SONY KV20FS12
Service Manual

39

Audio without K Board
Overview
The BA-5 chassis can contain one of two types of audio circuits depending on the model. One type uses the K board and the other does not. Screen size is not a factor in the use of a K board. This section discusses models that do not use the K board. There are also two different audio amplifiers that can be used by the BA5 chassis. The one shown here is used in all models that output 5Wx2 or less. The other audio output, which will be shown in the Audio With K Board section, is used for models with at least 10Wx2 power output.

there are headphones plugged into the headphone jack J401, the audio will not be returned back. This may be useful for troubleshooting in the event of a bad headphone jack. The back signal to CN407 is then applied to the R+ or L+ speaker terminal depending on which channel you are looking at. The R- and L- speaker terminals are connected to the � outputs of IC401 Audio Amp.

Volume Control and Muting
Volume control is adjusted by varying a PWM waveform output from the Syscon ICs O Vol pin. This signal is connected to CN2009/10 and applied through D403 to a filter network consisting of C440, C442 and R432. These components smooth the PWM signal to a DC voltage. This DC voltage is applied to IC401/1 VC1 and IC401/7 VC2. The DC voltage determines the level of the signal output. The mute circuit is also connected to IC401/1 VC1 and IC401/7 VC2. When mute is called for, a HIGH signal is output from CN2002/9 Mute. This HIGH turns Q410 ON and grounds the VC1 and VC2 terminals. This reduces the audio to no volume. It should be noted here that these sets have the Auto Mute feature. If a BA-5 chassis were tuned to a weak station, the audio would be muted. When this occurs, No Signal will also be displayed on the screen. Q411 is a Power Off Mute transistor. It is configured so that when the set is turned OFF, Q411 turns ON. This is possible because the + 9 volt line holds voltage longer than the +12 volt line. When power is disconnected and voltage remains on the 9-volt line, current flows between the B-E junction of Q411. This turns the transistor ON and grounds the VC1 and VC2 lines.

Switching
Models without the K board use separate audio switches, IC405 and IC406, for each channel. The left channel is switched by IC405 and the right channel is switched by IC406. These switches each have three audio inputs and two-control line inputs. The control lines are the same ones used in video switching on 24� and under models. The following table shows the state of the control lines for the input selected:

Input Tuner Video 1

IC405/406 S1 0 Volts 3.3 Volts

IC405/406 S2 0 Volts 0 Volts

Video 2 0 Volts 3.3 Volts Whichever input is chosen is output from its respective IC at pin 7.

Audio Amp
The output from the switching circuit is applied to a RC network on each channel. After passing through the RC network, the signals enter IC401 Audio Amp. IC401 Audio Out amplifies the input signals and outputs them as a differential pair. These + signals are routed to CN407 and then to CN408 both on the A board. This method is used because the Audio amp is in the rear and the headphone jack is in the front of the A board. The signal then passes through the headphones and back to CN407. If