The ACT-40 Amateur Transmitter is housed in ACT-40-C black-crackle finish cabinet rack having overall dimensions of 19 inches wide, 15 inches deep, 26% inches high. The weight of the transmitter is approximately 164 pounds. A schematic diagram of the complete transmitter is shown in Figure 19.
The ACT-40-R (Figures 2 and 3) is the basic unit of this transmitter and is a complete 40-watt C-W telegraph transmitter when used with the required R-F unit accessories as mentioned above and suitable antenna tuning equipment. The chassis is 17 inches long, 14 inches wide and 2 inches high. It is fastened to a panel 19 inches long, 8% inches high, Vs thick and is suitable for mounting in a standard rack.
The R-F portion of the chassis consists of a crystal-controlled oscillator using an RCA-47 pentode, a buffer or doubler employing an RCA-802 pentode and a power amplnier using two RCA-801 triodes in a cross-neutralized push-pull circuit. A filter circuit to minimize key clicks is included in the unit. A 0-200 milliammeter and a four-point selector switch are mounted on the front panel. By operating the selector switch, the oscillator plate current, the buffer plate current, and the power-amplifier grid and plate current may be read. Neutralization is accomplished by means of a "neut" stick or insulated screwdriver inserted through a hole in the front panel. A special neutralizing capacitor is used so both 801's are neutralized simultaneously. A switch on the front panel opens the power-amplifier plate return circuit for neutralization or for preliminary tuning. A combination of battery and grid-leak bias is employed for the buffer/doubler and power-amplifier circuits to keep the plate current at a safe value when excitation is removed. The R-F unit is capable of delivering 40 watts when properly loaded by an antenna. The full carrier output can be modulated 100% with the ACT-40-M Modulator Unit for phone operation. An adustable antenna-coupling coil is an integral part of the final-amplifier tank-coil assembly. Antenna coupling adjustment need be made but once so long as the antenna constants remain fixed.
The power-supply portion of the chassis consists of a plate transformer, filament transformer, an RCA-83 mercury-vapor rectifier tube, filter reactor and capacitor, and "bleeder" resistors to provide proper plate and screen voltages for all the tubes in the R-F unit. The transformers are designed for 115-125-volt, 60-cycle a-c, and the primaries being tapped for these voltages. Connection to the a-c source is made by means of terminals located on the rear apron of the chassis. The main power switch mounted on the front panel closes the primary circuit to the filament transformer. This switch must be closed before power can be made available through the plate transformer switch, also operated from the front panel. Convenient outlets mounted on the rear apron of the chassis are so connected that a-c supply voltage to other units may be plugged in and energized through the main power and plate transformer switches. Thus the plate voltage to the R-F modulator or other circuits cannot be applied prior to turning on all filaments. A green pilot light indicates that the filaments are on and the transmitter is ready for operation.
The ACT-40-M Modulator Unit (Figures 4 and 5) consists of two high-gain stages of speech amplification, each using an RCA-57 as a pentode resistance-coupled audio-amplifier, a push-pull driver stage using two RCA-45's, and a Class B amplifier using two RCA-801's. An RCA-83 is used as a rectifier to deliver plate voltage to the Class B stage, and to the speech amplifier/driver stages by means of a voltage divider. A clamp is provided on the chassis to hold the batteries for the Class B stage bias supply and for the RCA-45 driver stage. Approximately 61l/2 volts bias is required for the RCA-801 stage and 56 volts for the RCA-45 stage. The overall gain of the modulator unit is 101 db.
The input circuit to the first RCA-57 is of the high-impedance type, a 250,000-ohm potentiometer being used in the grid circuit. This type of circuit is suitable for use with a crystal microphone or any other high-impedance microphone. Since there is no input transformer, chances for hum pickup are minimized. When a carbon microphone or other low-impedance microphone is used, it is necessary to employ an input transformer or some other suitable means of coupling. The audio gain is regulated in this stage. The first stage is resistance coupled to another high-gain stage, which is resistance coupled to the 45 driver stage. A phase inverter circuit across the primary of the driver transf ormer eliminates the necessity of a transformer to couple the 57 to the 45 tubes. The 45 stage is transformer coupled to the 801 grids. The 801 s will deliver approximately 33 watts of audio power to a 4000-ohm load such as is presented by the Class C stage of the R-F unit. The secondary of the modulation transformer is designed to carry 125 milliamperes at 500 volts.
A switch on the input volume-control is connected in series with the primary of the plate transformer, so that when the gain control is turned to the "Off" position the modulator plate and speech-amplifier plate-voltages are removed. A terminal board on the rear apron of the chassis provides a convenient means of connecting the modulator unit to its load. Two cords and plugs are provided, one for the a-c input to the filament transformer primary and one for the plate transformer primary.
The ACT-40-A Antenna Unit (Figure 6) consists of two variable condensers, a 0-2.5 ampere thermocouple R-F ammeter and a special knife switch on the back of the panel. By means of the switch a number of circuits for tuning the antenna may be used as illustrated in Figures 7, 8 and 9. Two stand-off insulators are mounted on the panel so that an antenna and ground, or two-wire transmission line may be connected to the unit. Two heavy copper-wire bus connectors are supplied for connections to the R-F unit.
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