10. Current transformer T1204 provides power output indication on the front panel metre and
also provides part of the ALC feedback voltage to the gate of the FET microphone
amplifier Q113. Part of the ALC is developed from voltage at the output. This advanced
ALC circuit senses power and so the output power is almost constant for loads between 25
and 100 Ohms,.
D. METERING
1.
On receive the battery voltage is monitored on an expanded scale to permit determination
of battery condition.
2.
The metre is automatically switched to monitor antenna output current on transmit.
E. MISCELLANEOUS
1.
Q108 shorts out receiver B+ when switching from receive to transmit, to prevent feedback
from the speaker to the microphone during the switching interval.
2. Operate/Tune
Switch
When in tune position a DC current is passed through double balanced mixer FL102
unbalancing it and permitting the LFO frequency to pass through it, through Diodes D101
and D102 (turned on by the Tune Button) into the 2nd double balanced mixer FL103. The
LFO input to FL103 produces the suppressed carrier frequency at the output which is used
for antenna tuning. The Tune Button can also be used to tune the transmitter into a dummy
load resistor.
3. Supply
Protection
If the supply voltage exceeds about 18 V the zener diode D1 will conduct and blow the fuse
F1. (This should not occur with any of the power options offered for the SBX-11A). If the
supply polarity is reversed the zener diode will conduct in the forward direction and blow
the fuse.
