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 LG 42PC5DC Plasma TV Switch Mode Power Supply Operation and Troubleshooting

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هـانـى الـنجار(( LCD ,LED,PLASMA , Receivers))

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تاريخ التسجيل : 14/11/2009
الموقع : EGYPT

مُساهمةموضوع: LG 42PC5DC Plasma TV Switch Mode Power Supply Operation and Troubleshooting   الإثنين فبراير 20, 2012 5:50 pm

Another difference is that the VA and VS Controls have circuit symbol
numbers reversed between the boards. VR901 on one board is VS and on
the other it will be VA.


AC Voltage is supplied to the SMPS Board at Connector CN1 from the AC
Input Filter. Standby 5V is developed from the Circuits of Transformer
T201, RC401, RC402 and Q422. Standby 5V is then routed to 2 places, the
Micro Processor IC4015 on the Main Board and to the Sub Micon U702 on
the SMPS, where AC – ON / DETECT is developed and sent from Pin 1 of
Connector P1 to IC4015. With the presence of both 5V STB and AC – ON/
DETECT, IC4015 is active.

When IC4015 receives an “ ON “ Command from either the Keyboard or
the Remote IR Signal, it outputs a high to RL-ON which enters the SMPS
Board at Pin 2 of P1 and M5V-ON. The high RL-ON Voltage is sensed by the
Sub Micon U702 which causes the Relay RL102 to close turning on the
380V Supply developing the 19V,12V and 6V Voltage Sources.

The 3.4V line at P3 (Pins 1 and 2) also P2 (Pin 7) becomes active
when Pin 13 the 3.4V-ON line of connector P1 goes high turning on the
3.4V Supply.

The M5V-ON Line located on the SMPS at Connector P1 (Pin 7) is sensed by U702 the Sub Micon IC turning on the 5V VCC line.

The last step to bring the supply to “Full Power” occurs when IC4015
brings the VS-ON line high at Pin 5 of Connector P1 the SMPS Board which
when sensed by the U702 Sub Micon IC turns on the VA and VS Supplies.

The Operating Sequence discussed above can be confirmed by using the
Oscilloscope in the XY Mode. Start by connecting Channel 1 of the
Oscilloscope to 5V STB and Channel 2 to AC DET the delay between 5V STB
and AC DET can be noted. Next Connect Channel 1 to RL ON and Channel 2
to VS ON again the delay between the 2 voltages can be noted. Connect
Channel 1 to RL ON and Channel 2 to M5V-ON the 2 supplies come up at the
same time proving the 5V VCC line becomes active before the VS ON line.

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.

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.

Listen for 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 U702 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
19V, 12V and 6V to the Main Board. If the Relay closed and these
supplies failed suspect a problem with the SMPS.

3.4V-ON will activate the 3.4V Supply to the Main Board, at this time
Audio should be present as the Tuner and Signal Processing circuits are

The next step of operation calls for the M5V-ON line from the Main
Board to the SMPS to go high. A high on the M5V-ON Line activates the 5V
Vcc line. Loss of 5V Vcc results in no Raster, no Reset, no Y, Z,
Control or X Board operation. Loss of 5V 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 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 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.


This test can confirm the proper operation of the SMPS without the need to exchange the board.

The Operation of the Power Supply can be confirmed by removing the
SMPS Board and placing it in the Service Position. Place two 60Watt
lamps in series across pin 9 (VS) and 5 (Ground) of P812.

Apply AC Power and check for 5V STB and AC-ON / DETECT at P800, if
present remove AC Power and install a 100Ω¼ watt resistor from 5V STB to
RL-ON on Connector P800. Apply AC Power, the AC Relays should close and
19V, 12V and 6V should be Present. If ok remove AC Power.

Solder a 100Ω¼ watt resistor from 5V STB to 3.4V-ON at P800. Apply AC
Power the 3.4V Supply should now be present. If ok remove AC Power.

Install a 100Ω¼ watt resistor from 5V STB to M5V-ON on Connector
P800, 5V Vcc should now become active, If present remove AC Power.

Install a 100Ω1/4 Watt resistor from 5V STB to VS-ON on Connector
P800, VS and VA should now be present. This test will confirm the normal
operation of the SMPS.

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LG 42PC5DC Plasma TV Switch Mode Power Supply Operation and Troubleshooting
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