TM 11-5820-921-40-1
2-47. When the power amplifier is in the Tune Mode, operation of the circuits in the XMT RF signal path
is identical to that previously described, with the following exceptions:
1. Tune Relay 1A7A1K2 is also energized, switching the dummy load (series resistor 1A7A1R1 and
shunt resistor 1A7A1R2) in series with the tank circuit output. This assures a constant loading
for tuning the tank circuit regardless of the impedance presented by the antenna coupler. The
power amplifier can tune load impedances exceeding the 25 to 75 ohm worst case range, including
reactive loads.
2. The ALC (Automatic Level Control) system loop gain is automatically increased to reduce the
XMT RF power output to about 40 watts. A current-derived ALC signal called Ip SENSE is
provided to limit drive until sufficient RF voltage is generated to control the system.
3. The reactive elements in the tank circuit π network are not initially correct for matching the tube
output impedance to the nominally 50 ohm load, but the proper elements are automatically
selected in the Tune Mode by the bandswitching of the shunt elements and the fine servo tuning
of inductor 1A8A5L1.
2-48. Control and sequencing of circuit operation in the Tune Mode are described in the following para-
graphs. The explanation includes command and control signals to the receiver-transmitter and the antenna
coupler.
2-50. Subsequent paragraphs 2-51 through 2-112 describe the operation of power amplifier circuits as
they are used during the normal receive-transmit operation, i.e., system tuned at the desired frequency.
2-52. The ON-OFF switching of the +26.5 Vdc nominal primary power line in the power amplifier is
controlled remotely by the receiver-transmitter. As shown in figure 2-4, +26.5 Vdc (DIRECT) is distributed
to receiver-transmitter RT-1209/URC via connector 1A14J2-L and Amplifier-Converter AM-6879-URC
2-53. The dc power ON command from the receiver-transmitter (called PA ON/OFF) is a ground on pin
N of connector 1A14J2. This ground is applied to the base of transistor 1A3Q1, turning the transistor
"ON". This applies a positive voltage through resistor 1A3R6, transistor 1A3Q 1, and diode 1A3CR3 to the
base of transistor 1A3Q3 driving the transistor into saturation and completing the current path from the
+26.5 Vdc line thru the coil of relay 1A14K 1. When the PA ON/OFF relay is energized, the +26.5 Vdc is
connected from DC PWR connector 1A4J3 through the normally closed contacts of circuit breaker
1A14CB2 to all user circuits within the power amplifier.
a.
Reverse Polarity Protection - The correct power line connections at DC PWR connector 1A14J3 are:
(1) Pins A and B positive (+26.5 Vdc)
(2) Pins C and D negative (ground)
If the polarity of these connections is accidently reversed, dc power to the power amplifier circuits cannot
be switched on. Diode 1A3CR2 on the system control PWB assembly prevents a negative voltage from
being applied to switching transistors 1A3Q1 and 1A3Q3, The transistors cannot be damaged nor can
PA ON/OFF relay 1Al4K 1 be energized.
b.
Over-voltage "Protection - If the primary supply line dc voltage exceeds +33 Vdc, the power amplifier
is automatically switched OFF to prevent damage to circuits and components. The tripping voltage
is sensed via voltage regulator 1A3VR2 and resistors 1A3R3, R4, and R7. The trip point is adjusted
