COLLINS AN/FRT-6

GENERAL DESCRIPTION

PURPOSE

Radio Transmitting Set AN/FRT-6, is designed for shore-based communication service. The set is capable of providing an output of 40 kilowatts of continuous wave radio telegraph, frequency shift telegraph, or facsimile transmission on any frequency from 4.0 to 26.0 mc. The principal function of the equipment is to effect communication with reliability and precision from shore to ship or point to point. Due to its high r-f power output, the transmitter is intended primarily as a means of transmitting intelligence over long and difficult circuits.

BASIC PRINCIPLES OF OPERATION.

Operation of Radio Transmitting Set AN/FRT-6 is based upon the principle of generating, at an exact sub-multiple of the desired output frequency, a low energy radio frequency signal, multiplying the frequency, and amplifying the power of this signal to produce the desired radio frequency carrier. This carrier is then interrupted, or the frequency shifted, in accordance with the intelligence to be transmitted. The resultant radio frequency energy is then radiated from a suitable antenna.

The required low energy excitation is derived from one of two alternate exciter units incorporated within the transmitter. These may be further augmented by a Frequency Shift Keyer. These two units are RF Oscillator O-91/FRT-5, which is a stabilized master oscillator, and RF Oscillator O-92/FRT-5, which is a ten channel crystal controlled oscillator. This low energy signal is then passed through a buffer stage and thence into two frequency multiplier stages where the frequency of the signal is multiplied by a factor of 2, 4, 6, or 8. Following the two multiplier stages, the signal power is amplified to 15 kw in an Intermediate Power Amplifier and then further amplified from 15 kw to 40 kw in the power amplifier stage. The intelligence may be superimposed on the carrier wave in one of two ways, viz., off-on keying or frequency shift keying. The former is produced by interrupting the carrier in the low energy buffer and 1st multiplier stages. Frequency shift keying is accomplished by passing the signal through a frequency shift keyer prior to its entering the buffer amplifier. The frequency shift keyer then shifts the carrier frequency a small amount, linearly with respect to the keying signal.

Protection for operating and maintenance personnel, as well as protection against equipment damage caused by overloads or other normal equipment faults, is as complete as is possible with existing design methods.

GENERAL DESCRIPTION OF UNITS.

a. INTRODUCTION. - Radio Transmitting Set AN/FRT-6 is contained within four handsomely styled sheet metal cabinets, which bolt together to form a single unit. In addition, there are four small sheet metal enclosures which house the two power transformers and the two control units required.

Several features, new and unique to equipment of this power class, have made this a compact yet easily serviced equipment. A single type medium power triode is used throughout the power stages, a pair in the intermediate power amplifier and six in the final power amplifier. The anode voltage requirements of this tube are relatively low thereby allowing the use of intermediate voltage rectifier tubes and smaller than usual filter and power supply components. The tubes and tank circuit components are forced air cooled, thereby eliminating the customary bulky water cooling system.

The externally mounted plate transformers are dry type, air cooled and fully enclosed in grounded sheet metal cases. Connecting leads are brought in through grounded conduit and connected internally thus eliminating the usual requirement for additional protective enclosures. The high current primary circuits are fed directly to the plate transformers through the external control units thereby eliminating the space and service problems presented by large high current wiring and control components located within the main enclosure.

All resonant circuits are continuously variable, motor driven, and servo positioned thus eliminating the usual complication of switching or shorting and awkard mechanical drives. Servo positioning allows the controls to be grouped at a convenient and logical location while the resonant circuit is placed where it best suits the requirements of the circuit.

The upper front door of each of the four cabinets has a large glass window which affords a continuous view of all of the tubes contained within the bay. The upper front door, itse]f, provides access to all of the tubes within the bay as well as some of the adjustment controls not directly associated with tuning. These doors are hinged at the top and counter balanced for ease of operation. They are further provided with dual interlocks, both positive electrical and automatic mechanical in nature, which remove all dangerous voltages and ground the circuits as soon as the door is opened. Status lights, illuminated meters, and the main control switches are mounted in these upper doors. Identical interlocks are also provided on the large double rear doors. All interlocks for all the units are wired in series so that opening any door immediately removes the high voltage from the circuits in all of the bays. The cabinets each have a non-interlocked lower front door which provides access to the control relays and timers and several infrequently used controls. The lower door also contains a small drop door to provide a convenient means of access to the control panels of each of the bays, without having to open the lower doors.

b. LIST OF UNITS. - Radio Transmitting Set AN/FRT-6 may be thought of as consisting of the following major and minor units.

RF Unit T-225/FRT-5 which contains:
1. RF Oscillator O-91/FRT-5
2. RF Oscillator O-92/FRT-5
3. Frequency Shift Keyer KY-45/FRT-5
4. Power Supply PP-454/FRT-5
5. Servo Amplifiers
6. Servo Power Supply
7. Patch Panel
8. Electronic Keyer
9. Control Panel
10. Buffer Amplifier, Frequency Multipliers, and Driver Amplifier
11. Intermediate Power Amplifier
12. Coupling Network
RF Unit AM-350/FRT-6 which contains:
1. Servo Amplifiers
2. Servo Power Supply
3. Control Panel
4. Power Amplifier
5. Antenna Coupling Network
Power Supply PP-490/FRT-5 which contains:
1. Control Circuits
2. Low Voltage Supply
3. High Voltage Supply
4. Bias Voltage Supply
Power Supply PP-515/FRT-6 which contains:
1. Control Circuits
2. High Voltage Supply
3. Bias Supply
Power Control C-587/FRT-5
Power Control C-598/FRT-6
Power Transformer TF-122/U
Power Transformer TF-124/FRT-6

c. MAJOR AND MINOR UNITS DETAILS. - In more detail, the major and minor units of Radio Transmitting Set AN/FRT-6 consist of the following:

(1) RF UNIT T-225/FRT-5. - RF Unit T-225/FRT-5 is the left center bay of the four main bays. It contains, on a special mounting space the units described under paragraphs (a), (b), (c), (d), (e), (f) and (g) below. This space is a pivoted, double width, 25" high standard equipment rack. Pivoting this rack provides easy access to the rear of all the units mounted thereon. RF Unit T-225/FRT-5 contains as permanent equipment the circuits and units described under paragraphs (h), (i), (j), (k), (l) and (m).

(a) RF OSCILLATOR O-91/FRT-5. - RF Oscillator O-91/FRT-5 is a stabilized variable frequency oscillator which derives its stability from a 100 kc crystal standard. Utilizing a permeability tuned oscillator, it provides a frequency stabilized output in the range of 2-4.5 mc. It is mounted in the equipment rack which occupies the lower part of the r.f. bay.

(b) RF OSCILLATOR O-92/FRT-5. - RF Oscillator O-92/FRT-5 is a crystal controlled 10 channel oscillator. With proper crystals, the oscillator provides an output from 2-4.3 mc. Plate and filament voltages for this unit are derived from the main transmitter. It is mounted in the equipment rack of the r-f bay.

(c) FREQUENCY SHIFT KEYER KY-45/FRT-5. - Frequency Shift Keyed KY-45/FRT-5 is used to provide a frequency shift which is linear with respect to the keying voltage. In a telegraph system, it causes the transmitter to emit one frequency for a mark signal and another for a space signal. Because of its linearity with respect to the keying voltage or current, it may also be used for photo and teleprinter transmission. This unit is mounted in the equipnient rack of the r-f bay.

(d) POWER SUPPLY PP-454/FRT-5. - Power Supply PP-454/FRT-5 is a dual supply furnishing plate and filament voltages to HF Oscillator O-91/FRT-5 and Frequency Shift Keyer KY-45/FRT-5. It also supplies a regulated bias to the keyer. It is mounted in the equipment rack of the r-f bay.

(e) SERVO AMPLIFIERS. - There are six identical interchangeable, plug-in Servo Control Amplifiers mounted in a common enclosure on the equipment rack in the r-f bay. One of these units is associated with each of the six tuned servo-positioned circuits in RF Unit T-225/FRT-5. Each contains a voltage amplifier, a relay control tube, and a pair of motor control relays.

(f) SERVO POWER SUPPLY. - The Servo Power Supply furnishes voltage to all of the Servo Amplifiers, and drive motors located in RF Unit T-225/FRT-5. It is mounted on the lower left side of the equipment rack, immediately below the Servo Amplifier Unit.

(g) PATCH PANEL. - The remaining unit in the equipment rack is the Patch Panel which contains the two Oscillator output jacks, the Keyer input and output jacks, the transmitter input jack and several spare jacks. Three cables with connectors on each end are supplied with the unit to facilitate setting up any desired type of excitation and keying.

(h) ELECTRONIC KEYER. - An Electronic Keyer is mounted on the right side of the Control Panel of RF Unit T-225/FRT-5. It receives the keying impulses and accomplishes off-on keying of the transmitter proper. Provision has been made for keying of the transmitter by negative, positive, polar negative, or polar positive impulses for mark.

(i) CONTROL PANEL. - The control panel carries the basic transmitter tuning and checking controls and is located behind the lower front door. Easy access to these controls is provided, however, by a small drop door located at the top center of the main lower door.

(j) BUFFER AMPLIFIER, FREQUENCY MULTIPLIERS, AND DRIVER AMPLIFIERS. - The buffer amplifier, frequency multipliers, and driver unit are contained within a sub-assembly which is mounted to the left of the control panel. The unit is hinged so that it may be tilted forward for easy access to the interior. Inside are located a broadband buffer-amplifier, and two frequency multiplier stages. The driver tube is mounted on top of the assembly.

(k) INTERMEDIATE POWER AMPLIFIER. - The intermediate Power Amplifier is located on the top deck of RF Unit T-225/FRT-5. It is a push-pull, cross neutralized, grounded cathode amplifier with both grid and plate circuits resonant and continuously variable from 4.0 to 26.0 mc. The resonant grid circuit, which also provides the plate tank for the driver stage, is contained within an air tight enclosure located behind the control panel. The power amplifier tube sockets are mounted on top of this enclosure. Cooling air for the PA tubes is forced into this enclosure by the blower, located in the lower rear of the cabinet, and upward through the tube sockets to cool the tube radiators. The resonant plate tank circuit is located directly behind the Intermediate Power Amplifier tubes and is readily accessible through the large double rear doors.

An alternate provision is made for operating the intermediate and final power amplifier as linear amplifiers driven by a single side band suppressed carrier exciter such as the Western Electric D-156000 twin channel unit. The grid circuit of the intermediate power amplifier is arranged to be modified so that it presents a 200 ohm balanced load to the exciting single side-band transmitter. Approximately 500 watts of peak exciting power are required.

(l) COUPLING NETWORK. - The coupling network occupies the top of RF Unit T-225/FRT-5. It is a resonant circuit continuously variable across the entire frequency range and is suspended, as a unit, on rails or tracks from the top of the cabinet. Magnetic coupling from the plate tank coils to the inductors of this coupling network is therefore varied by the positioning of the coupler on its rails.

(m) GENERAL. - The upper front door of RF Unit T-225/FRT-5 has a large glass window to provide continuous visibility of the Intermediate Power Amplifiers. All meters directly associated with tuning or adjusting the r-f stages in the bay are mounted above the window. The filament, D.C. controls, and colored status lights are located below the window on the upper front door. A ventilating blower cooling the Intermediate Power Amplifier tubes is located on the floor of the unit. Dual interlocks for personnel protection are incorporated on all but the lower front door.

(2) RF UNIT AM-350/FRT-6. - RF Unit AM 350/FRT-6 consists of the entire right center bay of Radio Transmitting Set AN/FRT-6. It contains the following units and circuits.

(a) POWER AMPLIFIER. - The Power Amplifier is located on the top deck of RF Unit AM 350/FRT-6. It consists of six forced air-cooled triode tubes operating in a push-pull grounded grid circuit. Three independent tubes, identical to those used in the Intermediate Power Amplifier, are mounted in a special common socket, and operated in parallel for each side of the push-pull circuit. No neutralization is necessary because of the inherent isolation between the input and output circuits of the grounded grid amplifier. The cathode or input circuit of the Power Amplifiers is an extension of the output coupling circuit of the Intermediate Power Amplifier, and is therefore located in RF Unit T-225/FRT-5.

Special r-f chokes are inserted in the filament leads to prevent them from short circuiting the input circuit. These chokes and a variable input capacitor are located in the enclosure directly above the power amplifier tubes. This enclosure will be referred to as the cathode box or enclosure throughout this book. A pair of special grid contact and by-passing assemblies are fastened to the bottom surface of this cathode box.

Anode, grid, and filament connections are so arranged that the tubes can be inserted as plug-in units from the bottom of the anode or plate casting. Since this would be a rather awkward procedure by hand, a special platform arrangement which holds three tubes has been devised. This platform is raised or lowered by a hand-wheel operated elevator to plug-in or disconnect the tubes. Cooling air for the tubes is forced upward through these elevators from the air chamber upon which they are mounted. A single large blower is located on the floor in the rear of the cabinet. The plate tank and output coupling circuits occupy the upper half of the rear portion of the cabinet. The plate tank circuit consists of two oppositely wound variable inductors and two variable vacuum capacitors all mounted with their axes vertical. These four variable elements are driven simultaneously with the proper speed ratio by a motor driven chain and sprocket arrangement located on the shelf below the tank circuit.

(b) ANTENNA COUPLING NETWORK. - The output network consists of a similar pair of variable coils and a single variable vacuum capacitor arranged to be varied simultaneously by a tuning motor. The network is suspended, as a unit, on rails from the top of the cabinet and moved forward or backward to vary the coupling to the plate tank coils.

(c) CONTROL PANEL. - The control panel carries the Power Amplifier, Antenna tuning, and transmitter checking controls. It is located behind the lower front door of the bay. Easy access to the controls is provided, however, by a small drop door located at the top center of this main door.

(d) SERVO AMPLIFIERS. - There are three identical interchangeable servo amplifiers mounted in an assembly rack in the center of the lower part of RF Unit AM-350/FRT-6. The servo amplifiers, in conjunction with their tuning motors, serve to position the three tuned circuits in this bay, viz., PA PLATE TUNING, ANTENNA LOADING, and ANTENNA TUNING.

(e) SERVO POWER SUPPLY. - The Servo Power Supply furnishes voltage to the three Servo Amplifiers located in RF Unit AM-350/FRT.6. It is mounted at the bottom center of the assembly rack in the r-f bay.

(f) GENERAL. - A large glass window in the upper front door of RF Unit AM-350/FRT-6 provides continuous visibility of the Power Amplifier tubes. Meters directly associated with tuning or adjusting the r-f stages in the bay are mounted above the window while the filament and DC voltage controls and the colored status lights are mounted below the window. On the floor of the bay, readily accessible through the rear doors, is the blower which provides the forced draft ventilation for the Power Amplifiers. All doors except the lower front have dual interlocks to afford adequate protection for the operating and maintenance personnel.

(3) POWER SUPPLY PP-49O/FRT-5. - Power Supply PP-490/FRT-5 consists of the left hand bay of Radio Transmitting Set AN/FRT-6. It contains the following units and circuits.

(a) CONTROL CIRCUITS. - A vertical panel inside the lower front door of Power Supply PP-490/FRT-5 mounts all of the control and overload relays and the timers which are associated with both Power Supply PP-49O/FRT-5 and RF Unit T-225/FRT-5. The control circuits, of course, are also interlocked with those of the remaining two bays to afford complete and adequate operation and protection. The lower front door is not interlocked and may be opened at any time without exposing open or unprotected circuits. A small drop panel on this door provides convenient access to the controls most often used in connection with Power Supply PP-490/FRT-5 and RF Unit T-225/FRT-5.

(b) LOW VOLTAGE SUPPLY. - 600 volts DC positive to ground for the operation of the frequency multiplier and buffer stages, the driver screen, and RF Oscillator O-92/FRT-5 is provided by a common low voltage supply. Two Xenon gas filled 3B28 rectifier tubes for this supply are grouped at the end of the rectifier shelf in Power Supply PP-490/FRT-5.

(c) BIAS SUPPLY. - 400 volts DC negative to ground for biasing the frequency multipliers, driver, intermediate power amplifier, and off-on keyer is provided by a common supply located in the Power Supply PP-490/FRT-5 unit.

(d) HIGH VOLTAGE SUPPLY. - The six 4B32 rectifier tubes mounted on the tube shelf are part of the plate voltage supply for the Intermediate Power Amplifiers. The high voltage transformer for this supply is mounted external to the unit. The filter components are located on the floor of the bay and are readily accessible through the large double rear doors.

(e) GENERAL. - Access to, and continuous visibility of the rectifier tubes is provided by the upper front door with its large window. Meters associated with the high voltage DC, the primary AC voltage and the Intermediate Power Amplifier filaments are located above the window. Status lights and controls are mounted below the window. A blower for circulating air in the power bay is mounted on the floor of the unit and is readily accessible through the rear doors. No special cooling is required for the rectifiers since they contain the inert gas Xenon and hence can operate efficiently and properly in spite of extremely wide temperature variations. All doors except the lower front are provided with dual interlocks for the protection of personnel.

(4) POWER SUPPLY PP-515/FRT-6. - The right hand bay of Radio Transmitting Set AN/FRT-6 is designated as Power Supply PP-515/FRT-6 and contains the following units and circuits.

(a) CONTROL. - A vertical panel behind the lower front door mounts the control and overload relays and the timers which are associated with both Power Supply PP-515/FRT-6 and RF Unit AM-350/FRT-6. They are, as mentioned previously, interlocked with those of the other two bays. The lower front door is not interlocked and may be opened at any time without exposing open or unguarded circuits.

(b) HIGH VOLTAGE SUPPLY. - Twelve Xenon gas filled 4B32 rectifier tubes are associated with the high voltage supply which is used to furnish the plate voltage to the Power Amplifier stage. The tubes are mounted on the top deck of Power Supply PP-515/FRT-6 and are readily accessible through the upper front door. The high voltage filter components are mounted on the floor of the bay and are accessible through the rear doors. The high voltage supply transformer is mounted external to Power Supply PP-515/FRT-6.

(c) BIAS SUPPLY. - Four 3B28 rectifier tubes are used as the rectifiers of a high current low voltage (1.5A at 400V) bias supply for the power amplifiers. All of the components for this supply are located within this cabinet.

(d) GENERAL. - The rectifier tubes are in constant view through the large glass window in the upper front door of the bay. Meters and controls for the unit are mounted above and below the window respectively. A blower to provide proper circulation of the air is mounted on the floor of Power Supply PP-515/FRT-6 and is accessible through the rear doors. Again no special cooling is required for the Xenon tubes. Dual interlocks, both automatic mechanical and positive electrical in nature, are incorporated on all but the lower front door to provide adequate protection for personnel.

(5) POWER CONTROL C-587/FRT-5. - Power Control C-587/FRT-5 includes the primary line input terminals for the 230 volt power to all parts of the transmitter except the final amplifier high voltage power supply plate transformer primary. The following components are included: a fused 3 phase circuit to the transmitter for filaments, blowers, etc.; a combination overload protecting breaker and start-stop contactor; a delta-wye switch for the intermediate power amplifier plate transformer primary; and a set of series line resistors for tune-up purposes with a shorting contactor for normal operation. These components are housed in a heavy gauge sheet metal box with a large front access door. This box is stamped with a wide variety of knock-out discs for conduit connection of the wiring. It is provided with four large mounting holes in its rear corners so that it may be wall mounted and connected directly to the primary line buss by means of rigid conduit.

(6) POWER CONTROL C-598/FRT-6. - Power Control C-598/FRT-6 consists of the primary line combination overload protecting breaker and start-stop contactor mounted in a heavy gauge sheet metal enclosure. This enclosure is identical to the one which houses the C-587/FRT-5 power control except that it does not have the ventilated resistor cage mounted on top. It is designed to be mounted on the wall near the main plate transformer with rigid conduit type wiring.

(7) POWER TRANSFORMER TF-122/U. - Power Transformer TF-122/U is a separate unit to be mounted on the floor separate from the cabinets. It is a three phase, dry type, air cooled unit endosed in a ventilated sheet metal case. It supplies the high voltage for the Intermediate Power Aniplifier plate voltage supply.

(8) POWER TRANSFORMER TF-124/FRT-6. - Power Transformer TF-124/FRT-6 is a separate unit to be mounted on the floor isolated from the cabinets. It is the main plate transformer of the power amplifier high voltage supply. This transformer is a three phase dry type air cooled unit enclosed in a ventilated grounded metal case. It contains a fixed transformer and a motor operated variable transformer which raises or lowers the output voltage at the will of the operator. The voltage may be varied under load, from 40% below to 10% above nominal.

(9) REDUCED POWER OPERATION. - The two left hand units of this equipment may be operated as the equivalent of an AN/FRT-5 15 kw transmitter completely independent of the 40kw power amplifier and its supply. A control circuit isolation switch, mounted on the relay panel of the Power Supply PP-515/FRT-6, isolates the 40 kw control circuits and allows the intermediate amplifier supply to be operated alone. The intermediate power amplifier output circuit is disconnected from the power amplifier input and connected to the transmission line to the antenna to complete the power cut back. This power cut back may be used as an emergency measure or for interim operation on circuits which do not require the full transmitter power.

TECHNICAL SPECIFICATIONS
Total Cubical Contents
Total Weight
Frequency Range	4000 to 26000 kilocycles
Frequency Multiplier Range	4 to 26 megacycles
First Frequency Multiplier	4 to 13 megacycles
Second Frequency Multiplier	4 to 26 megacycles
Type of Frequency Control	Crystal or Stabilized Oscillator
Type of Emission	a. A-1 (carrier ON-OFF) b. Frequency shift
Keying Speed	Up to 400 wpm both off-on and frequency shift emission
Nominal Carrier Output for each type of Emission	a. 40 kilowatts for A-1 emission, into a fixed 600 ohm load with a maximum SWR of 2 to 1 b. 40 kilowatts for frequency shift emission into fixed 600 ohm load with maximum SWR of 2 to 1
Power Factor of Equipment	a. Starting: 82.5% (High Voltage Off) b. Standby: 80.5% (Key Open) c. Normal Operation: 96%
Power Source Requirements	a. Voltage: 207 to 253 volts; designed for a center voltage of 230 volts b. Frequency: 60 cps ±5% c. Number of Phases: 3 Phase d. Input Power: Key Down: 86 kw; Key Up: 156 kw; High Voltage Off: 11.2 kw
Crystal	Type AN/CR27-U with fundamental frequency between 2.0 and 4.2 mc required (Not Supplied by Contractor)

TUBE COMPLEMENT
RF stages		AF stages and modulator		Rectifiers
Number	Type	Number	Type	Number	Type
8	3X2500A3			18	4B32
1	4-400A			8	3B28
2	807
1	6AG7

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	LBR	LIBERIA