The collector current to the PA output transistors is monitored as a voltage
drop across R5 (0.015 Ohm/10W) on the DC input board and fed to op amp U2
and Q14 on the 9 MHz mixer/IF board. U1B compares the set power level (as
determined by the front panel RF PWR control) to the preset current (as set
by R54 with the RF PWR control fully clockwise) and controls a pin-diode
attenuator consisting of D18/D19. If the PA collector current exceeds a
preset level of 22 amps the RF drive available at connector-27 (TXD OUT) is
reduced to protect the PA transistors.
Essentially, the PA is capabable of operating into a dead short or open, or
anything between, until the safe collector current point is reached. You
can always maintain a lower output power under high SWR conditions by
backing off on the RF PWR control.
73, Dave, K1FK
Fort Kent, ME
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