TM 11-5820-921-40-1
to+33Vdc by variable resistor 1A3R3. When the supply voltage exceeds +33 Vdc, transistor
1A3Q2 is biased into saturation, shorting the collector of transistor 1A3Q1 to ground. If the
system is turned "ON" 1A3Q3 switches off and de-energizes PA ON/OFF relay 1A14K1. If the
system is turned OFF transistor 1A3Q3 is prevented from being switched ON by the PA ON/OFF
command signal (ground) from the receiver-transmitter.
c.
Overtemperature Protection - Whenever the heat sink (case assembly) temperature exceeds +96C,
the normally open contacts of thermostat 1Al4A3S2, (on Dc-to-Dc Converter 1A14A3) close,
grounding the collector of transistor 1A3Q1. Transistor 1A3Q3 switches OFF, de-energizing PA
ON/OFF relay 1A14K1. Dc power to the PA is interrupted and cannot be switched ON until the
heat sink temperature has cooled to at least 80C (the thermostat contact opening temperature).
2-56. In normal operation, system switching from receive to transmit is controlled by the depressing the
PTT (push-to-talk) switch on the handset. This action switches the receiver-transmitter into the Transmit
Mode and provides a ground on the KEYLINE signal input, connector 1A14J2-K, of the power amplifier.
The KEYLINE signal (ground) is applied to the base of transistor 1A3Q6 through pin 18 on System Control
PWB Assembly 1A3. Transistor 1A3Q6 is turned on, applying a positive voltage to the bases of transistors
1A3Q7 and 1A3Q8. When the base of transistor 1A3Q7 goes positive, the transistor turns ON, applying a
ground to the T/R LINE at pin 12 of System Control PWB Assembly 1A3. This energizes the high voltage
relay 1A13K1. The +1600V SENSE signal causes the RF input relay 1A1K1 to energize, connecting the
receiver-transmitter to the RF driver input. It also: (1) energizes the RF output relay 1A7A1K1, connec-
ting the output tank circuit to connector 1A14J4 (input to the Antenna Coupler), (2) energizes the HV ON
circuit by turning on transistor 1A2Q8 on DC Control PWB Assembly 1A2, and (3) lights XMT indicator
1A14A4DS3 on Indicator Assembly 1A14A4.
2-57. When the base of transistor 1A3A8 goes positive, it turns on, applying a ground to the CPLR
KEYLINE signal at pin 22 of System Control PWB Assembly 1A3. The CPLR KEYLINE signal is applied
to the antenna coupler via connector 1A14J1-K, allowing the antenna coupler to tune.
2-58, Receive to transmit switching is automatically prevented whenever any faults occur or during the
60-second warm-up period immediately after dc power in the power amplifier is switched on. This is
accomplished by grounding the collector of transistor 1A3Q6, preventing the transistor from turning ON.
The CPLR FAULT signal is fed back from the antenna coupler through connector 1A14J1-B to pin 9 of
System Control PWB Assembly 1A3. Any PA FAULT is applied to pin 11 of 1A3. The GND DURING
WARMUP signal from DC Control PWB Assembly 1A2, pin 19, is applied to pin 20 of System Control PWB
Assembly 1A3. With any of these signals at a ground level, transistor 1A3Q6 is prevented from turning on,
which prevents RX-TX switching.
2-60. The receiver-transmitter must provide 8 dB 2 dB attenuation of RF output per volt at ALC input
level, starting at 0.6 Vdc maximum and ending at 6.0 Vdc minimum. This requirement establishes the
following levels for the power amplifier:
≅
+0.94 Vdc
MIN ALC
ALC≅
+2.6 Vdc
PRESET
OPERATE ALC ≅
+2.0 Vdc
≅
OPERATE ALC plus (+) 0.4 Vdc
TUNE ALC
(3 dB power attenuation)
2-61. Due to the slow attack time of the ALC and the need to protect the power amplifier against large
bursts of rf, the MIN ALC and the PRESET ALC values were chosen to keep the ALC voltage level higher
than any operate or tune ALC level. Thus, the decaying ALC voltage places the power amplifier in its
