LG 42PG20 Training Manual page 23

Understanding the Power On Sequence when Troubleshooting a possible Power Supply
Failure will simplify the process of isolating which circuit board failed to operate properly.
In this Section we will investigate the Power on Sequence and examine ways to locate
quickly where the failure occurred.
Check the Power On LED for Operation. A Red LED indicates a presence of 5V STB and
AC-ON/DETECT. Failure of the Power ON LED to light is an indication of loss of 5V STB
or AC ON/ Detect remember the 5V STB and AC-ON/DETECT are developed on the
SMPS and sent to the Main board. Check LD703 for Green glow.
When Power is pressed, look for LD703 to change to Amber. Listen for a Relay Click. The
click of the Relay is an indication of RL-ON going high. RL-ON is sent from the Main board
to the SMPS and when present, the IC701 controls the relay operation. RL-ON going High
and no relay is a failure of the SMPS. RL-ON staying low is a failure of the Main board.
Relay Operation means that the SMPS if working properly will output the 16V Supply to
the Main board. This voltage will allow the Tuner, Audio and Video Circuits on the Main
board to function, and if connected to an Antenna Input, Audio would be present. If the
Relays closed and these supplies failed suspect a problem with the SMPS.
The next step of operation calls for the M5V_ON line from the Main board to the SMPS
to go high pin 21 of P813. A high on the M5V_ON line activates the 5V VCC line. Loss
of 5V VCC results in no "Raster", no Display Panel Reset, no Y, Z, Control or X-board
operation. Loss of 5V VCC and M5V_ON going high could be caused by any of these
boards or failure of the SMPS. M5V_ON staying low indicates a problem on the Main
board.
VS-ON is the last step of the Power Sequence and is responsible for bringing the VS
and VA voltages up. The VS-ON signal pin 20 of P813 is sent from the Main board to
the SMPS as a high, VS and VA and full operation of the Display Panel are now enabled.
Loss of VS-ON results in loss of VA and VS and no Raster, no Panel Display Reset but
audio would be present. If VS-ON went high and VS and VA where missing the problem
CIRCUIT DeSCRIPTIONS
PDP Training - Fall 2008
4
2
P
G
2
0
23