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LG Plasma TV Unable to Turn On-Repaired
This faulty LG plasma TV model no: 42PA4500/chassis 23PA was unable to turn on manually or with the remote control.
When the power supply board output connector voltages were measured with a multi-meter the Va and Vs voltages were low. With power off, all other connectors to the power supply board were removed and power on again, the connector output voltages were found to be normal i.e, Va=55v,Vs=205v and V standby=5V. Suspect either the Y sustain, Z –sustain boards or both are high chances of failure due to the high operating power and temperature with MOSET or IGBT drivers.
Next with the power off, I reconnect all the connections back to the power supply board except for the connector to the Y-sustain board. When the power is turn on again, I am a able to activate the remote control. The sound came on and the TV channels can be activated. The Y-sustain board is confirmed to be faulty. The Y-sustain board with part no: EAX64286001 is rare and expensive in the market, so the best solution is to trouble-shoot on components level.
Check with the service manual there is little or limited information available for the Y sustain, Z sustain, control and Y buffer boards. Surprisingly, there are so many defective components which were shorted when tested with a multi-meter. These components were measured found to be shorted:
- Q709-KF7N60 (MOSFET)
- Q712-30F124 (IGBT)
- Q703-RJP63K2 (IGBT)
- Q706-SUM5N25 (MOSFET)
- D735-RF2001 (DIODE)
- Q708-RJP30H2A (IGBT)
After changing the defective parts, I though the repair was completed but it is far from over. A quick check on the Y-buffer board with multi-meter on diode range with one lead on the Y buffer positive supply track and the lead run down on all the leads of the Y buffer COF connectors to confirm whether any buffer ICs are shorted. Since the Y-buffer board is tested ok, the repaired Y-sustain board was re-assembled back on the chassis.
When power is on, the TV sound came back as before but there is still no picture. After a few minutes, the sound cannot be heard anymore and the remote cannot be activated. The Y-sustain board was re-checked for possible fault but no fault detected.
Next I check the Z sustain board and found a few IGBT short circuited ( Q7 ,Q9 and Q10) with part no. RJP30H2A. The reason why the short was not detected earlier because when the Y sustain was faulty, the power supply Va supply to the z sustain board was not normal or low instead of 55v.
The repaired Y and Z sustain boards were assembled back to the chassis, I expected the picture to turn on but it turned out to be disappointed again with no picture. The only good sign is that the TV channel sound is back. All the voltages measured on the Y and Z sustain board are within the specifications as marked on boards. I suspect that the Y-sustain board is not switching on because the large heat sink on the board is not warm with the power on for a few minutes.
Next I focus on trouble shooting the control board (ERB7482101), and then I realized that I have overlooked the blinking led on the board. The led does not blink or light up at all and this confirmed that the control board is abnormal. I noticed that the control board IC 703 has a small burnt hole on the top of the packaging.
With an oscilloscope 10X probes connected to the Y and Z sustain test points (see photo 1 & 2), there were no Y and Z sustain waveforms present on the test points. This confirmed that the control board was really faulty. I managed to purchase a used control board from the internet at a reasonable good price. After changing the control board the picture finally turn on but with unusual black patch on the screen as shown (photo 7). It is due to the Y sustain waveform still not normal or out of specification. Now the LED on the control board blinks.
Now it is time to re-align and confirm the Z and Y sustain board wave-forms. It is more accurate and easier to perform it with an oscilloscope. When the TV was switched on for 10 minutes, the picture disappeared again with another fault arises. This time Q704 and Q707 of the Y sustain board were found to be shorted. After changing these two components the picture comes back and no more failure after a few hours of test run.
Z-sustain board alignment
The z-sustain board alignment is much easier than the Y-sustain board. Firstly, with a multi-meter +ve and –ve probe across R151, adjust VR1 to read 120V as per labeling on the panel. The Z-sustain waveform can be check with an oscilloscope 10x probe connected to the capacitor as shown in photo 1. The signal VS_DA from the control board (photo 4) must be used as oscilloscope external trigger signal for the Y and Z sustain waveform alignment. Photo 5 show the Z waveform characteristic captured on the oscilloscope. It is not perfect but I think it is ok.
Y sustain board alignment
Connect the oscilloscope scope probe on the Y-buffer board as shown in photo 1. Photo 6 show the oscilloscope display of the Y waveform SET_UP setting was faulty or low. These resulted in black patches appeared on the picture as shown in photo 7.
It is important to have the Y and Z waveforms with the correct phasing and characteristic to deliver a normal perfect picture.
I am not sure what has happened to this TV with so many faults. It may be due to excess heat produced due to aging and deterioration of the heat conduction pads which caused the MOSFET/IGBT to overheat. I decided change the heat pads and add an extra exhaust fan on the back cover since the TV is a wall mount unit to improve the air circulation. The fan had stabilized the heat sink temperature to around 70 deg. C maximum (according to specification maximum junction temperature for MOSFET or IGBT is 150 deg.C).
The TV is being tested for about a week, operating more than 8 hours daily and without any problems. I think the repair is considered successful.
This article was prepared for you by SK Wong from Malaysia and he graduated from London with a degree in Electronics and Electrical Engineering. He has more than 40 years of experience in the consumer product manufacturing. With the passion to repair electronic products from young , he has been doing it part time until now.
Currently, he can provide turnkey solutions for consumer products including services from idea or concept to design , prototyping , finished products (including electronic,mechanical and plastic parts) and manufacturing.
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