Panasonic KX-F900 Service Manual page 181

KX-F900
(3) Ringer Circuit
If the ringer volume is set to low and the key is entered occurs, an alarm tone is output from pin 45 of the CPU
and input to Q212. This causes Q213 to turn off and results in a softer beep tone.
If the ringer volume is set to high, Q213 turns on and results in a louder beep tone.
Circuit Diagram
SPEAKER
C
_CPU
ALARM
CPU ALARM
VOLUME
(4) Reception Signal Circuit
The receiver circuit comprises expander IC202, side tone control IC IC208, and a speaker amplifier.
After being adjusted to the appropriate level by VR202, the signal passes through a 3 kHz LPF and an ex-
pander built into IC202. It is then input to side tone IC IC208. The side tone IC is connected to the microphone
amplifier. If a large input is input to the microphone, the gain control built into IC208 lowers the gain to reduce
the output of the speaker amplifier. If there is no large input being input to the microphone, the amplifier in
IC208 is set to standard gain. Consequently, the sound of the received audio signal becomes fainter when the
user is talking in a loud voice and the side tone level is lowered. When the user talks more softly, the received
audio signal is audible at the standard level.
Also, in addition to the input from the microphone, the ACG signal from the RF block is input to the side tone IC.
When the base unit and portable handset are separated from each other, causing the signal to become weaker,
the DC voltage rises and this voltage is input to pin 5 of IC208. When the DC voltage input to pin 5 rises, the
gain control built into IC208 lowers the gain. Consequently, the reception level is lowered when the reception
signal is weak and there is more noise. This prevents the noise from becoming too noticeable.
The reception signal passes through receiver volume selector switch Q217, and then drives the receiver
speaker.
RXVOL H : LOW LEVEL
L : HIGH LEVEL
SPK MUTE H : SPEAKER ON
L : SPEAKER OFF
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