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1947-48 Theatre Catalog, 6th Edition, Page 487 (471)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 487
Page 487

1947-48 Theatre Catalog, 6th Edition, Page 487

New Instruments to Help Theatre Servicers

Special Tools Aid Checks on Sound Systems

And Test Reels Make for Better

The perfection of projection and sound in Americas theatres is maintained only through the eternal vigilance of the projectionist, the theatre manager, and the serviceman.

Little there is that the theatre manager can do, save to be so familiar with what should be that he is able to detect the deviations in the highest possible standard. The projectionist can do more, yet he is- largely restricted to what might be termed the grosser adjustments of the equipment. Thus it is up to the serviceman, whose training, education, and experience have given him the scientific understanding and the technical know-how to keep the projection-booth equipment, and particularly the sound system, at the peak of operating perfection.

That such periodic servicing may be more precise and that the available data can better be interpreted so that proper adjustment can better be made, the research laboratories of manufacturing companies are constantly devising new tools for servicemen. The following sections pertain to some of these recent aids to services of sound and projection equipment.


The production of a new, highly sensitive vacuum tube audio voltmeter for use in industry, laboratory, and radio servicing, designed to measure a.c. voltages over ranges of frequency and amplitude far beyond the limits of ordinary a.c. voltmeters, has been announced by the Engineering Products Department of the Radio Corporation of America.

The unusual sensitivity of the new audio voltmeter (Type WV-73A) makes it possible to measure the electrical conductivity of switches, circuit breakers, relays, buses and grounds, in addition to transmission losses in lines and circuits and the response of special filters and compensators.

The new instrument is also applicable for testing radio receivers and sound systems and is used to measure gain and noise level in power amplifiers and ripples voltages in power supplies. The meter is also used to locate sources of frequency distortion and faulty amplifier components in receivers, phonographs, and public address systems.

The audio voltmeter includes an input circuit with low capacity and high resistance which makes it particularly suitable for measuring voltages in highimpedance circuits. This makes it possible for the electronic meter to measure the response of audio frequency power amplifiers and loudspeakers without disturbing their frequency characteristics. The range of the meter is 20 cycles to 20 kilocycles.

The main components consist of a precision attenuator, a three-stage high 1947-48 THEATRE CATALOG

gain stabilized amplifier, a balanced diode rectifier, a special d.c. microammeter, and a regulated power supply.

The voltage to be measured is fed to the attenuator through a shielded cable attached to a jack on the front panel. The attenuator consists of an eleven position switch connected to non inductive resistors, arranged in such an order that consecutive switching ranges overlap by 10 decibels. The meter can measure one one-thousandth of 21 volt, to one thousand volts.

From the attenuator, the voltage is fed to the high-gain amplifier, which employs a conventional feedback circuit to obtain stabilization and sharply reduces the input capacity of the first tube. Output voltages from this amplifier are fed to a balanced diode rectifier in order to produce d.c. for energizing the meter.

The high-level rectifier is designed to produce an output voltage that is pros portional to the average value of the full wave, thereby giving a meter reading that agrees very closely with an RMS meter for all usual distorted waveforms. Because a balanced diode rectifier is used, the meter indicates the true value of both halves of the wave, avoiding the polarity or turn-over error of half-wave circuits.

Other applications of the new RCA audio voltmeter include: use as an audio amplifier which gives high gain with essentially perfect fidelity, and whose sensitivity makes it especially adaptable for use with microphones having low output; bridge measurements which readily indicate the null point at either high or low audio frequencies; measurement of the output of a phototube with


sensitivity that will indicate extremely slight variations in light intensities to which the tube is exposed; and measurement of currents as low as one onethousandth of an ampere, or if a 0.1 megohm external resistor can be used, currents as low as 0.0001 milliampere can be measured.


A new miniature crystal rectifying probe, which adapts the VoltOhmyst and Chanalyst for circuit testing of teles vision, frequency-modulation, and other VHF applications within the sensitivity ranges of these instruments was announced by the Test and Measuring Section of the Engineering Products Department of the Radio Corporation of America.

The new probe (Type MI-8263), which may be used with all models of the VoltOhmyst, employs a germanium crystal to rectify applied a.c. voltages which are then measured by the dc. circuit of the meter. The meter reading is proportional to the positive peak of the applied a.c. voltage. Excellent linearity is obtained with the probe, even at the ends of the meter scale, and, because of its low input capacity and high impedance, this accessory is especially suited for accurate reading Without regard to load resistance.

Designed to facilitate testing in difficult places, the probe has a long, narrow ipoint. Its body is constructed of durable insulating material and reinforced at the end with a metal shield to eliminate capacity effects when held for testing. A detachable ground lead with an alligator clip is provided for

THE AUDIO VOLTMETER measures a.c. voltages over ranges oi frequencies and amplitude far beyond the limits of ordinary meters of this type. The unusual sensitivity of the new RCA unit makes it possible to measure electrical conductivity of switches. circuit breakers, relays, buses, and grounds, in addition to measuring the transmission losses in lines and circuits and the response of iilters and compensators.
1947-48 Theatre Catalog, 6th Edition, Page 487