X-2 Technical Service Manual Technical Description 4-19
Publication No: 15-02047
PA Gain Control
In the absence of any ALC inputs, the output of IC3/B holds the base of V8
to 3.6 V. With V9 base referenced to 5 V, V8 cuts off setting V9 to
maximum gain. An ALC control signal that causes any of the transistors V4
to V7 to conduct, results in the output of IC3/B rising to reduce the gain of
V9, and therefore controls the gain of the PA.
The output voltage of the RF bridge T1, L1, R1, R2 and capacitor divider
C3, C4, C5 and C6 is rectified by D1 for the forward power and D2 for the
reflected power. The addition of L2 (Link X not fitted) allows for a
reduction in output power at the frequencies above 12 MHz.
The output of the forward rectifier D1 is applied via divider R12 and R13 to
R15 to input of V4 and determines the PEP power output level (set by SOT
R14). D1 is also connected via R17 to input V5 and in conjunction with C68
sets the average power level to approximately 60 W.
The output of the reflected rectifier D2 is applied via divider R18 and R19 to
input V6 and takes control of the ALC when the reflected power exceeds 10
watts ( > 2 : 1 VSWR ).
Peak output collector swing is monitored by R57
[G11]
, D8 for V18 and R54,
D7 for V19. The diode cathodes are ORed and applied to V7
[F4]
via divider
R55 and R56. Transistor V7 conducts and limits the collector peak to 42 V
to prevent damage to the output transistors.
Battery Voltage
The battery voltage is monitored by V3. When the supply volts (A line)
drops to about 12 V the voltage at the base of V3, set by the resistor divider
R8 and R9, falls to 4.3 V causing V3 to conduct. This causes a rise in
voltage across R15 (part of resistor chain) and changes the threshold of the
forward ALC detector, reducing the output power. As the control is linear,
further reduction in supply volts continues to lower the output power.
Heatsink
When the heatsink exceeds 80°C, PTC resistor R22 rapidly increases in
value, reducing the ALC threshold level to comparator IC3/B. This lowers
the output power and prevents the heatsink from exceeding 90°C.
n
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