Audio Preamplifier; Audio Ic - GE Mastr II Maintenance Manual

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CIRCUIT ANALYSIS
LBI-4732
NOT PRESENT
IN EC-ICOM
REG
IOV
COMPENSATION
~v
~--------------~2~------------------------------------,
COMPENSATION
I
IC Ul
COLD END COMP
I
I
I
I
HOT END COMP
I
I
I
- l
I
I
24K
I
I
COMPENSATOR~
OUTPUT
24K
I
I
I
I
OSCILLATOR
CIRCUIT
\
+IOV
..._ ____ _. ________ .._. ____ ._____,MID-RANGE
A-
RF
OUT
FREQ
SELECT
L __ _
A-
_____J
L __
TEMP.
COMPENSATION
__CAP_A_C_IT-OR _ _
J
RC-2444
Figure 3 - Equivalent ICOM Circuit
increases, the equivalent resistance de-
creases and the compensation voltage de-
creases. The decrease in compensation volt-
age 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 volt-
ages from the 10-Volt regulator. Should all
of the ICOMs shift off frequency, check the
10-Volt regulator module or check output of
5C-ICOM.
AUDIO PREAMPLIFIER
The transmitter Audio Preamplifier is
not part of the transmitter and is covered
in the station Maintenance Manual.
AUDIO IC
The transmitter audio circuitry is con-
tained in audio IC-Ul02. A simplified draw-
ing of the audio IC is shown in Figure 4.
Audio from the Station Preamplifier at
pin 12 is coupled through capacitor Cl to
the base of Ql in the operational amplifier-
limiter circuit.
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
the resistance in the network (pin 9).
The output of Q3 is coupled through a
de-emphasis network (RIO and C3) to an act-
ive 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 and station gain Control Rl4 on
the Audio Preamplifier are contained in the
modulation adjustment section of the Trans-
mitter Alignment Procedure.
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