LG 42PG20 – Plasma TV – How to adjust Va-Vs voltages – How to enter Service Mode

Service Mode – Va & Vs Voltage adjustments – Voltage Chart - LG42PG20 Plasma TV

SERVICE MENU

The service menus can be used to make adjustments, change color alignment, and get software versions. There are two service menus.  The Adjust and Instart menu can be accessed using the service remote (Service remotes are available from LG parts). They can also be accessed by holding down the menu button on the TV and the remote until the user menu disappears. The menus alternate between Adjust and Instart every time the menu button is held down. If the TV asks for a password, enter 0000 (four zeros).

POWER SUPPLY OPERATION

AC Voltage is supplied to the SMPS board at Connector CN101 from the AC Input Filter. Standby 5V is developed from 340V source supply (which during standby measures 159V hot ground). This supply is also used to generate all other voltages on the SMPS.  The 5V (standby) voltage is routed to the Sub Micon circuit (IC701) on the SMPS and through P813 to the Main board or Micon (IC100). LD703 will glow green to indicate STBY 5V has arrived.  AC detect Pin 18 of P813 is generated on the SMPS by monitoring the AC input and rectifying a small sample voltage. This AC Detect Voltage is routed to IC701 on the Sub Micon on the SMPS and the Micon (IC100) located on the Main board. It is used as a basic “SMPS OK” signal.

When the Micon (IC100) on the Main board receives an “ON“ command from either the keyboard or the Remote IR Signal, it outputs a high to RL-ON. This signal first turns on a DC level shifter Q706 which creates 5V General. LD703 now glows amber indicating 5V General has been generated. This 5V General now provides the pull up voltages that supply the output circuits to the SMPS. The RL-ON enters the SMPS board at Pin 19 of P813. The RL-ON Voltage is sensed by the Sub Micon (IC701) circuit which causes the Relay Drive Circuit to close Relay RL101. this brings the PFC source up to full power by increasing the 159V standby to 340V. At this time the 16V source becomes active and sent to the Main board via P813.  The next step is for the Micon (IC105) on the Main board to output a high on M5V_ON Line to the SMPS at P813 Pin 21 which is sensed by the Sub Micon IC (IC701) on the SMPS turning on the 5V VCC line. The last step to bring the supply to “Full Power” occurs when the Micon (IC100) on the Main board brings the VS-ON line high at Pin 20 of P813 on the SMPS board which when sensed by the Sub Micon IC (IC701) turns on the VA and VS Supplies (VA is brought high before VS).

Note: 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.  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 could be caused by a failure on the SMPS or a circuit using these voltages. A resistance check should narrow the possible failures quickly.

POWER SUPPLY STATIC TEST
This test can confirm the proper operation of the SMPS without the need to exchange the board. This Power Supply can operate in a No Load State. This means that by applying AC power to SC101 and all other plugs disconnected, this power supply will function. Simply removing P813 (Lower Right Hand Side of the board), will cause the “AUTO” Pin 22 to go high from its normal low state allowing the Power Supply to go to full power on mode when AC Power is Supplied. Be careful after this test and make sure the VA and VS lines have discharged before reconnecting the supply cables.  If the Y-Sus and Z-Sus boards are working normal, “Display Panel Reset” will be visible when the SMPS comes up to full power. Shorting the Auto Pattern Gen. Test points at this time should result in test patterns on the screen.

Vs - Va ADJUSTMENTS
This Power Supply will come up and run with “no” load (P812 pulled). Pull P813, apply AC power, and the Power Supply starts. Y-Sus/Z-Sus runs “Yes” both Y and Z waveforms.

Use Full White Raster