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18
Theory of Operation
fm audio Processor Signal Paths
Voice band audio normally enters the Balanced Input at P1-1 and P1-2 on the subrack connector
while microphone audio enters at P1-3. Potentiometer R31 sets the balanced compression level of
U1B while R2 sets the microphone compression level of U1A (see Figure 3).
Each amplifier has a dynamic range of 25 dB. Jumper JU17 is only installed when using very low
input levels (-18 to -25 dBm) and allows better tuning range for R31. The output of the compression
amplifiers are normally set for pre-emphasis (6 dB/Octave) but can be set for a flat audio response
using jumpers JU1 and JU2.
C4
1.0uF
R2
5K0
P1
FB1
3
MICROPHONE INPUT
P1
4
P3
3
+8.0V SWD
P2
3
+9.5V CONTINUOUS
P2
4
P1
1
BALANCED INPUT
R30
604R
P1
2
1
X
JU12
Y
C51
P2
3
1.0uF
1
TONE/DIGITAL I/P
R46
4K53
NOT INSTALLED FOR DIGITAL MOD.
P2
2
SUBTONE INPUT 1
hi gh = voi ce
l ow = dat a
hi gh = w i deband sel ect
Microphone and Balanced Input Circuitry
MT-3 FM Transmitter Mainboard Instruction Manual
IM20-MT3TXMN
+8.0V SWD
+9.5V CONTINUOUS
JU36
1
3
X
2
Y
R1
C13
R56
47uF
0R0
0R0
MICRO P HO NE AGC P RE AMP
U1
a
SA571D
9
THD
Var G
Trim
TP11
MICRO P HO NE INP UT
Output
LE V E L CO NTRO L
C1
3
1.0uF
2
11
Rect Cap
R3
C35
1.0uF
1
15
Inv In
Rect In
R39
15K4
R41
C5
137K
10nF
R59
20K0
13
Vcc
8
THD
TP4
Trim
Output
C30
1.0uF
6
R3
Rect Cap
R62
15K4
2
Rect In
C9
R51
GND
JU17
10K0
1.0uF
4
R5
1M00
3
BALANCE D
2
R31
INP UT LE V E L
5K0
R64
CO NTRO L
100K
BALANCE D AGC P RE AMP
1
JU11
2
3
AUXILIARY INP UT
LE V E L CO NTRO L
4.0V
1
JU10
S UBTO NE 1 INP UT
CO NTRO L LE V E L
C25
MICRO P HO NE
TP12
1.0uF
CO MP RE S S IO N
14
CO NTRO L
10
PRE-EMPHASIS
R60
16
18K2
R61
C37
18K2
3
3.3nF
12
2
R63
5K0
R74
C36
C26
1
330nF
N/I
4.7uF
C42
680nF
FLAT AUDIO
U1b
SA571D
C2
1.0uF
BALANCE D
3
CO MP RE S S IO N
Var G
TP3
CO NTRO L
7
C15
3.3nF
R4
1
18K2
R6
18K2
1
5
Inv In
2
R38
C8
5K0
C19
R73
4.7uF
330nF
3
N/I
C7
680nF
R76
16k2
TP6
C32
1
R49
R48
N/I (22nF)
X
100K0
16k2
JU33
2
2
JU5
Y
C16
3
1.0uF
R57
SEL
4.0V
9
R82
C14
8
10K0
10nF
10
U2
c
MC33174
TP2
C10
10nF
3
R42
5K0
2
1
R8
2K21
3
Y
2
JU1
X
R9
49K9
1
PRE-EMPHASIS
R7
2K21
3
Y
2
JU2
X
R10
49K9
1
FLAT AUDIO
R65
100K
TWO PORT POLARITY
U2
d
3
R52
Y
MC33174
100K
JU14
13
2
14
X
12
1
PRE-EMPHASIS
R11
C38
3
Y
2K21
3.3nF
JU9
2
X
R40
49K9
1
FLAT AUDIO
C39
R43
680nF
11K8
JU16
B0313
FIGURE 3
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