New metered variac autotransformers

Metered Yaria.es are portable testing devices, each consisting of a Yariac autotransformer, a voltmeter, and an ammeter or a wattmeter or both. Switching, fuses, and 3-wire power cord are also provided. These handv, compact assemblies have many uses both in the laboratory and on the test bench, among them overvoltage and undervoltage tests, trouble shooting, and measurements of voltage, current, and power.

Meters are shielded from the transformer stray field, permitting an over-all accuracy of 3% of full scale. The on-off switch disconnects both sides of the line. Make-before-break switches permit the dual-range ammeters and voltmeters to be switched under load.

Three new models are now available, in addition to the two (Types W5MT3A and W5MT3W) previously announced.1 For convenient reference, all five available models are listed below.

The new Type W10MT3A and Type W10MT3W duplicate in the 10-ampere rating the features of the 5-ampere models previously announced. The Type W5MT3AW offers the advantage of all three meters — reading load volts, amperes, and watts — in a single assembly.

'••NW Metered Variaes. Types W5MT3A, W5MT3W," General Ra<tio Experimenter, 33, 5, May, 1959.

Type,

Input Vol trine

Output Voltn f/e*

Output Meter Banoes Current

(Amperes) Amperes ty&ts

Code Word

Price

W5MT3A

120

0-140

0-5

0-5

None

CABAL,

$89.00

W5MT3W

120

0-140

0-5

None

0-150 0-750

CA BOB

112.00

W5MT3AW

120

0-140

0-5

0-5

0-150 0-750

CABEX

150.00

W10MT3A

120

0-140

0-10

0-2 0-10

DOGEN

1 10.00

W10MT3W

120

0-140

0-10

0-300 0-1500

DOGII)

138.00

♦Voltmeter ranne is 0-150

Type W5MT3AW Metered Variac.

♦Voltmeter ranne is 0-150

Type W5MT3AW Metered Variac.

Type W10MT3A Metered Variac.

Type W10MT3A Metered Variac.

General Radio Variac

View of the Type W5LMT3 Variae® Autotransformer. The 3-wire to 2-wire adaptor shown at the right is furnished.

type w5lmt3 variac autotransformer

The popular Type W5L Variae^ Autotransformer is now available in :i portable model, Type W5LMT3, with ease, handle, on-off switch, overload protector, 3-wire cord and plug, and 3-wire outlet. Maximum output voltage is limited to input line voltage, which permits a maximum current of 9.2 amperes and an output rating of 1.1 kva. Rated current of 7.1 amperes can be drawn at any point on the winding: maximum current can be drawn at, or near, maximum voltage only. A load drawing not more than maximum current at maximum voltage can be controlled over the full range of output voltage (() to 120).

The combination of portability, high rating, 3-wire plug, and built-in overload

SPECIFICATIONS

Input Voltage: 120. Driving Torque

Output Voltage: 0-120. gin-em").

Maximum Current: 9.2 amperes. including handle and cord.

Load Rating: 1.1 kva. Net Weight: pounds (3.75 kg).

Type Code Word Price

View of the Type W5LMT3 Variae® Autotransformer. The 3-wire to 2-wire adaptor shown at the right is furnished.

protector make this Variae a most useful device for general testing in the laboratory and shop.

10-20 ounce-inches (700-1400

W5LMT3

Variae1*1 A utotra n sf or m er

COTOS

-

$34.50

VB-2

Replacement Brush

.75

new resistance decade boxes with improved switches

After having for several years fulfilled on a special basis many requests by customers for decade resistance boxes containing steps of 0.01 ohm, we are now adding to the line two new items, Type 1432-T: 1,111.1 ohms, 5 dials, in steps of 0.01 ohm, and Type 1432-T: 111.1 ohms, 4 dials, in steps of 0.0 1 ohm.

The 0.01-ohm-per-step decade greatly enhances the usefulness of a multi-dial decade resistor. If, in a particular application, the lowest decade is used as the last of several decades, its relative inaccuracy (±2%) is swamped out by the high resistance in series with it. 10veil when this lowest decade is used with only one or two others, it still can be very useful in making precise adjustment to a desired circuit condition, as, for instance, a null balance in an im-

Null Balance
Figure 1. View of the Type 1432-T Decade Resistor.

pedance bridge. Used in an ac bridge to sot the resistive balance, its high resolution permits a corresponding sharpening of the null, so that the desired reactive balance can be set with greater precision.

While the 0.01 ohm-per-step Type 510 Decade-Resistor Unit had been manufactured for some time on a special basis for selected uses, it became possible to catalog it for separate sale (as Type 510-AA) and to use it in decade resistance boxes only when the low-residual-resistanee version of the Type 510 Switch, about to be described, came into production.

This basic change in the Type 510-P3 and -P3L Switches was made for the purpose of reducing the switch resistance, which is stated as being between 0.002 and 0.003 ohm in our current Catalog P. I'se of two silver-bearing alloys as the materials from which to make the contact buttons, switch brushes, and take-offsprings has resulted in a decrease of this resistance by a factor of three or four to one. This means that the switch resistance on a normal switch will not exceed one milliohm, even under extreme conditions of resistance variation.* (See Figure 2 for typical variation of resistance vs. time for old and new metal combinations.)

Since the two new alloys referred to above are high-copper alloys, having small silver additions rather than a large proportion of zinc, they appear redder in color than the earlier alloys. Because the high-copper alloys are more subject to corrosion by atmosphere or finger marking, a number of corrective measures have been taken. The brushes are nickel-plated, except on the contact surfaces, to maintain appearance. The takeoff springs are silver-plated to assure persistence of the originally low contact

*Ili this connection it should be pointed out that the switch resistance tends to increase under circumstances where the switch has not been moved for a long period of time. This is true of even the best switch materials, such as silver. It is always prudent, after a box has stood with one or more switches in a Riven position for any considerable period, to (live the switches several swings back and forth over the full range of their motion. This will dissipate by friction any foreign products which may have built up the switch resistance during the period of inactivity. This should become as much a matter of habit as brushing one's teeth or cleaning one's spectacles.

DA*S ON CONTACT

Figure 2. (Left) Change in switch resistance as a function of time when left at rest. When switch is rotated, the resistance drops im mediately to its original value. (Right) Change in switch resistance as a function of cycles of operation for bronze and silver-bearing-copper alloys.

/bronze

silver -6earinc

coppe

r

resistance, which improves their solder-ability at the same time. New lubricants, otherwise similar to what had been in use, but having strong antioxidant properties, are employed. That used at the contact-button-brush interface is Master Lubricant's 11101, available for

Type maintenance purposes from our Service Department in four-ounce tubes at 75 cents per tube. This lubricant is equally satisfactory for use on older Type 510 Switches having phosphor-bronze brushes and Tobin-bronze contact buttons. — p. k. McElroy

Code Word Price

1432-T

1,11 1.1

ohn

ns in steps of 0.01

devil,

$120.00

1432-U

111.1

oh n

ns in steps of 0.01

dewin

95.00

a quartz-crystal frequency discriminator

Note: This discriminator is not built for sale by the General Radio Company. The description is published for the information of those who may wish to build their own. —Ed.

During the development of low-noise frequency-multiplier systems, a sensitive frequency discriminator was devised to measure the dynamic frequency modulation present in the output signal from the multiplier. The discriminator, which is described here, provides a sensitivity adequate for the measurement of deviations of ± 1 x 10_y or less when a narrowband wave analyzer is used to select the modulation-frequency components.

Figure 1. Block diagram of the discriminator.

Figure 1. Block diagram of the discriminator.

The circuit makes use of the steep phase-vs.-frequency characteristic of a quartz crystal at series resonance to provide the required sensitivity. A balanced phase-detector circuit is used to reduce the effects of amplitude modulation. The block diagram of the circuit shows that it belongs to the class of frequency discriminators in which the input signal is applied to a phase detector through one channel with negligible phase shift and through a second channel with a phase shift that, is a function of frequency.

The circuit is similar to the Foster-Seeley discriminator circuit insofar as the balanced phase-detector circuit is concerned. The inherent tuned-circuit phase shift, however, which is the basis of the operation of the Foster-Seeley discriminator, is negligible compared with the phase shift produced in the series-resonant crystal as the input frequency varies about the series-resonant frequency. The slope of the phase-shift characteristic in the quartz crystal branch is larger in comparison with that in the LC circuit by approximately the ratio of the Q's of the resonant elements,

100,000

or approximately ~ = 1000. Hence the phase characteristic of the quartz crystal dominates that of the inductance-capacitance circuit.

DIFFERENTIAL BALANCING CAPACITOR E.F JOHNSON i9maiI

RESISTANCE BALANCE

DIFFERENTIAL BALANCING CAPACITOR E.F JOHNSON i9maiI

RESISTANCE BALANCE

Foster Seeley Discriminator

CENTER FREQUENCY IO pf ADJUSTMENT

Figure 2. Circuit schematic of the discriminator.

CENTER FREQUENCY IO pf ADJUSTMENT

Figure 2. Circuit schematic of the discriminator.

output volts

Amplitude Discriminator

OÉVIATION (CYCLES)

voltam - o pvs

OÉVIATION (CYCLES)

Figure 3. Re* sponse characteristic of (left) the 10-Mc discriminator and (right) the 1 -Mc discriminator.

voltam - o pvs

The principal series resonance of the crystal presents a very low impedance in the signal path, whereas the shunt resonance of the crystal with its own electrode capacitance presents a high impedance. Hence the shunt-resonant point, which would otherwise represent a possible spurious response, has negligible influence.

The frequency of the series-resonant response of the quartz crystal is adjustable by means of the adjustable series capacitor. The bandwidth of this response is affected by the shunt capacitor, connected between the center-top of the coil and ground. A large capacitor here produces a narrow-band response (maximum Q) as it does in any crystal filter circuit.

The response of the discriminator is limited to modulation frequencies within the pass band of the crystal branch. Modulation-frequency side bands beyond the bandwidth of the crystal element are not passed. The 10-Mc discriminator described has been used to measure (>0-cvele and 100-eycle modulation, and to estimate 120-cycle modulation. It is not effective for higher modulation rates. A 1-Mc discriminator has also been built but is not effective for modulation rates above approximately 10 cycles sec, and hence is not useful for the measurement of power-supply-related modulation.

INTERKAMA 1960

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