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RINGER CIRCUIT
When a ringing voltage is sensed on the line,
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path. When operating voltage is reached, the
oscillator starts and U201 detects the ring
frequency. After a valid ring frequency is
applied to the U201 pin 25 RFD (Ring Frequency
Discrimination) port, the ring tone generator of
MO (Ring Melody Output) port.
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Ring frequency passes through DC blocking
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capacitor C201 or C234 (for Switzerland or
Austria) and zener diode ZD205 or ZD209 (for
Switzerland or Austria) to U201 pin 25 RFD
port.
Line ring AC voltage passes through DC
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blocking capacitor C201, current limit resistor
switch to FET202 (BS170) pin3, C217,ZD207 and
The ring frequency discriminator of U201
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assures that only signals with a frequency
between 13 Hz and 70 Hz are regarded as valid
ring signals.
continuously, the ring melody generator (pin 28,
MO) is activated and remains active as long as a
valid ring signal is present.
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output pin 28 (MO). This is the piezo buzzer
drive signal.
The 3 basic melody frequencies are: Fl = 880 Hz,
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rate is set to 4 which means that the sequence of
second.
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set by T/P switch (SW202).
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typical -14 dBm) at U201 pin 4 MFO (DTMF
Generator Output). This signal is levelled by
pin 9 MFI (DTMF Amplifier Input).
Line dial signals appear at U201 pin 39 LS (Line
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Current Sense Input).
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Dial pulses originate at U201 pin 27 (DPn),
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which toggles Q205, driving Q201. The resulting
intermittent voltage interrupts the telephone
line.
Pulse MB ratio is set by U201 pin 2 MDS port.
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the line during make period of pulse dialling.
SPEECH CIRCUIT
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Handset transmitter circuit
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Transmit (condenser mic.) of handset is filtered
by R213, R242, R243, R244, C221, C223, C224,
by U201 pins 32 and 33 (Ml, M2).
. Handset receiver circuit
Dynamic recever of handset is filtered by R245,
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line.
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pin 7 STB.
Circuit Description
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