RCA 50-SW / MI-7330

GENERAL DESCRIPTION

50-SW

THE 50 SW - A NEW TRANSMITTER FOR INTERNATIONAL BROADCASTING

The 50 KW transmitter shown above is being manufactured in quantity by RCA for the use of the United Nations all over the world. Development was started about a year before Pearl Harbor when it became evident that high-power international broadcasting would become increasingly important both here and abroad. Today twelve of these transmitters are in operation, providing educational and entertainment services, as well as accurate news and information to the populations of many countries. By the end of 1943, another two will have been installed and, during 1944, another group of ten.

Already on the air are stations at Rio de Janeiro, Brazzaville in French Equatorial Africa, Leopoldville in the Belgian Congo, and in England. Under way are stations in Australia, Canada, the USSR, and a group of stations in this country under OWI direction. Two of the stations in this country will be operated by the Columbia Broadcasting System and three by the National Broadcasting Company.

The proved circuit design features of the RCA 50-E Broadcast Transmitter were applied to the 50 SW. Rectifier and audio circuits are alike and the basic simplified control circuits are similar. Radio frequency circuits, of course, are entirely new, for the requirements of international broadcasting introduced problems not usually encountered in domestic medium-frequency transmitters For instance, it is very desirable to be able to set up or change frequencies quickly to any spot in the range from 6 to 22 megacycles. In the 50 SW this is provided for by including two radio frequency channels, each complete from crystal to output amplifier stage and each contained in a separate compartment with separate interlocking systems so that one may be entered for work while the other is on the air. A single power supply and a single modulator unit are so arrnaged that they may be switched to either radio frequeny unit, the changeover requireing no more than five seconds.

The modulator and power supply units are arranged in the center with the two radio frequency channels at the left and right. The farthest doors on each side provide access to the fronts of the exciter units as is illustrated in one of the smaller pictures on the next page. The innermost doors on each side lead into the two R-F compartments. The high-power stages are arranged in line behind the exciters so that these doors open into a space which runs along the side of each amplifier. The center door opens into a control compartment which is shown in the illustration at the left. From this control compartment there is an interlocked door leading into the rectifier compartment. A number of photographs taken inside these compartments are shown in the following pages.

photo RCA 50-SW

CONTROL CIRCUITS

All control relays, contactors, and distribution switches are centralized on panels in the control compartment located behind the middle door of the transmitter. This door is not interlocked so that it may be entered during operation for check-ups on the operation of the control circuits. The window at the right provides a view of the rectifier compartment.

photo RCA 50-SW

HIGH-VOLTAGE RECTIFIER

DC power from the 1.5 kv., 5 kv., and 10 kv. rectifiers may be switched to either of the radio frequency units. The high-voltage (10 kv.) rectifier, shown in the illustration, is similar to that used in the RCA 50-E Broadcast Transmitter. A spare tube, with filament on, is constantly ready for use. It can be switched in to replace any of the six regular tubes. A bias rectifier on the modulator is the only additional d-c power supply required for operation of the complete transmitter.

photo RCA 50-SW

DUMMY ANTENNA

The portable assembly illustrated at the left is capable of dissipating the full 75 KW (50 KW modulated 100%) output from the transmitter. It can be set up for any resistance between 300 and 600 ohms at any frequency between 6 and 22 megacycles. It is particularly valuable for use during initial adjustment on a new operating frequency.

photo RCA 50-SW

MODULATOR

High-level class "B" modulation of the 50 KW carrier is provided. The low-power audio system is similar to that used in the RCA 50-E transmitter and in the 5-E transmitter. A cathode-follower driver, along with highly stabilized feedback circuits, provides low-distortion operation. The modulator unit proper, shown in the illustration, uses two RCA-880's, the same type of tube as used in the power amplifier.

photo RCA 50-SW

EXCITER

There are four crystal positions in each R-F channel. The crystal oscillator is followed by a doubler, three intermediate stages, and a driver stage using two RCA-827-R air-cooled Radiotrons. Low-power intermediate stage are tuned and reset by means of tap switches and variable capacitors. Excitation ratios are controlled by capacity dividing circuits. Adjustment si simplified by the lack of transmission lines for inter-stage coupling.

photo RCA 50-SW

POWER AMPLIFIER

The power amplifier proper is made up in two units for easy installation. The front section, which butts against the rear of the exciter unit, contains the two RCA-880 power amplifier tubes with associated water insulating coils, variable tank capacitor, variable neutralizing capacitor, filament transformers, and seal air blower. Cooling water is supplied directly to the tube jackets through short ceramic pipes of small cross-section, thus reducing radio frequency power loss in the water to a negligible amount. A motor-driven variable tank capacitor is used for tuning over a small frequency range. It consists of a single hinged plate at ground R-F potential operating in conjuction with two differentially variable plates which are attached directly to the tube jackets. The differential variation is utilize for balancing the plate currents of the two tubes in push-pull. Spurious frequeny circuits are minimized by the lack of any inductance between the plates of the tubes and the capacitor plates. The same holds true for the fixed neutralizing capacitor plates which are attached directly to the tube jacket.

photo RCA 50-SW

TANK CIRCUIT

The rear section of the power amplifier unit contains the tank coils and output circuits. A rectangular coil made up of 1" copper pipe covers the frequency range from 6 to 14 megacycles. Two turns are required for the lower frequency range. The lower turn is variable by means of a motor-driven control so that the tank capacitor tuning range can be augmented by variable inductance as well. Above 14 megacycles, the 1" copper pipe coil is replace by a hairpin-type inductor of 2" copper pipe which serves to cover the range from 14 to 22 megacycles. A shorting bar on the hairpin is set at the proper point for the frequency desired.

photo RCA 50-SW

OUTPUT COUPLING

The output tuning circuit uses inductors similar to the tank circuit and a motor-driven balanced variable capacitor to form a parallel tuned tank coupled to the plate tank. Output to a 300 to 600 ohm balanced transmission line is taken directly from the two hot plates of the variable capacitor. A motor-driven arrangement provides means for raising or lowering the complete assembly of output coupling coils and variable capacitors, thus allowing for a variation of output coupling without affecting either the output circuit or plate tank tuning. This feature allows for quick compensation during operation when sudden weather changes cause variations in the transmission line impedance. All five motor tuning control keys are located on the front panel wher the controlled effect can be noted on panel instruments.

MI-7330

50 KW SHORT WAVE BROADCAST TRANSMITTER

photo RCA MI-7330

The Short Wave Broadcast Transmitter described herein embodies a number of new developments from the RCA Laboratories which combine to produce an efficient and economical high power equipment for broadcasting in the International Short Wave Bands. Among these developments are new type high-frequency power tubes, simplified automatic control circuits, all a-c operation, new designs of high power audio frequency transformers, and new high~frequency tank circuit designs.

The transmitter is designed with two separate r-f channels, each capable of operating at any chosen frequency in the range from 6 to 22 megacycles. Rapid change can be made between any two frequencies by changing r-f channels, and any third frequency can be set-up on either channel while the other is in operation. Each r-f channel is provided with its own output terminals, which may be connected directly to an antenna transmission line, or to a transmission line switching system external to the transmitter.

High level Class B plate modulation of the output r-f amplifier is used because of its high overall efficiency. In this system the power required is reduced for average program modulation, increasing momentarily at a syllabic rate on modulation peaks. The output r-f amplifier employs two tubes, in a push-pull circuit. The modulator also uses two tubes, and is excited by a series of low power push-pull Class A audio stages. New methods of transformer and circuit design and the use of stabilized feed-back in the audio system give outstanding quality of modulation.

The control system has been designed to provide full automatic operation with greatly simplified circuits, using rugged, reliable components. Improved overall transmitter efficiency results in lower power consumption, also smaller power supply units and smaller cooling system equipment. With all a-c operation, motor-generator sets and rotating power apparatus have been eliminated. Power transformers may be located out-of-doors if desired, and, while, a two floor building is recommended for convenience in making piping and high voltage connections, a single floor layout can also be used readily. High reactance type filament transformers are empIoyed, eliminating the need for complicated starting resistor control on the filament circuits of the larger tubes.

The cooling system is of the closed type, using distilled water, and includes a storage tank, regular and spare circulating pumps and surface type cooler. The distilled water system is made up entirely of nonferrous metals and porcelain, and contains no organic materials which might contaminate the water and lower its insulation resistance.

The power amplifier uses two tubes connected push-pull in a high efficiency Class C circuit. Amplitude modulation of the carrier is accomplished in the plate circuit by the Heising system, using a high efficiency, Class B modulator. The modulator is driven by a chain of three audio amplifiers in cascade, the last amplifier exciting the modulator tubes through a "cathode-follower" circuit which results in smaller components and simplified amplifier design. The modulation system employs the latest exclusive RCA technique in determining the constants of the transformer, reactor and capacitors, to obtain optimum performance with smaller and less costly equipment.

In the mechanical design of the equipment great care has been taken to use rugged components and to arrange the various parts to facilitate operation and maintenance of plant. The disposition of the equipment and the arrangement of the enclosure and partitions allow observation of the various units in operation while at the same time reducing operating hazards to personnel to a minimum. With maximum convenience, the Transmitter Enclosure also presents an attractive modern appearance, leading the field in functional industrial styling.

All transmitter tubes except the final r-f amplifiers and the modulators are air-cooled. These latter are water-cooled, using a closed distilled water system, designed to reduce water losses to a minimum. Regular and spare circulating pumps are both connected to provide immediate change-over in case of necessity.

Individual instruments are provided in all important tube circuits to permit accurate observation of the performance of all parts of the equipment. Instruments connected with power supply, rectifier voltages, and the final r-f amplifier and modulator tubes are mounted at appropriate points on the front enclosure. Instruments for the r-f exciter, as well as tuning controls, are located inside the enclosure door in front of each r-f channel. An inner door is interlocked, so that no high voltage circuits are exposed when checking or adjusting the transmitter.

Several functions are combined in the control circuits, including safety to personnel, protection of equipment, complete automatic or manual control of operation, and automatic change from one r-f channel to the other. In respect to protection of personnel, the electrical door interlocks are so connected as to remove all dangerous voltages and ground all d-c circuits when the doors giving access to the transmitter are opened. The interlock circuits are so arranged that power is required to maintain operation. A failure of control circuit power or a break in the interlock or grounding relay circuits will result in shut down.

Protection to equipment is provided for by time delay relays which prevent application of bias or plate voltage to rectifiers or power tubes until the filaments have been properly heated, also by high speed overload protection, by means of individual relays in the plate circuit return of power tubes and in the primary feed circuits to rectifier plate supply transformers.

Individual control switches are provided so that the various filament, bias and plate circuits may be turned on and off separately during tests or "trouble shooting". These switches are located in groups at appropriate places on the transmitter enclosure. If these control switches are placed in the "on" position in advance, then the transmitter can be put into full operation by throwing the station start switch, which will turn on filaments and, after necessary delay, the bias and plate supplies. Provision is also made for either automatic reclosure or lockout operation. In the latter case, an overload will result in shutting off of plate circuit power. In the former case, the plate power will be reapplied automatically after an overload, unless three overloads occur in rapid succession, in which case shut down will result.

The transfer feature is controlled by a time sequence relay which, when the "transfer" control is operated, will remove plate voltages, automatically operate the necessary power supply transfer contactors, and then reapply plate voltages.

The console is provided with indicator lamps for all important circuits, also with switches for shutting down the high voltage supplies in case of emergency. It is also provided with a volume indicator and various switches and gain controls for handling incoming programs to the transmitter.

The whole equipment is designed in conformity with the most up- to-date practices of good engineering, and will conform to standards of the American Institute of Electrical Engineers and of the United States Federal Communications Commission.

TECHNICAL SPECIFICATIONS
Type of Emission	A3 - Telephone
Power Output (300 to 600 ohm balanced line)	50 KW
Frequency Range	6 to 22 MC
Frequency Stability	10 parts per million guaranteed
Power Supply Requirements	2300 volts, 50 cycle, 3 phase. Regulation, from no load to full transmitter load, not to exceed 5% of total. Voltage variation due to regulation and other causes should not exceed 10% of nominal 2300 volts. Average program requirements 120 KW - Power Source should be capable of supplying 200 KW. A separate 115 volt, 50 cycle supply for the crystal heaters is required.
Power Input: Average program level Modulation Factor = 1	Approximately 120 kW Approximately 150 KW
Type of Modulation	High level class "B"
Audio Frequency Input 600 ohms: Average program level Modulation factor = 1.0 Audio Frequency Response	0 VU approx. 6 VU Uniform within ±1.5 DB, 30 to 10,000 cycles
Audio Frequency Distortion Residual Noise Envelope Distortion	60 db below 100% modulation 4% r-m-s at 1000 cycles and 90% modulation
Specifications may change without notice

TUBE COMPLEMENT
RF stages		AF stages and modulator		Rectifiers
Number	Type	Number	Type	Number	Type
4	880	2	880	6	857B
4	827-R	8	828	8	872A
4	810	2	1603	2	836
4	828
4	802

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	AUS	AUSTRALIA	B	BRAZIL
	CAN	CANADA	COD	CONGO DEMOCRATIC REPUBLIC
	COG	CONGO REPUBLIC	MRC	MOROCCO
	PAK	PAKISTAN	G	UNITED KINGDOM
	USA	USA