Service manual

CB-27
Audio Development
Audio Processing
The CB chassis uses IC1400 to process audio. This IC features true MTS (Multi-channel Television
Stereo) sound, dbx noise reduction, and SAP (second audio program) processing. Its I
2
C bus at pins 9 and
10 allows adjustment for volume, balance, tone control, front surround sound, SAP switching, source
selection, and stereo separation. IC1400 is powered by +9VSB at pin 24 and grounded at pins 22 and 11.
CB television receivers make use of at least 2 audio inputs to the audio processor. A third set of inputs is
employed on IQBxxB44W type models for audio boost purposes. A description of this process is below.
The composite audio comes from a switch in the IF section through C1436 into pin 19. This allows selec-
tion of composite audio from either the main or the PiP IF source. Refer to the IF circuit description for an
explanation of this switching. IC1400 processes this audio, separating SAP, left, and right audio. The left
and right channels are then output on pins 6 and 7. Alternately, the audio processor can use audio input
from an external source at pins 39 and 40.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2425
36
26
27
28
29
30
31
32
33
34
35
37
38
39
40
41
42
43
44
45
46
47
48
BASSL2
TRE-R
TRE-L
SURRIN
SURROUT
LSOUT-R
LSOUT-L
NC
SDA
SCL
DGND
SAD
MAININ
MAINOUT
NC
PCINT1
PCINT2
PLINT
COMPIN
VGR
IREF
GND
SAPTC
VCC
BASSL1
BASSR2
BASSR1
SURRTC
TVOUT-L
TVOUT-R
AUX2-L
AUX2-R
AUX1-L
AUX1-R
VLTC
VLDC
VCAWGT
VCATC
VCAIN
VEOUT
VETC
VEWGT
VE
SAPIN
SAPOUT
NOISETC
STIN
SUBOUT
IC1400
C1401M
C1402M
C1404M
C1416
C1435M
PROV
C1434M
PROV
R1401
R1402
R1425M
C1421
C1420
R1406M
C1418
C1436
C1413
R1404M
C1425
R1421M
R1422M
+9VSW
C1433M
C1430
C1403M
C1406M
C1431M
C1409
C1410
R1409M
C1432
C1415
C1414
R1405M
C1417
C1427
C1426
C1428
C1419
C1422
C1424
R1407M
R1408M
* Not used
in all models
To Audio
Boost Circuit *
Switched
Audio In
Composite
AudioInFrom
IF Circuit
Clock &
Data Lines
To Audio &
External Amps
<<Figure CB-27>>
All audio input and output pins should meter 4.0 DC volts. Signal at pin 19 should have a strength of 245
mVrms. All other audio inputs and outputs on IC1400 should scope at 490 mVrms. This includes the audio
boost I/O described now.