LBI-3999 CIRCUIT ANALYSIS
6410 and Q411 operate as a push-pull
Class AB audio PA stage. The PA output
is
coupled through audio transformer T410 to
the loudspeaker. The yellow and white
tertiary windings of T410 supply balanced
feedback to the collector of Q7. The feed-
back winding minimizes distortion and pre-
vents the pick-up of external electrical
noise.
Base bias for the PA stage and the
elimination of crossover distortion
is
controlled by bias adjust potentiometer R43.
The potentiometer
is
set
at the factory as
shown in STEP
1
of the receiver Test Pro-
cedure.
Do not adjust bias adjust poten-
tiometer R43 unless PA transis-
tors Q410 and Q411 have been
re-
nlaced.
Audio high and low are also present at
centralized metering jack 5442, and can be
used as shown in STEP
1
of the receiver
Test Procedure. The output stage provides
5
watts at
less
than
5%
distortion into a
3.5-ohm load at the receiver output
term-
inals (3.2-ohm load at the Control Unit).
SQUELCH (A426)
Noise f the audio-noise amplifier
operates the squelch circuit. With no
carrier present in the receiver, this noise
is
coupled to the base of noise amplifier
Q8 through a high-pass filter which attenu-
ates frequencies below
3
kHz. The filter
consists of C30, C31 and R45, as
well
as
C34 and L3 in the collector circuit of Q8.
The gain of Q8
is
determined by the Squelch
control, which varies the bias on the base
of Q8. Thermistor RT2 keeps the critical
squelch constant over wide variations in
temperature.
The output of noise amplifier Q8
is
rectified by diodes CR5 and CR6, and filter-
ed by C36 and C37 to produce a negative DC
voltage. This DC voltage
is
applied to the
base af DC amplifier Q9, turning
it
off.
When turned off, the collector voltage of
Q9
rises
to approximately 8 volts, turning
on DC amplifier Q10. When conducting, the
collector voltage of Q10 drops to almost
ground potential, which removes the base
bias to audio amplifier Q6 and audio driver
Q7, turning them off
.
When the receiver
is
quieted by a
signal (unsquelched), the noise in the
re-
ceiver
is
reduced, turning DC amplifier
Q9 on and DC amplifier Q10 off. This
allows the audio stages to conduct so that
sound
is
heard in the speaker.
Resistor R53 connects from the
emitter
of audio driver Q7 to the
emitter
of noise
amplifier Q8, providing a hysteresis loop
in the squelch circuit. When a weak signal
opens the squelch, the signal level may be
reduced by 4 to 6 dB without the squelch
closing. This
limits
squelch "flutter" or
"picket-fence" operation.
With audio driver Q7 conducting, a
positive voltage through R53 helps to
re-
duce the gain of noise amplifier Q8. This
positive feedback provides a quick, positive
switching action in the squelch circuit.
When the receiver squelches, audio driver Q7
turns off and
its
emitter
potential drops
to zero. This reduces the DC feedback
through R53 to the
emitter
of noise ampli-
fier Q8. Reducing the feedback causes Q8
to conduct harder, turning the audio stages
off quickly.
Keying the transmitter removes the +10
volts from J19, turning off DC amplifier
Q9 and turning on Q10 to mute the receiver.
CHANNEL GUARD (A428/A429)
Channel Guard Board Model 4EK16A10
is
a fully transistorized encoder-decoder for
use in the MASTR Professional Series mobile
and station combinations. The tone freq-
uencies are controlled by plug-in tone net-
works that are made with precision components
for excellent stability and reliability.
The tone frequencies range from 71.9 to
203.5 Hz.
Encoder (A428)
Keying the transmitter removes the
re-
ceiver mute +10 volts, and forward biases
feedback control diode CR5, causing
it
to
conduct. When conducting, the diode shunts
R39 which reduces the impedance of the
positive feedback loop (R39, R35 and C19).
This provides the necessary gain to the base
of Q5 to permit oscillation.
The encoder tone
is
provided by selec-
tive amp-oscillator transistors Q5 and Q6
which oscillate at a frequency determined
by the tone network. Negative feedback
applied through the tone network to the base
of Q5 prevents any gain in the stage except
at the desired encode frequency.
Starting network R45, C21, C22 and CR6
provide an extremely fast starting
time
for
the encoder tone. Keying the transmitter
removes the receiver mute +10 volts, causing
a pulse to be applied to the base of Q6 to
quickly start the oscillator. Thermistor-
resistor combination R.32 and RT1 provides
temperature compensation for the oscillator
output. Limiter diodes CR3 and CR4 keep
the tone amplitude constant.
Emitter-follower Q7 follows the oscil-
lator circuit. The encoder tone
is
taken
from the
emitter
of Q7 and applied to an
