LG Plasma TV Unable to Turn On-Repaired

By on May 12, 2016
repair lg plasma tv

 

 

 

 

 

 

 

 

This faulty LG plasma TV model no: 42PA4500/chassis 23PA was unable to turn on manually or with the remote control.

lg plasma tv repair

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.

component level repair

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:

  1. Q709-KF7N60 (MOSFET)
  2. Q712-30F124 (IGBT)
  3. Q703-RJP63K2 (IGBT)
  4. Q706-SUM5N25 (MOSFET)
  5. D735-RF2001 (DIODE)
  6. 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.

plasma tv repair

Photo 1

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.

Z SUSTAIN BOARD

Z SUSTAIN BOARD COPPER SIDE

Photo 2

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.

scope external trigger

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.

z sustain board alignment

Y sustain board alignment

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.

sustain board alignment procedure

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.

skwong

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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http://www.jestineyong.com/how-to-repair-samsung-lcd-tv-with-vertical-lines/

 

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27 Comments

  1. Parasuraman S

    May 12, 2016 at 10:26 pm

    Amazing, that is all what I can say! You have depth of knowledge and expertise in trouble shooting, and further, persistence, that brought the result! Hat's off to you! I have never handled a plasma TV so far, and after reading this, do not wish to do so.........

    Likes(6)Dislikes(0)
    • skwong

      May 13, 2016 at 10:58 am

      Thanks for your support, to be honest with you this is the 1st plasma tv i repaired. I have read yr repair articles frequently and you are very good in crt tv repair.From your experience I feel you have the capability to pickup plasma tv repair. From my personal view I feel that plasma TV is bit more difficult to repair than LCD or CRT tv.

      Likes(5)Dislikes(0)
      • Robert Calk

        May 14, 2016 at 12:52 am

        The authors of the Plasma TV books that I have claim that they are actually easier to troubleshoot than LCD and CRT TV's because problems are more easily diagnosed by just observing what is happening on the screen.

        Likes(1)Dislikes(0)
  2. Paris Azis

    May 12, 2016 at 10:30 pm

    Very interesting repair and presented in detail. I am impressed with the number of the failing components and your "hunting" patience.
    Well done!

    Likes(6)Dislikes(1)
    • skwong

      May 13, 2016 at 10:59 am

      Thanks

      Likes(1)Dislikes(0)
  3. Albert van Bemmelen

    May 13, 2016 at 12:45 am

    Thank you Mr. SK Wong. Indeed a very well presented and successfully
    completed repair! Even when the LG Service Manual is present, it still
    no doubt was a hell of a job. Only a Pro like you would have been able
    to take all the pitfalls. Add to that the adjusting of many of the
    potentiometer controls to correct the Screen you've done. The TV owner
    may count himself lucky it was you who still could save the Plasma screen!

    Likes(4)Dislikes(0)
    • skwong

      May 13, 2016 at 11:11 am

      Thank you for your comment. I fully agree with you, the service manual will not provide you with the complete information. Sometimes can be very frustrating where service instruction not properly illustrated. The service manual may have written for internal use and basic information are norm and all technical persons are equipped with the knowledge.

      Likes(4)Dislikes(0)
  4. Robert Calk

    May 13, 2016 at 1:22 am

    Well done! Thanks for sharing the repair with us.

    Likes(2)Dislikes(0)
  5. Anthony

    May 13, 2016 at 4:20 am

    Great repair and fantastic diagnostic skills Mr S K Wong ! Thanks for sharing your very interesting stories here !

    Kind Regards

    Likes(3)Dislikes(0)
  6. Mohamed Sittiq

    May 13, 2016 at 12:42 pm

    Dear Sir,

    Absolutely fantastic work done

    Thanks
    MOhamed Sittiq M

    Likes(2)Dislikes(0)
  7. Yogesh Panchal

    May 13, 2016 at 3:35 pm

    Sir,
    Thanks for sharing your experience .

    Likes(1)Dislikes(0)
  8. suranga bandara, Suranga Electronics

    May 13, 2016 at 4:18 pm

    Dear Sir,

    Good Service information and very well presented.
    I think you are an expert in this Field.
    Thanks.

    Likes(1)Dislikes(0)
  9. macdonald makwemba

    May 13, 2016 at 9:44 pm

    Thanks for sharing repairing tip

    Likes(1)Dislikes(0)
  10. Humberto

    May 13, 2016 at 10:22 pm

    Hi Mr. skwong, very interesting repair, at the end you saved a device from the dump.

    Likes(2)Dislikes(0)
  11. beh

    May 14, 2016 at 6:02 pm

    this is a super article and yes this is destiny of all irreparable boards that must be replaced by a healthy one thanks SK .you did not said how much you paid?

    Likes(2)Dislikes(0)
  12. Sameer

    May 15, 2016 at 6:00 pm

    Really outstanding repair with a an excellent indepth explanation.You really need experience for this repair.Honestly some techs like myself would have dumped it.How long did you work on this plasma ?

    Likes(0)Dislikes(0)
    • skwong

      May 16, 2016 at 12:57 pm

      I spend 3 to 4 days of trouble-shooting including understanding plasma tv from the internet since it is the 1st faulty plasma tv I encountered. I managed to complete the repair in about 2 to 3 weeks due to the time spend on sourcing the faulty parts and waiting for it to arrive. I always commented that sourcing for parts are more challenging than the trouble shooting.

      Likes(1)Dislikes(0)
  13. tino choolun

    May 20, 2016 at 2:12 am

    fantastic article thumb up
    tino

    Likes(1)Dislikes(0)
  14. Andrew vickers

    July 1, 2016 at 5:49 am

    Thank you very informative had same tv but powered up no picture after fault finding found 2 sun45n25 on ysus board faulty purchaced 2 of them fitted and tv is now working purfect again thank you again for the time and effort you put in to fix your tv cheers andy

    Likes(1)Dislikes(0)
  15. Bruno Lima

    July 25, 2016 at 6:57 pm

    Hello!

    Thanks for the article. When you say that the MOSFET and IGBT where shorted? You tested them on the board? You read a direct short between pins with the multimeter in diode mode?

    Please share the way that you have tested them.

    Again, thanks for the wonderful article.

    Likes(1)Dislikes(0)
  16. skwong

    August 2, 2016 at 11:07 am

    Hi Bruno,
    When I tested the MOSFET and IGBT with a multimeter on the diode mode, it indicate shorted (almost zero ohm) on the 3 leads i.e gate,source and drain which make the trouble-shooting much easier and straight forward. If it is not direct short sometimes it can be confusing . Not all IGBT and Mosfet can be tested accurately for normal function with a multi-meter.

    Likes(2)Dislikes(0)
    • Bruno Lima

      August 2, 2016 at 6:47 pm

      Thanks for the information.

      Likes(1)Dislikes(0)
  17. Russ Hill

    May 27, 2017 at 3:12 pm

    Q706-SUM5N25 (MOSFET) part number is wrong, its sum45n25.

    Likes(3)Dislikes(0)
  18. BAHRUZI

    July 13, 2017 at 6:58 am

    TV LG 42"PA4500 PLASMA; VS=206,VA55,6,VZB130, AFTER REPLACE RJP30K2,KF7N60,RJP30H2A TV ON,BUT APPEARENT ABNORMAL AS LIKE PHOTO 7,
    AND DIM.

    Likes(0)Dislikes(0)
  19. Luigi

    September 15, 2017 at 6:06 am

    I have the same model of LG 42PA4500 TV with the same problem by disconnecting the Y-sus the power supply is no longer in protection, if you can advise me what component I have to test, beyond those in these photos. Thank Luigi

    Likes(0)Dislikes(0)
  20. Benno

    April 20, 2018 at 8:06 pm

    Q707 is the same as Q708 or?
    got the same tv allmost all mosfets/diodes are shorted on the Y board,
    Z board "multimetered" ok for now.

    Likes(0)Dislikes(0)
  21. Arnold

    May 26, 2018 at 7:38 am

    on thissame tv i found Q707,Q708,fried n i replaced them so the tv played for about 1 hour bt i could feel that its overheating n next sound was kabooooommmm n the tv was dead so this time it fried+Q703 n Q704 n im back to square one. what opened my eyes here is...becouse the set isi old i must install a fan to cool down the mosfets n i guess the problem of this stressful tv will b solved. thanks for the revelation

    Likes(0)Dislikes(0)

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