Audio Ic - GE MASTR II Maintenance Manual

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CIRCUIT ANALYSIS
LBI4898
NOT PRESENT
IN EC-ICOM
REG COMPENSATION
IOV 15V
.----\---------{l\----------{2~-------------------,
COMPENSATION
I
IC Ul
COLD END COMP
~~
I
I
HOT
END
COMP
I
I
I
- l
I
I
24K
I
I
COMPENSATOR~
OUTPUT
24K
I
I
I
I
OSCILLATOR
CIRCUIT
+IOV
FREQ
SELECT
L __
L _ _
Figure 3 - Equivalent ICOM Circuit
The hot end compensation circuit does
not operate at temperatures below +55°C.
When the temperature rises above +55°C, the
circuit is activated. As the temperature
increases, the equivalent resistance de-
creases and the compensation voltage de-
creases. The decrease in compensation vol-
tage increases the capacity of the varactor,
decreasing the output frequency of the ICOM.
SERVICE NOTE: Proper ICOM operation is de-
pendent on the closely-controlled input vol-
tages from the 10-Volt regulator. Should
all of the ICOMs shift off frequency, check
the 10-Volt regulator module.
AUDIO IC
The transmitter audio circuitry is con-
tained in audio IC Ul02. A simplified
drawing of the audio IC is shown in Figure 4.
Audio from the microphone at pin 12 is
coupled through pre-emphasis capacitor Cl to
the base of Ql in the operational amplifier-
limiter circuit. Collector voltage for the
transistorized microphone preamplifier is
supplied from pin 11 through microphone col-
lector load resistor Rl8 to pin 12.
The operational amplifier-limiter cir-
cuit consists of Ql, Q2 and Q3. Q3 provides
limiting at high signal levels. The gain
of the operational amplifier circuit is fix-
ed by negative feedback through Rl9, R20
and the resistance in the network (pin 9).
The output of Q3 is coupled through a
de-emphasis network (RlO and C3) to an
active post-limiter filter consisting of C4,
C5, C6, Rll, Rl2, Rl3, Rl5, Rl7, and Q4.
Following the post-limiter filter is
class A amplifier Q5. The output of Q5 is
coupled through MOD ADJUST potentiometer
Rl27 to the phase modulators.
SERVICE NOTE: If the DC voltages to the
Audio IC are correct and no audio output can
be obtained, replace Ul02.
For radios equipped with Channel Guard,
tone from the encoder is applied to the
phase modulators through CHANNEL GUARD MOD
ADJUST potentiometer Rl28, and resistors
RllO, Rl21 and Rl24. Instructions for set-
ting Rl28 are contained in the modulation
adjustment section of the Transmitter Align-
ment Procedure •
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