Modem Troubleshooting - LG LRM-519 Service Manual

10. MODEM TROUBLESHOOTING

No Dial-up
connection
YES
Connection Phone
line
YES
NO
IC307 (5,21-pin)
+3.3V input OK
YES
NO
J304(between 3,and
4 Pin) 40~52V OK
YES
NO
IC304 (3-pin) Clock
Out 2048KHz OK
YES
DTE Setup & Connection (8-bit, no
parity bit, one stop bit and 19.2kbps)
YES
NO
"AT" Response OK
YES
Modem OK
0V -- QE
0V
-- DCT
0V
-- RX
0V
-- IB
0.8V
-- C1B
0.7V
-- C2B
~2.3V
-- VREG
~1.0V
-- RNG1
IC905 Check
Phone Line Check
IC304 (1-pin) Clock
Input 27MHz OK
YES
Re-soldering IC304
Check IC309 Refer
to Fig 7-10 OK
YES
Change IC309
ON-HOOK
DCT2 -- 0V
IGND --
DCT3 --
0V
QB --
0V
QE2 --
0V
SC --
0V
VREG2 --
0V
RNG2 --
~1.0V
< Fig 7-10. IC309 Typical Voltage >
NO
Check OSC500
NO
Re-soldering IC309
OFF-HOOK
1.6V
-- QE
3.4V
-- DCT
2.5V
-- RX
0V
-- IB
0.8V
-- C1B
0.8V -- C2B
2.3V
-- VREG
1.0V
-- RNG1
3-12
DCT2 --
2.2V
IGND --
0V
DCT3 --
1.6V
QB --
2.8V
QE2 --
2.1V
SC -- 0V
VREG2 --
1.8V
RNG2 --
0.9V