COLLINS 207A-1

GENERAL DESCRIPTION

1. GENERAL DESCRIPTION AND PURPOSE

The Collins 207A-1 is a 10-kilowatt AM or 15-kilowatt cw transmitter designed for landbased operation in the range 4 - 26 megacycles. It may be used for medium-power AM transmission, telegraphic on-off or frequency shift keying at speeds greater than 400 words per minute, and for boosting the power output of a single-sideband transmitter to a peak envelope power of 8 kilowatts. The transmitter is designed for higher than average program modulation.

This equipment is the latest development in medium-power transmitters. It incorporates many new features that improve its reliability and simplify adjustment procedures. An outstanding feature is the system of motor-driven, servo-positioned, continuous-tuning resonant circuits with which the transmitter may rapidly be tuned to any frequency within its range by means of six easily accessible front panel tuning controls.

The 207A-1 is equipped with the Collins 40G R-F Crystal Oscillator as a standard exciter. However, any one of several different standard rack-and-panel type exciters may be used. For example, the Collins Frequency-Shift Keyer 709A-1 may be used in conjunction with Stabilized Master Oscillator 708A-1 and Power Supply 707A-2.

There is no need for a transformer vault or other protective enclosure since all the components which mount external to the transmitter are enclosed in sheet-metal housings. Wiring between these units and the transmitter is in grounded conduit or flexible metal tubing. The transmitter and all components are adequately ventilated and air cooled.

2. BASIC PRINCIPLES OF OPERATION

The Collins 207A-1 is a master oscillator-power amplifier type transmitter in which a low-power radio-frequency signal is multiplied in frequency and amplified sufficiently to produce a high-power radio-frequency carrier. The carrier is then modulated or interrupted or its frequency is shifted in accordance with the intelligence to be transmitted. The resulting radio- frequency energy is then radiated from a suitable antenna.

The low-power excitation may be supplied either by the ten-channel crystal controlled oscillator which is included in the transmitter or by any one of several different standard exciters. This low-power signal passes through a buffer amplifier and two multiplier stages where the frequency is increased by a factor of 2, 4, 6, or 8. The radio-frequency energy is then amplified to 15 kilowatts in the power amplifier stage.

Intelligence is added to the carrier wave by the modulator stage for amplitude modulated (AM) transmission. For code (cw) transmission the intelligence is added by on-off keying or frequency-shift keying. Off-on keying is accomplished by interrupting the carrier in the low-power buffer and first multiplier stages. Frequency-shift keying is accomplished by passing the signal through a frequency-shift keyer before it enters the buffer amplifier. The keyer then shifts the carrier frequency in accordance with the keying signal.

The transmitter is equipped with safety and overload devices which afford complete protection to personnel, and which protect the equipment from damage by overloads and other possible equipment troubles.

3. GENERAL DESCRIPTION OF UNITS

a. INTRODUCTION. - The Collins 207A-1 Transmitter is contained in three sheet-metal cabinets which bolt together to form a single unit. In addition, three smaller sheet-metal enclosures contain the modulation transformer and reactor, the power transformer, and the power control unit.

Several features unique to transmitters of this power class have made the 207A-1 a compact, yet easily serviced piece of equipment. The two medium-power triodes in the power amplifier stage require relatively low plate voltage. This allows the use of smaller than usual power supply components. The tubes and tank circuit components are forced air cooled thereby eliminating the need for a bulky water cooling system.

The externally mounted transformers are dry type, air cooled and fully enclosed in grounded sheet metal cases. Connecting leads are brought in through grounded conduit and are connected internally, thus eliminating the usual requirements for additional protective enclosures. High-current primary circuits are fed directly to the plate transformer through the external control unit; thus the space and service problems usually presented by high-current wiring and control components within a main enclosure are eliminated.

All resonant circuits are continuously variable, motor driven, and servo positioned. This system avoids the usual complications involved in band- switching and mechanical drive. Servo positioning allows the controls to be grouped at a convenient location, while the resonant tank is placed where it best suits the requirements of the circuit.

The upper front door of each of the three cabinets has a large glass window which affords a view of tubes within the bay. The upper front door provides access to these tubes as well as some of the adjustment controls not directly associated with operating the transmitter. These doors are hinged at the top and counterbalanced for ease of operation. They are further provided with dual interlocks, both positive electrical and automatic mechanical, which remove all dangerous voltages and ground the high voltage as soon as the door is opened. Status lights, illuminated meters, and the main control switches are mounted in these upper doors. Interlocks for all the units are wired in series so that opening any door immediately removes high voltage from 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 access to the control panels of each of the bays when the lower doors are closed.

b. LIST OF UNITS. - Collins 207A-1 Transmitter consists of the following units.

R-F Unit, which contains:
1. R-F Oscillator 40G
2. Servo amplifiers
3. Servo power supply
4. Patch panel
5. Electronic keyer
6. Control panel
7. Buffer amplifier, frequency multipliers and driver amplifier
8. Power amplifier
9. Antenna coupling network
10. Space and switching for additional exciters
Power unit, which contains:
1. Control circuits
2. Low voltage supply
3. Bias voltage supply
4. High voltage supply
5. Primary voltage regulator
Modulator unit, which contains:
1. Audio amplifier and driver
2. High power modulator
3. Modulator bias and audio amplifier power supply
PA plate power control unit
PA plate transformer
Modulation network

c. MAJOR AND MINOR UNITS - DETAILS. - In more detail the major and minor units of the 207A-1 transmitter are:

R-F UNIT. - The r-f unit is the right end bay. It contains on a special mounting space the units described under paragraphs a., b., c., d., and j. 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. The upper front door of the r-f unit has a large glass window to provide continuous visibility of the power amplifiers. All meters directly associated with tuning or adjusting the r-f stages in the bay are mounted above the window. The filament controls, d-c controls, and colored status lights are located below the window on the upper front door. A ventilating blower cooling the power amplifier tubes is located on the floor of the unit. The r-f unit contains as permanent equipment the circuits and units described under paragraphs e., f., g., h., and i..
1. R-F OSCILLATOR 40G. - R-F Oscillator 40G is a crystal-controlled 10-channel oscillator. With proper crystals, the oscillator provides output in the range 2 - 4.3 mc. Plate voltage for this unit comes from the main transmitter power supply. Filament voltage and power for the crystal oven are supplied from a separate 110 v. source. The oscillator is mounted in the equipment rack of the R-F bay.
2. SERVO AMPLIFIERS. - Six identical, interchangeable, plug-in servo control amplifiers are 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 servopositioned tuned circuits in the r-f unit. Each contains a voltage amplifier, a relay control tube and a pair of motor control relays.
3. SERVO POWER SUPPLY. - The servo power supply furnishes voltage to all of the servo ampifiers and drive motors located in the r-f unit. It is mounted on the lower left side of the equipment rack immediately below the servo amplifiers.
4. PATCH PANEL. - The remaining unit in the equipment rack is the patch panel, which contains eight jacks - one is the transmitter input jack and one is the crystal oscillator output jack. The other six may be used as required.
5. ELECTRONIC KEYER. - An electronic keyer is mounted to the right of the r-f unit control panel. It receives the keying impulses and controls the transmitter carrier. The keyer may be actuated by polar or neutral keying impulses. This unit is hinged so that it may be tilted forward for access to the interior.
6. CONTROL PANEL. - The control panel carries the basic transmitter tuning controls and is located behind the lower front door. Easy access to these controls is provided by a small drop door located at the top center of the main lower door.
7. BUFFER AMPLIFIER, FREQUENCY MULTIPLIERS, AND DRIVER AMPLIFIER. - The buffer amplifier, frequency multipliers and driver unit are contained within a subassembly which is mounted to the left of the control panel. The unit is hinged so that it may be tilted forward for access to the interior. In this unit are located an untuned buffer-amplifier, and two frequency multiplier stages. The driver tube is mounted on top of the assembly.
8. POWER AMPLIFIER. - The power amplifier is located on the top deck of the r-f unit. 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 is the plate tank for the driver stage, is located within an enclosure 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 tubes. The resonant plate tank circuit is located directly behind the power amplifier tubes and is readily accessible through the large double rear doors.
  
  Provisions are made for operating the power amplifier as linear amplifier to be driven by a single-sideband suppressed-carrier exciter such as the Western Electric D-156000 twin-channel unit. The grid circuit of the power amplifier can be modified so that it presents a 200-ohm balanced load to the output of the single-sideband transmitter. Approximately 500 watts peak exciting power are required.
9. ANTENNA COUPLING NETWORK. - The output network occupies the top of the r-f unit. It is a resonant circuit continuously variable across the entire frequency range and is suspended on tracks from the top of the cabinet. Magnetic coupling from the plate tank coils to the inducctors of this coupling network is varied by positioning the coupler on its rails.
10. SPACE AND SWITCHING FOR ADDITIONAL EXCITERS. - Space for auxiliary exciters is provided in the lower front of the r-f bay. A space of 19-1/4 inches is available on one half of the panel and 10-1/2 inches on the other.
POWER UNIT. - The power unit is the center bay of the 207A-1 transmitting set. Access to the rectifier tubes is provided by the upper front door. Meters in the high-voltage dc, primary ac, and the power amplifier filament circuits are located above the window. Status lights and power 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 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 door are provided with dual interlocks. The power unit contains the following units and circuits:
1. CONTROL CIRCUITS. - A vertical panel inside the lower front door of the power unit mounts all of the control and overload relays and the timers associated with both the power unit and the r-f unit. The control circuits and interlocks are interlocked with those of the other bays. The lower front door is not interlocked and may be opened at any time without exposing open or unprotected high-voltage circuits. A small drop panel on this door provides access to the controls most often used in connection with the power unit, the r-f unit, and the modualtor unit.
2. LOW-VOLTAGE SUPPLY. - A low-voltage supply provides 600 volts d-c for operation of the frequency multiplier and buffer stages, the driver screen, and R-F Oscillator 40G. Two xenon gas filled 3B28 rectifier tubes for this supply are located at the end of the rectifier shelf in the power unit.
3. BIAS SUPPLY. - A supply located in the power unit provides 400 volts dc negative to ground for biasing the frequency multipliers, driver, power amplifier, and on-off keyer.
4. HIGH-VOLTAGE SUPPLY. - The six 4B32 rectifier tubes mounted on the tube shelf are part of the plate voltage supply for the power amplifiers. The high-voltage transformer for this supply is mounted externally. The filter components are located on the floor of the bay and are readily accessible through the large double rear doors.
5. PRIMARY VOLTAGE REGULATOR. - The primary voltage regulating system consists of a set of buck-boost transformers in series with the 230-volt input line, a motor-driven powerstat, and a voltage-sensitive motor control circuit. The voltage regulating system maintains a constant output voltage of 230 over a line frequency range of 50-60 cps and an input voltage variation of ±10%.
  
  All components of the voltage-regulating system are located in the power bay. The buck-boost transformers are located underneath the blower, the powerstat is mounted on the floor of the cabinet next to the blower, and the motor-control circuit components are mounted on a sub-panel behind the control panel of the power unit.
MODULATOR UNIT. - The modulator unit is the left-hand bay of the 207A-1 transmitter. It contains the following units and circuits.
1. AUDIO AMPLIFIER AND DRIVER. - Audio amplifier and driver stages are located on the rear wall and floor of the top deck of the modulator unit, respectively. The audio stage uses a twin triode, type 12AX7, operated as a balanced push-pull voltage amplifier to provide 800-1110 volts peak swing for the modulator grids. The driver tubes are mounted on the floor of the top deck through which cooling air is forced from the blower compartment. All three tubes are readily accessible through the upper front door.
2. MODULATOR. - The high-power modulator stage utilizes two type 8C25 triodes in a push-pull circuit adjusted for class B1 operation. One tube is located on the left side of the top deck of the modulator unit. The other modulator tube is mounted on the right side of the top deck. Cooling air is forced upward through the tube sockets and cooling fins from the blower compartment below. Both tubes are accessible through the upper front door.
3. MODULATOR BIAS AND AUDIO AMPLIFIER POWER SUPPLY. - The 3-kilovolt supply uses two xenon-filled type 4B32 rectifier tubes. This supply furnishes 270 plate volts for the audio stage, 1700 volts for the driver plate, and from -670 to -1100 volts for modulator bias. The rectifier tubes are mounted in the rectifier compartment of the modulator unit and are accessible through the rear doors.
PA PLATE POWER CONTROL UNIT. - The PA plate power control includes the primary line input terminals for the 230-volt power to all parts of the transmitter. The following components are included: A fused 3-phase circuit to the transmitter for filaments, blowers, etc.; a combination overload breaker and start-stop contactor; a delta-wye switch for the 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-guage sheet metal box with a large front access door. This box is stamped with a variety of knock-out discs for conduit wiring. The box has four large holes in the rear corners so that it may be wall mounted and connected directly to the primary line bus by means of rigid conduit.
PA PLATE TRANSFORMER. - The PA plate transformer is to be mounted on the floor separate from the cabinets. It is a three-phase, dry type, air-cooled unit enclosed in a ventilated sheet metal case. It supplies high voltage for the power amplifier plate supply.
MODULATION NETWORK. - The modulation network is to be mounted separate from the main cabinets. It houses the modulation transformer coupling capacitors, the modulation reactor, a low-pass filter, and the AM-CW relay. The modulation transformer and reactor are dry type, and all connections to them are made inside the sheet metal case. Access for inspection is provided by a hinged top which is electrically and mechanically interlocked.

TECHNICAL SPECIFICATIONS
Total cubical contents	344 cubic feet (not crated)
Total weight	8253 pounds (not crated)
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 variable frequency oscillator
Type of emission	a. A3 (amplitude modulation) b. A1 (carrier on-off) c. Frequency shift
Keying speed	Up to 400 wpm for both on-off and frequency shift emission.
Keying input circuit impedance	25,000 ohms - one line grounded
Nominal carrier output for each type of emission	a. 10 kw for AM emission into a fixed 600-ohm load with a maximum swr of 2 to 1 b. 15 kw for A1 emission, into a fixed 600-ohm load with a maximum swr of 2 to 1 c. 15 kw for frequency-shift emission into a fixed 600-ohm load with a 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 requirement	a. Voltage: 207 to 253 volts; designed for a center voltage of 230 b. Frequency: 60 cps ±5% or 50 cps ±5% c. Number of phases: 3 d. Input power: AM unmodulated: 25 kw 100% modulated: 35 kw CW key down: 25 kw CW key up: 4.5 kw High voltage off: 3.3 kw e. Regulation tolerance: ±5% voltage variation from no load to 38 kw
Crystal	Type AN/CR-27/U with fundamental frequency between 2.0 and 4.2 mc (not supplied by contractor)
Modulation method	High level (class B1 modulators)
Modulation capability	100 per cent
Audio input level for 100% modulation	+10 ±2 dbm into 150/600 ohm input impedance
Audio frequency response	Equal to or better than 2 db between 150 and 7500 cps and ±4 db from 50 to 10,000 cps
Audio frequency harmonic distortion	Less than 5% rms for fundamental frequencies 50 to 7500 cps for values of modulation from 0% to 90%
Carrier frequency stability	±5 cycles per megacycle
Carrier noise level	AM - 50 db below 100% modulation over band of 70 to 7500 cps Cw - Less than 3% equivalent amplitude or phase modulation
Carrier shift	5% maximum at 90% modulation
Ambient temperature range for satifactory operation	0 to 50°C (32°F to 122°F)
Elevation above sea level for satisfactory operation	6000 feet maximum
Specifications may change without notice

TUBE COMPLEMENT
RF stages		AF stages and modulator		Rectifiers
Number	Type	Number	Type	Number	Type
2	3X2500A3	2	4-250A	4	3B28
1	4-400A	2	8C25	6	4B32
1	6AK5	1	12AX7	2	2050
4	6X4	2	4B32
2	6AG7
2	807
7	12AU7
6	12AX7
2	VR90

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