The PA control circuits provide the following functions:
ON-OFF switching of primary power to the dc-to-dc converter, voltage regulator, antenna coupler
and other circuits within the power amplifier unit, on command from the RT.
Rx-Tx (Receive-Transmit) switching of the RF signal as required for Receive mode and Transmit
mode operation. In the Receive mode, both the 1A7A1K1 relay and the lA1K1 relay are de-
energized and the receive RF signal is bypassed around the power amplifier circuits to the input of
the receiver-transmitter. In the transmit mode, both the 1A7A1K1 relay and the 1A1K1 relay are
energized, connecting the tube tank circuit output to the antenna coupler and the transmitter
output to the first driver stage.
Inhibits transmit operation if: (1) the antenna coupler does not tune to a preset VSWR during a
preset time, (2) the PA tank circuit does not tune correctly, (3) the system (PA and coupler) do not
tune in less than 20 seconds, (4) the high voltage overload is tripped, or (5) the bandswitch motor
Samples the RF power output to provide a proportional positive-going ALC (automatic level
control) signal to the RT (receiver-transmitter) for controlling the TX output of the receiver-
transmitter. The ALC loop gain is preset to maintain a 400 watt output with variations in gain of
the signal path.
The power circuits provide the necessary operating voltages for the circuits in the PA and are
derived from the +26.5 Vdc (nominal) input from the power source. These voltages are +1600 Vdc from a
transistorized dc-to-dc converter and high voltage module, -14 Vdc from Meter Board PWB Assembly 1A1,
+19 Vdc from series type regulator 1A14Q3 and DC Control PWB Assembly 1A2, and filament voltages to
the output amplifier tubes from filament regulators 1A14Q4 and 1A14Q5 and DC Control PWB Assembly
1A2. The voltage regulators are self-protected against shorts while the dc-to-dc converter is overload
protected. In case of temperatures over 96C the complete power supplies are disabled by thermostatic
switch 1A14A3S2 until the PA has cooled down to a safe temperature. The primary power supply and
system wiring are protected from short circuits in the PA by 50 ampere (normally closed) circuit breaker
The cooling circuit consists of a vane axial fan, 1A14B1, which is external to the sealed case-cover
assembly, and Static Power Inverter Assembly 1A12, which is thermostatically y controlled. The blower is
automatically switched on by 1A14A351 when the heat sink temperature exceeds +55C and switched off
when the unit has cooled to about +54C. When the system is operated in an intermittent Tx condition,
the blower does not start. Under continuous Tx conditions and high ambient temperatures the blower runs
1-10. The PA contains circuits and indicators for monitoring operation. Meter 1A14M1, normally set to
indicate forward power output in transmit, and six front panel indicator lamps provide the means for
monitoring PA status without opening the unit. The indicators RDY and XMT are green, TUN and FIL
fault are amber, and PA and CPLR fault are red.
1-11. For testing and troubleshooting the PA, meter (1Al4M1) has eleven test positions selectable from
TEST SWITCH 1A1S1. All dc voltages, the filament current of each tube, the ALC, the cathode level
(RFK), tube plate current (Ip), reflected RF power (REF), and forward RF power (FWD) can be measured
using TEST SWITCH 1A1S1 and meter 1Al4M 1. Indicator lamps (GREEN) on Band Control PWB
Assembly 1A6 show which band is being used. An overload lamp on DC Control PWB Assembly 1A2 shows
that the PA fault is caused by excessive plate current when it is lighted (red) at the same time the front
panel PA fault lamp is lighted (red). In addition all modules and pc boards have test points for monitoring
and fault location.
1-12. Refer to chapter 2 for a detailed description and analysis of power amplifier operation.