Panasonic KX-FT21BX, Service Manual

Page: 99 / 206

TROUBLESHOOTING GUIDE
- 73 -
KX-FT21LA
(1) Defective ITS (Integrated telephone system) section
Following the ITS section or NCU section, search for the route between the microphone and
the telephone line (sending) or between the telephone line and the speaker (receiving) where the signal disappears.
Check the components at that point.
Does pin 99 of IC1 on
the digital board change
from high to low level?
With the unit on-hook (standby),
does a bell ring when pins 3-4
of PC1 on the analog board
are shorted? Is a current flowing between pin 4
of PC1 on the analog board and
pin 56 of IC1 on the digital board? Check the path between pin 4
of PC6 and pin 19 of IC4.
Check that the DTMF signal is
being output from pin 3 of IC5
on the digital board.
Check that the DTMF signal is
being output at the points on the
DTMF circuit send route.
Check the bell detect circuit
(R37, C35, PC1, etc.)
on the analog board. Check the path over which the
bell signal is sent to the speaker.
Check IC5 and pin 70~81 of
IC1 on the digital board.
Replace Q3.
Does pin 99 of IC1 on the
digital board change from
high to low level?
Check IC1 on the
digital board.
Does the base Q3 on the
analog board change from
high to low level?
Check RLY1 on the analog
board.
Check the conduction between the base
of Q3 on the analog board and pin
99 of IC1 on the digital board.
1 No handset and monitor transmission/reception
2 No pulse dialing
3 No ring tone
4 No tone dialing
YES
YES
NO
NO
NO
NO NO
YES
YES
YES
NO NO