Audio Ic - GE MASTR II Maintenance Manual

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CIRCUIT ANALYSIS
LBI-4896
NOT PRESENT
IN EC-ICOM
REG
IOV
COMPENSATION
5V
~------------~2~----------------------------------~
- -
- -
- l
COMPENSATION
IC Ul
I
COLD END COMP
'}
r
24K
I
I - - - - - - - ,
COMPENSATOR~
HOT END COMP
\
I
L __
24K
fREQ
SELECT
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
voltage increases the capacity of the var-
actor, decreasing the output frequency of
the ICOM.
SERVICE NOTE: Proper !COM operation is
dependent on the closely-controlled input
voltages 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
contained 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 ampli-
fier-limiter circuit. Collector voltage
for the transistorized microphone pre-
amplifier is supplied from pin 11 through
microphone collector 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
fixed by negative feedback through Rl9, R20
and Rl26.
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
coupked 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
Alignment Procedure •
3