Service manual

CA-22
CA CIRCUIT DESCRIPTIONS
Audio Amplification
There are three audio circuit configurations used in the various CA chassis: mono, stereo, or MTS (Multi-
channel Television Stereo). While few CA receivers are monophonic sets, the mono audio circuit bears
mentioning due to the fact that it is used in each of the three circuit variations.
IC804, the audio amplifier IC and heart of the mono audio circuit, is capable of both mono and stereo audio
amplification. IC804 has dual audio inputs (pins 2 and 5) and, naturally, dual outputs (pins 10 and 7).
These outputs drive 8-ohm, 5-Watt speakers. The audio amplifier utilizes +18 volts audio (+18 AUD) as its
power supply on pin 9. Its characteristic voltage gain is 50 dB. However, to eliminate potential noise
problems, R854M and R864M are used in conjunction with feedback capacitors C852 and C862 to limit
voltage gain. Such an arrangement allows use of a higher power audio signal that is less prone to noise
interference. Maximum output to the speakers should not be more than 3 Watts. Pin 3 of IC804 connects
the Mute line from pin 14 of the microprocessor to the audio circuit. A high signal on the base of Q800 will
turn that transistor on, lowering voltage on pin 3 and causing the amp IC to mute the audio.
1
2
5
6
487
1039
IC804
R861M
C861
R862M
C865MR852MC855M
C852
R854M
R864M
C862
C843
+18VAUD
W140
C842
C841
Q800
Mute
1
2
3
4
C854
C864
C853
C863
R853
R863
R843
R851M *
C851*
Left Audio
*(MTS or
Stereo Only)
To TV
Speakers
9S4
Composite
Audio or Right
Audio
<<Figure CA-21>>
When IC804 is used as a mono amplifier, composite audio comes through C861 into pin 2. When used in a
stereo or MTS receiver, pin 2 serves as the right audio channel input, and pin 5 as the left. C861 and C851
(for non-mono sets) are used to pass audio but block any DC. Surface mounted capacitors, C855M and
C865M, are used to set the gain cut frequency to approximately 13 kHz. Voltage dividers consisting of
R861M and R862M for the right channel and R851M and R852M for the left channel limit the maximum
signal input to 42 mVrms. Maximum output from the stereo processor is normally 490 mVrms, which
would be too high an input for IC804.