SECTION V - CIRCUIT DESCRIPTION
Refer to SBX-11A schematic while reading the following description. For location of P.C. board
components see the component layout diagram of the printed circuit board.
A. OSCILLATORS
1.
The receiver channel oscillator is Q104. Its frequency is varied by the CLARIFIER C501.
Trimmer capacitors on each channel permit setting each crystal on frequency while the
CLARIFIER is centered.
2.
The transmitter channel oscillator is Q106. It is similar to the receiver channel oscillator
but does not have a CLARIFIER frequency control. Each crystal has a trimmer capacitor.
3.
The LFO operates at 456.5 kHz. C138 is used to adjust the oscillator to the correct
frequency.
B. RECEIVER
1.
Q101, a FET emitter follower, couples the signal from the receiver input filter to the mixer
U101. In U101 the signal is amplified and mixed with the channel oscillator frequency -
producing the I.F. frequency which is fed to U102 through T101.
2.
The signal is further amplified by U102, then passes through transformer T102 and
sideband filter FL101.
3.
U103, which has two outputs (pin 6 and 8) amplifies the signal from FL101. The output
from pin 8 is rectified by D106 and D107 and the resultant DC controls the current through
Q102 which supplies the forward AGC bias for U101, U102, and U103.
4.
The output from pin 6 is fed to T103 then to the balanced modulator/
demodulator FL102 through T-R switch diode D102. In FL102 this I.F. signal mixes with
the LFO signal from Q107 to produce the audio signals.
5.
The audio is fed to volume control R1, to Q103 the audio preamp, and to U104 the audio
output, then to the speaker via C125.
