55LH90 (CHASSIS:LA92C) - LG TV Service Manual (repair manual). Page 44

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Check JK1206
Can you see the normal waveform?
JK1206 may have problem. Replace this Jack.
Check the input (Pin R1,R3,P2 and Pin V1,V2,U1)
of LGE3159(IC100).
Measure waveform at C101,C102,C103 and C115,C116,C117
because it’s more easy to check.
Can you see the normal waveform?
LH40/LH55 Trouble shooting guide – Component
YES
NO
Video
NO
YES
YES
Check all power domain of LGE7329 (IC801)
(3.3V, 1.9V_CORE,1.26V) 
& crystal output (X800 / 12MHz)&FRC_RESET
Measure Power & waveform at L800,L801,L802,L803,
L804,L806
because it’s more easy to check.
Can you see the normal waveform?
This board has big problem because FRC chip(LGE7329) have some troubles.
After checking thoroughly all path once again, You should decide to replace 
LGE7329.
Although you replace it, you cannot see normal display, decide to replace 
Main chip(LGE3159)
This board has big problem because Main chip(LGE3159) have some troubles.
After checking thoroughly all path once again, You should decide to replace LGE3159 or not.
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