TM 1 1 - 5 8 2 0 - 9 2 1 - 4 0 - 1
2-28, Indicator lamps (DS1 thru DS8) on Band Control PWB Assembly 1A6 show which band is presently
being used, while an OVERLOAD indicator (DS1) on DC Control PWB Assembly 1A2 lights when the PA
FAULT is caused by excessive plate current. This lamp will be lit at the same time as the PA FAULT lamp
when an OVERLOAD FAULT occurs. In addition all modules and pc boards have test points that are
easily accessible for monitoring and fault location.
2-30. Subsequent paragraphs 2-31 through 2-48 are a detailed description of the RF amplifier.
RECEIVE MODE RF SIGNAL PATH (See figure 2-2)
2-32. In the receive mode of operation, none of the power amplifier circuits are required for operation and
are bypassed. The RCV RF signal from the antenna (through Antenna Coupler CU-2064/GRC-1 93)
enters the power amplifier through "N" type 50-ohm coaxial Connector 1A14J4 and is routed through
the normally closed contacts (pins 6 and 8) of relay 1A7AK1. The RCV RF signal is then routed through
a coaxial cable (RG-188) from the A2 output of the ALC Module to pin 20 of Meter Board PWB Assembly
1A1. Here the RCV RF signal is routed through the normally closed contacts (pins 6 and 8) of relay
1AlK1 to pin 18 of Meter Board PWB Assembly 1 Al. From pin 18 of Meter Board 1Al, the RCV RF
signal is routed through a coaxial cable (RG-158) to the "N" type 50-ohm coaxial connector 1Al4J5 and
then to Receiver-Transmitter RF-1209/URC. Both relays (1A7AlK1 and 1A1K1), which are de-energized
during the receive mode, are controlled by the T/R LINE signal from System Control PWB Assembly 1A3.
2-33. Dc power is applied to all necessary power amplifier circuits during the receive portion of the
Rx-Tx mode of operation so that the power amplifier is ready for immediate operation during the transmit
portion of the Rx-Tx mode. When the radio set is in the Rx-Only mode, all power amplifier circuits are
de-energized because a PA ON signal is not sent to the power amplifier from the receive-transmitter.
l 00 milliwatts, nominal) from the
receiver-transmitter is applied to the 1A14J11-A1-A1 input of RF Driver Module 1A11 through the RT
RF input connector 1A14J5 and the closed contacts (pin 5 to pin 2) of energized relay 1A1K1. Relay
lA1K1 is energized by the +1600V SENSE signal which charges capacitor 1A1C9 across resistors
lA1R18 and 1AlR20. When the charge across capacitor 1A1C9 reaches a sufficient amplitude,
transistor 1A1Q5 is biased on, applying a positive bias to the base of transistor 1A1Q4. This biases
1A1Q4 on, applying a ground through 1A1Q 1 to pin 7 of relay 1 Al K 1 and energizing the relay.
2-37. The XMT RF signal from relay 1 A 1 K 1 is applied through a resistive attenuator, comprising
lA11R2, R3 and capacitor lA1lC1, to the base of first RF amplifier transistor 1 Al 1 Q 1. The attenuator
reduces the nominal <_l 00 milliwatt input signal by less than 1 dB and presents a constant 5 O-ohm termina-
tion impedance to the receiver-transmitter output stage. Transistor 1 A 11 Q 1 is operated as a common-
emitter class A amplifier whose output is transformer coupled through 1A11T1, to the push-pull high
level driver stage, formed by transistors 1A11Q2 and Q3. The DC collector voltage of transistor 1A11Q1
is fed through the primary of transformer 1 Al lT1 while the base bias is established by resistors 1A11R4
and R6. Both collector-base feedback (through selective network 1Al 1R5, C2, and L2) and emitter
degeneration (through resistor 1A11R7) are used to stabilize the first stage gain in the presence of supply
voltage variations and transistor parameters over the 2 to 30 MHz frequency range.
2-38. As stated before, the output of the first rf amplifier stage is coupled to the bases of the class AB
push-pull output stage, formed by transistors 1A11Q2 and Q3, through impedance matching transformer
1 Al 1T1. +26.5 Vdc (nominal) is fed to the collectors of 1Al 1Q2 and Q3 through transformer 1A11T2.
The voltage drop across the resistor-diode combination of 1Al 1R 1 and 1Al 1CR1, which is caused by
the current through transistor 1 Al 1Q1, is used as the base bias for push-pull transistors 1A11Q2 and Q3.
