available by means of an indexed rotary switch connecting the meter to any of the following measuring circuits:

FIG. 26—Exterior view of Supreme Model 551 analyzer.

The first scale division of the 0-200 ohm range is 0.1 ohm, and at center scale the resistance reading is 3.5 ohms. This extreme open scale which can easily be read as close as 0.02 ohms is especially valuable when checking the resistance of shorted voice coils, filament windings on transformers, rosin joints, shorted turns in converter armatures, etc.

The megohmeter has two ranges 0-2/20 megs, which is operated from a self-contained power supply for high resistance and cable leakage testing.

FIG. 28—Panel view of Supreme Model 581 signal generator.

Supreme Model 581 Signal Generator.—This all wave r.f. oscillator has a range of 130 k.c. to 60 m.c. on 5 fundamental bands and 3 harmonically related bands.

Other noteworthy features includes a 400 cycle modulating oscillator which modulates the r.f. carrier the standard 30%; a beat frequency audio frequency oscillator having a 60/10,000 cycle range with less than 5% harmonic distortion; and an electronic frequency modulator or "Wobbulator."

This model is useful for alignment testing by the output meter (amplitude modulated r.f. signal) method or the visual cathode ray tube (frequency modulated r.f. signal) method; demodulation and detector testing; checking fidelity and overall response, and gain of audio and P.A. amplifier systems, band pass width; selectivity curves of if. amplifiers, etc.

FIG. 30—Internal connection diagram of Readrite Model 430 tube tester.

The whole circuit is very stable, using a modified electron coupled system, which will not drift due 'to changes in line voltages, ambient temperature or attenuator control operation.

The circuit shown in fig. 29 has incorporated in it two 6A7, one 84 and one 76 tube.

FIG. 31—Front view arrangement of devices in Keadrite Model 430 tube tester.

Readrite Model 430 Tube Tester.—The wiring diagram and panel view of this'type of instrument is shown in figs. 30 and 31 respectively.

This instrument is designed to test both metal and glass types of tubes.

The panel has five sockets and a direct reading "GOOD-BAD" meter scale, two selector switches, one load control knob, one a.c. voltage adjustment knob and one push button switch to indicate the condition of the tube under test.

The circuit is designed on the "emission" principle in that the meter indication depends on an emission test of the tube.

Cathode-leakage and short-circuit tests can also be made with this instrument.

J-'IG. 32—Panel view of Readrite Model 720-A point to point panel.

Beadrite Model 720-A Point-to-Point Tester.—This tester is equipped to handle both the glass and the glass-metal tubes. It may be used to measure resistance capacity and continuity, as well for voltage checking of any tube circuit.

The poirit-to-point tests are made through an eight conductor cable, which is plugged into the receiving set socket. Tester socket terminals are arranged according to R.M.A. standards, thereby making it unnecessary to remove chassis from cabinet when localizing faults. Arrangement of the different tube elements does not affect tests.

SOCKET A,B,C,D. SOCKET E

SOCKET A,B,C,D. SOCKET E

FIG. 33—Connection diagram of Readrite Model 720-A point to point tester.

 SELECTOR SWITCH CONNECTIONS OFF NO READING 7R BLANK 2R 15 D C. VOLTS 8R 150.MILLI-AMP 3R 150 9R 750 A.C. VOLTS 4R 300 10R 150 5R 600 11R 25 6R 15 MILLI-AMP. 12R 10 13R BLANK

FIG. 33—Connection diagram of Readrite Model 720-A point to point tester.

The tester is equipped with two meters; a d.c. meter having scale for reading 15-150-300-600 volts, 15-150 milliamperes and an o.c. meter for reading 10-25-150 and 750 volts.

Separate meter ranges made available by connecting a single . pair of jacks and using the selector switch. For diagram of connection and panel view, see figs. 33 and 32.

FIG. 34—Front view arrangement of Readrite Model 710-A tester.

Readrite Model 710-A Tester.—This instrument is used to test all parts of the tube circuits by plugging directly into the • receiving set socket.

It will handle sets equipped with either glass or glass-metal tubes.

There are three meters, a d.c. volt-meter which reads 0-20/60/300/600 volts, and has 1,000 ohms resistance per volt, a d.c. milli-ammeter scale 0-15/150 and an a.c. voltmeter, scale 0-10/140/700.

 SELECTOR SWITCH CONTACTS 7-PRONG SOCKET SELECTOR SWITCH CONTACTS 7-PRONG SOCKET 2-6 7 4-2 3-6 8 4-3 C-G-6 4-5 4 1-6 10 4-6 6-4 11 5-3 . 6 6 — 5 12 5-4

FIG. 35—Schematic diagram of connections in Readrite Model 710-A tester.

FIG. 35—Schematic diagram of connections in Readrite Model 710-A tester.

A special positive contact selector switch connects all d.c. circuits to the d.c. volt meter. Panel jacks are provided to make individual range connections for the three meters.

The panel view and connection diagram are shown in figs. 34 and 35.

Philco Model 025 Signal Generator and Radio Tester.—This instrument consists principally of a volt-ohm-milliammeter for both a.c. and d.c. service.

The a.c. and d.c. voltage scales are 0-10/30/100/300/1,000. Current up to 10 amperes may be read directly on the milli-ammeter by using a special shunt.

OUTPUT

OUTPUT

The circuit is designed for capacity and resistance measurements which values are recorded on special scales, although in reading capacity (Mfd.) a special calibration chart should be consulted.

For internal connection and exterior views of instrument, see figs. 36 and 37.

Readrite Model 557 Signal Generator.—This signal generator is equipped with coil combinations to obtain frequency band as follows:

Coil "A" covers the band from 110 to 295 K.C. Coil "B" covers the band from 275 to 840 " Coil "C" covers the band from 820 to 2,800 " Coil "D" covers the band from 2,500 to 8,500 " Coil "E" covers the band from 8,000 to 20,500 "

FIG. 37—Panel view of Philco Model 025 Signal generator and radio tester.

The operation of the oscillator is as follows: After determination of the frequency to be covered, select proper plug in the coil as shown under heading "Plug-in Coils"; place coil in 6-hole socket in shield can which is accessible by removing the nickle cap near the toggle switch marked "On-Off". Connect oscillator and set the attenuator to approximately 75 on the dial, after which the toggle switch marked "MOD-UNMOD" is set to position desired.

FIG. 38—Panel arrangement of Readrite Model 557 signal generator.

Generally speaking, all oscillator alignments are made with a modulated signal. Consult graph chart for the coil selected. Note dial setting for the frequency desired. Set dial pointer of frequency selector dial to the position as shown on graph. Turn oscillator power on by throwing the OFF-ON switch to the ON position and attenuate the signal to desired level by rotating the attenuator control so that a minimum signal is reached. If further reduction in signal strength is wanted, use jacks marked Minimum and Ground.

FIG. 39—Wiring diagram and coil arrangement in Readrite Model 557 signal generator.

Output Meter.—An output meter should always be connected to the radio output when using a signal generator. In order to avoid serious energy loss the output meter should be connected between the plate of output tube and chassis. If the output meter does not have a condenser there should be a condenser inserted in the output plate lead. This will prevent a burnout of meter. A .5 mfd. 400 volt condenser is suitable.

Vacuum Tube Voltmeters (General).—The vacuum tube voltmeter is an instrument used in service work for direct measurement across high impedance circuits, such as in the measurements of radio-frequency and audio-frequency voltages where the use of power consuming instruments would be unsatisfactory on account of the small current in the circuit.

For example, the impedance of an r.f. circuit such as is used in the first and second stage of a receiver may be as high as 2 or 3 megohms when adjusted to resonance with an incoming signal.

To make any measurement of potential across such a circuit it is obvious that a meter having a resistance of 3 to 4 megohms would be required, as a meter having a lower resistance might change the potential condition in the circuit it is desired to measure, too much, and hence make the measurement unsatisfactory.

It has been found that the only connection that could profitably be made across such a circuit without upsetting the circuit potentials would be that of another vacuum tube, the. connection being made across the grid and cathode of said tube.

Essentially, the vacuum tube volt meter as the name implies, is nothing more than a vacuum tube connected through a meter in its plate circuit to a suitable power supply.

The grid and the cathode of the tube are connected across the circuit to be measured, the potential of said circuit causing a change in,grid voltage on the tube and thus, a resultant change in plate current is indicated on the instrument.

As the vacuum tube is also a rectifier, potentials of any fre-quency placed across the grid and cathode of the vacuum tube voltmeter will result in a direct current deflection on the instrument in the plate circuit.

It is for this reason that the vacuum tube voltmeter can be used for measuring audio as well as radio frequency potentials provided the circuit is worked out correctly to cover this broad range of frequency.

Weston Model 669 Vacuum Tube Voltmeter.—Front view and internal connection of the instrument is shown in figs. 40 and 41 respectively. The principal characteristics of this type of instrument is as follows:

1. It has 6 self-contained ranges controlled by a rotary switch in the lower left hand corner, the full scale readings being 0-/1.2/3/6/8/12/16 volts. This meter is different from other multiple range vacuum tube voltmeters in that on all of these ranges only the grid to cathode impedance of the vacuum tube appears across the circuit to be measured.

2. The device operates directly from a 120 volt 60 cycle ax. line, a self-contained transformer and power supply providing the necessary direct current potentials. A neon regulator bulb is used to hold the d.c. grid and plate voltages fixed irrespective of variations in line voltage. Up to the present time the problem of eliminating variations in vacuum tube meter readings with line voltage fluctuations has been a serious problem. The use of this regulator bulb has therefore put measurements of this type on a different plane as readings in the vicinity of .2 to 1 volt were practically impossible without having some sort of regulation of supply voltages.

3. Tubes used in the instrument are a type 78 and a type IV, the former being the measuring tube and the latter the rectifier for the power supply. The 78 tube is mounted with the top projecting through the panel so that direct connection can be made to the grid cap using short leads. In the same way the grid is kept approximately 1 in. from any other metal surface and in this way input capacity is kept at a minimum.

4. A six range scale is provided, all a.c. readings being made ' directly without reference to curves or charts of any kind. The circuit has been worked out so that readings can be taken on 60 cycle lines without visible error, the frequency coverage of the device being from approximately 40 cycles up through receiver

>. 40—Panel view of Weston Model 669 vacuum tube voltmeter.

short wave ranges. On very high frequencies such as from 10 to 20 megacycles slight errors will occur due to tube capacity even though this has been kept at a very low value. Such errors, however, are not very great being of approximately the same order as attained on other instruments used in this frequency range.

Among the measurements which can be made on this instrument is analysis of oscillator performance on super-heterodyne receivers, measurements of gain per stage in all types of receivers, checking of resonance, automatic volume control measurements, etc.

FIG. 41—Schematic wiring diagram of Weston Model 669 vacuum tube voltmeter.

TYPICAL TUBE-VOLTMETER CIRCUIT

TYPE 954 (AS TRIOOE)

C| ~ 500JJJJp CONDENSER (MICA) R4r 40000"OHM POTENTIOMETER C2=l6 pf COND. FOR CALIBRA- TOR COARSE ADJUSTMENT

TION WITH AND MEASURE- IN BALANCING OUT

### MENT OF LOW FREQUENCIES PLATE CURRENT

C3=500 WJf CONDENSER (MICA) R5= 2000"OHM RES.(VARIABLE) M = MICR0AMMETER(50 OHMS APPROX) (ON POSITION I GIVES RANGE R|=2000-0HM RES.(WIRE WOUND) e< 0F 2 V0LTS RMS Rj- 5O0OO-OHM RES. (WIRE WOUND) |ON POSITION 2 GIVES RANGE R3=IOOOO-OHM RES.(WIRE WOUND) ^ 0F 14 V0LTS RMS

NOTE: LEADS b, C. d AND e RETURN INSIDE CABLE. LEAD o IS CONNECTED TO GROUNDED HOUSING.

CONSTRUCTION OF PROBE

### CONSTRUCTION OF PROBE

FIG. 42—Vacuum tube voltmeter arrangement in which 954 (on account of its small size) is placed at the point of measurement. In this way long leads and high input capacitances are avoided with the desirable result that measurements can be made at radio frequencies with a minimum effect on the constants of the circuit under measurement. (Courtesy Radio Corp. of America).

FIG. 42—Vacuum tube voltmeter arrangement in which 954 (on account of its small size) is placed at the point of measurement. In this way long leads and high input capacitances are avoided with the desirable result that measurements can be made at radio frequencies with a minimum effect on the constants of the circuit under measurement. (Courtesy Radio Corp. of America).

CHAPTER 31