TM 11-5820-921-40-1
normal operating range as opposed to starting the power amplifier in an overload (overdriven state). The
minimum ALC level was chosen to keep the attenuator inside its range during keyed intermittent voice,
CW, or data transmission periods. The OPERATE ALC is variable and may operate anywhere in the range
described above, thus allowing for any normal system variations due to environmental or electrical changes.
2-62. Prior to keying, the PRESET ALC voltage (≅ +2.6 Vdc) is developed on Meter Board PWB Assembly
1A1 by resistive divider 1A1R21 and R22. This PRESET ALC voltage is applied through connector
Xl A1-23, on Interconnection PCB Assembly 1A1, and connector 1A14J2-R to the receiver;transmitter.
When the PA is keyed, the T/R LINE is grounded. This inserts diode 1A1CR10 and resistor 1AlR23 into
the PRESET ALC circuit, causing the ≅ +2.6 Vdc PRESET ALC voltage to start decaying toward the
MIN ALC voltage of ≅ 0.94 Vdc. At the same time, the OPERATE ALC voltage (≅ 2.0 Vdc) is developed
in ALC Module Assembly 1A7. This is accomplished when the RF output of the final tank circuit is
sampled by capacitive divider network 1A7A1C1 and C2 and detected by diode 1A7A1CR1, a positive dc
signal is applied to pin 4 of operational amplifier 1A7A2AR1-1 and compared to the RF ALC voltage
applied to pin 3 of 1A7A2AR 1-1. This produces the OPERATE ALC voltage which is applied through
diode 1A7A2CR4 and resistor 1A7A2R9 to connector 1A7P1 -6. The ALC voltage is then applied through
connector 1A14J2-R to the receiver-transmitter to control the RF input to the power amplifier. Nominally,
the ALC voltage is +2.0 Vdc, for a power amplifier output of 400 watts 1 dB.
2-63. When the PA is operated in the Tune Mode, the TUNE LINE is grounded to accomplish two functions.
First, the grounded TUNE LINE energizes Tune Relay 1A7A1K2, connecting dummy load resistor 1A7A1R1
in series and resistor 1A7A1R2 in parallel with the power amplifier output, Second, it changes the thresh-
old voltage developed by voltage divider network 1A7A2R3, R4, R1 (adjustable), R12, R13, and R18.
The threshold voltage is compared to the IP SENSE signal (i.e., plate current of final amplifier tubes
1A14V1 and V2), to produce the TUNE ALC voltage output (OPERATE ALC +0.4 Vdc) at pin 9 of opera-
tional amplifier 1A7A2AR1-2. The increased ALC voltage is sent to the receiver-transmitter via 1A14J2-R,
to reduce the RF drive input of the power amplifier.
2-64. FAULT PROTECTION
2-65. The power amplifier contains circuits that prevent destructive secondary failures due to the following
circuit malfunctions or abnormal operating conditions:
a. REVERSE POLARITY PROTECTION - Described in paragraph 2-54a.
b. OVERVOLTAGE PROTECTION - Described in paragraph 2-54.b.
c. OVERTEMPERATURE PROTECTION - Described in paragraph 2-54.c.
d RECEIVE TO TRANSMIT RX-TX SWITCHING INHIBIT - (See paragraph 2-55).
The power amplifier cannot be switched into the Transmit Mode if any of the following conditions exists:
GND DURING WARM-UP -60 second initial power-on warm-up time has not elapsed. (See
e.
paragraph 2-5 1.)
f.
HF OVERLOAD FAULT - The+ 1600 Vdc is OFF due to tripping the De-to-De Converter overload
protection. (See paragraph 2-1 37.)
TUNE TIME FAULT - Tuning of the PA and the ANT CPLR was not completed in less than 20
g.
seconds, (See paragraph 2-1 24.)
h.
BAND SWITCH FAULT - The bandswitch motor 1A8A4B1 is running (either during a normal
frequency change or abnormally because of a malfunction in the control circuits). (See paragraph
L
i.
MIN GND - The power amplifier tank circuit has not tuned correctly, i.e., the 1 A8A5 L 1 variable
L
coil wiper has rotated all the way to the minimum inductance end of the coil and the MIN micro-
L
switch 1A8A3S2 has been actuated, grounding the MIN line. (See paragraph 2-88.)
ANT CPLR FAULT - The antenna coupler has not tuned properly (i.e., the tune cycle was not
j.
completed in 20 seconds or the VSWR is greater than 2:1). (See antenna coupler Technical Manual
TM-87117B-34/1,)
