Technical Description 4-18 X-2 Technical Service Manual
Publication No: 15-02047
4.6.4 Output Stage and Bias Regulator
The push-pull class B output stage V18
[E11]
and V19 is base driven from the
centre tapped secondary winding of transformer T3. The centre tap provides
the bias feed from the bias regulator.
The bias regulator V16 and V17 provides a constant voltage to the bases of
V18 and V19. Transistors V16 and V17 form a feedback voltage regulator,
the output voltage being the base-emitter voltage of V16, adjustable by
preset potentiometer R45. Zener diode V15 and resistor R47 cause the bias
to increase when the supply voltage drops below 11 V to reduce
intermodulation distortion.
Transistor V16 is mounted on the PA heatsink and provides temperature
compensation to the bias network.
The balanced to unbalanced impedance matching output transformer couples
the power amplifier output via the transmit/receive relay K7 to the band
filters.
4.6.5 Output Filters and Antenna 1 and 2 Select
The frequency range of 2 MHz to 18 Mhz uses five low pass filters selected
by relays K2 to K6 and operated by IC1. The filters are selected to remove
harmonics generated by the PA.
The output of the filter circuit passes through the Automatic Level Control
(ALC) RF bridge to antenna output J1 or J2 as selected by relay K1.
4.6.6 ALC Control
104-02908
Automatic Level Control is provided from the following sources:
• Forward power
• Reflected power
• Output stage collector swing
• Battery voltage
• Heatsink over-temperature.
Control Inputs
The ALC control inputs are applied to V4 - V7 and the ORed output is
connected via R20 to positive input (pin 5) of the ALC level comparator
IC3/B
[F5]
. The negative input (pin 6) is set to 3.6 V by the divider R22, R23,
and R24.
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