Service manual

CB-28
CB CIRCUIT DESCRIPTIONS
Audio Boost
Those sets that have the audio boost feature output audio signal on pins 43 and 44 to IC1401. This IC is
powered by the ±14AUD. It strengthens the audio signal and returns it to pins 41 and 42 of the audio
processor. From here, the boosted signal is output from IC1400 on pins 6 and 7.
OUT1 OUT4
-IN1 -IN4
+IN1 +IN4
V+ V-
+IN2 +IN3
-IN2 -IN3
OUT2 OUT3
10
9
87
6
14
13
12
11
5
4
3
2
1
IC1401
R1490M
R1491M
R1489M
C1484
R1488M
-14AUD
C1482M
R1486M
R1487M
C1479
C1478M
R1484M
R1485M
R1482M
R1483M
R1492M
C1483
C1477M
C1476M
R1477M
R1481M
R1478M
R1479M
R1480
C1475
R1475M
R1493M
R1476M
C1474M
C1473
+14AUD
C1481
C1480M
C1472M
R1474M
C1471M
R1472M
R1473M
R1471M
43
41
42
44
TVOUT-R
IC1400
AUX2-R
AUX2-L
IC1400
TVOUT-L
<<Figure CB-28>>
Audio Amplification
CB receivers employ two 8-ohm, 7-Watt speakers. These are driven by IC804, the stereo audio amplifier.
Although the audio amp is capable of driving 10-Watt speakers, the supply voltage has been reduced from
+36 volts DC to +22 volts. Normal gain of this IC is 42 dB. In order to drive the speakers at 5 Watts, a
signal strength of only 50 mVrms is necessary. Such a low strength signal is susceptible to noise interfer-
ence, so the gain is reduced to 32 dB by negative feedback from the output in order to allow for an in-
creased signal strength of 142 mVrms. R868 and R869 accomplish this reduction for the right channel and
R858 and R859 do so for the left channel. The output level from the amplifier is 425 mVrms. Cut-off
frequency is 14.3 kHz and is determined for right and left channels by C865M and C855M respectively.
IC Location Part # Notes
IC803 221-00170 All Models
IC804 F-53765 All Models
IC1400 221-01127 All Models
IC1401 221-00188 IQB27B44W
<<Table CB-7>>