Power Supply Generic Troubleshooting Tips - LG 42PQ30 Training Manual

Power Supply Generic Troubleshooting Tips

Power Supply Generic Troubleshooting Tips
Remember if a voltage is missing check for proper resistance before proceeding
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.
When Power is pressed, 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 operation of both Relays. RL-ON going
High and no Relay is a failure of the SMPS, RL-ON staying low is a failure of the Main Board or something between.
Relay Operation means that the SMPS if working properly will output the 17V and 12V Supplies to the Main Board.
These voltages 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 closes and these supplies failed suspect a problem with the SMPS
or an excessive load on the line.
The next step of operation calls for the M5V ON line from the Main Board to the SMPS to go high on P813 pin 21. A high
on the M5V ON Line activates the M5V line to the Y-SUS Board. Loss of M5V results in no "Raster", no Display Panel
Reset, no Y, Z, Control or X Board operation. Loss of M5V and/or 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 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 could be caused
by a failure on the SMPS or a circuit using these voltages. A Resistance check should narrow the possible
failures quickly.
37 Preliminary Information 42PQ30