Would appreciate a sanity check of my plasma TV failure analysis

[erikbrenn]

Hi,

My LG 60" plasma died on me, just like that. I got a great deal on it 3 years ago, I love the picture, so I'm spending some time to write up an repair-attempt analysis. Hopefully someone can verify my findings, or even better correct my conclusions.

I have seen a bunch of repair videos lately, hoping to learn some of the tricks of the trade, how to find shorts etc.

Here are my steps and my analysis. Any thoughts are welcome.

Problem description.
TV suddenly doesn't turn on, no reaction when pressing remote, neither when pressing the ON button under the TV.
When powering the TV I can hear the relay click (on?), and click again after a few seconds. It keeps on like that, becoming more frequent after 20-30 secs

At first, I reckoned the problem being a bad cap not being able to hold its charge etc.

Steps
I took out both the power supply board and the main board, as shown below.


The cable between the two boards carries the signals shown below.



Testing the powersupply board
Inspecting the board all the caps look ok, so I haven't tested them using my ESR yet, also because the powersupply boards seems to work ok.
I powered up the power supply board (main board cable disconnected)

- The relay is clicking after a couple of seconds, and stays like that, in standby mode I guess. Normal behaviour AFAIK
- All voltages on the power supply board measures ok, i.e:
  - the 5.2V and 17V lines on the connector to the main board
  - the VS (205V) and VA (55V) on the other connector going to the panel
 
Next, i figured I should apply some load to the 5.2V and 17V lines, so see if that triggers any problem with the supply.
According to the silkscreen on the supply board the 5.2V line is rated for 2.5A, and the 17V is rated for 1A



I used some low value ceramic resistors I have to load the lines with about 2A and 1A respectively, and everything seemes to work fine. Relays clicks on powerup of board, and things are stable. Current verified with multimeter.

Based on this I can't see how anything can be wrong with the powersupply board... ?
 
Moving forward, as soon as I connect the main board the clicking starts after a few seconds. I therefor shifted my attention to the main board.
I found a service manual and schematic of my TV's main board online:  https://www.manualslib.com/manual/1276870/Lg-60pb5600.html  (schematic start on page 16)


Testing the main board
There are 4 regulators on the main board, all located near the connector coming from the powersupply.
The picture below shows the connector, with rails and signals from the powersupply. The 5V_ST was measured ok on the powersupply, also with load.


The 5V_ST is fed to these two circuits:



To my understanding, things work like this:
The 5V_ST rail is used for two purposes:
  - To feed regulator IC603 which creates the 3.3V_ST rail
  - To feed the MOSFET Q604, which forwards the 5V_ST to the 5V rail whenver the relay (RL_ON) is activated (when TV is turned on).
     (The 5V rail is just fed to the other regulators providing some other rails)
 - The RL_ON signal comes from the main LG controller IC1202, which I guess is powered by the 3.3V_ST standby rail (pwr pin of ic not visible in schematic)

Based on this I figured something must be wrong with the 3.3V_ST rail so I checked the IC603/3.3V_ST regulator
When powering up the system I notice that:
 - IC603 becomes very hot. I put alcohol on it and it vaporized quite fast.
   I found that strange since it's not on a heat sink.
 - The output of the regulator is stable for a few seconds, then it drops to zero a brief moment when the relay is clicking
 - I turned off power and measured 10 ohms from output pin of regulator to ground, indicating a partial short on this rail

I took out the the IC603 regulator using my rework station, The 3.3V_ST rail still measures 10 ohm to ground, so regulator is not the short.
(But before I did this I ordered new regulator from digikey and replaced it, no change)

To determine the short I continued as follows:

I fed 3.3V from my powersupply to the 3.3V_ST rail, setting current limit to 1A.  The board then draws 800mA, so something must be getting hot.
I spent some time with alcohol, fingers and magnifying glass. The only thing that I have found to become hot is the main LG controller IC1202
I have checked all 15+ capacitors that schematic shows connecting the 3V_ST rail to ground, but could not see/feel anything suspecious.

So this is were I hit the wall.

Conclusions/further plans
My theory is that the LG controller has failed. It is the only thing that gets hot. The 3.3V_ST line pulls 800mA before voltage drops to zero a brief moment, which caused relay to click. Since the chip looses its power rail I guess it causes the RL_ON signal to go low and off the relay.

The IC is sold on Aliexpress, but unfortunately it's a BGA chip with lots of balls, there is no way I can do BGA work like that.
I cannot see that any of the caps on the 3.3V_ST line could be the problem as the heat dissipates in the IC.

Moving forward I will try to heat up the PCB underneath the IC, in the hope that any balls that have connected will separate again. Since the chip has a heatsink I reckon it gets quite hot under normal operations.

I would appreciate your comments on my analysis, and hope that this is useful for others.

Cheers,

erik

[erikbrenn]

Should also mention that I have searched for a replacement board (EAX65405606). No luck on ebay.  I found a polish sparepart reseller that seemingly has it (but polish language only). I sent them an email to check.

cheers.

[MagicSmoker]

I have a 55" Samsung plasma which I had to repair a few years ago and while I don't remember exactly what went wrong, I do remember getting a replacement board from this website: https://www.ftvparts.com/

It's in the US, obviously, but might help you anyway.

[erikbrenn]

Thanks for the tip, they don't have my board though.

Feels terrible to throw away this big TV just because of a blown chip.

[PKTKS]

Well  here in Brazil all things nowadays come mostly  via import.

And that is not cheap. Taxes will mostly double or triple the price
you see on any product - INCLUDING BGA MACHINERY.

So... it is not rare to have BGA done as services from a  third part.

If you have damn sure the chip is faulty and you have no interest
expending money on a (questionable really) BGA machine just
for the sake of doing proper thermal gradient reflow...

Well you just out source it. Try finding someone which would
do the BGA job by just sending chip and board.

Obviously you should check the qualified people to do so...

Paul

[shakalnokturn]

BGA balls don't usually meet by themselves (other than on a botched reflow) they usually crack.
I tend to agree with your conclusions and to not know if for your test you only fed the 3.3V to the mainboard.
How hot is very hot for IC1202?
To be on the safe side you could make sure that nothing else is getting too hot such as DDR (on the underside?) or main IC on T-con board.

[erikbrenn]

BGA balls don't usually meet by themselves (other than on a botched reflow) they usually crack.

Sounds right. I will try to heat up the board from below using my reflow tool as a last attempt. But wanted to await any pending comments on other things I might have overlooked before I continue with such invasive actions   

I tend to agree with your conclusions and to not know if for your test you only fed the 3.3V to the mainboard.

Didn't quite understand that.  I also tried to feed the 5V_ST rail with 5.2V before I took out the 3.3V regulator. It was the same result, but of course the regulator got hot in addition to the LG chip.  This makes sense sinse regulator is rated for 1A so it will handle a partial short on the LG chip causing a 800mA sink.

How hot is very hot for IC1202?

It draws 800mA so things get kinda hot after some time, I will bring out my infrared termometer when I come home (forgot I had it). But it has a heatsink so it made me wonder if this is normal. But 800 mA is just wrong isn't it, even if this is some kind of asic chip doing lots of stuff. I mean it's in standby mode..... 

To be on the safe side you could make sure that nothing else is getting too hot such as DDR (on the underside?) or main IC on T-con board.

The DDR is located just top-right of the chip. There is absolutely nothing on the underside of the board. 

Btw, I found a polish site that has the board. I contaced them I they only ship to polish addresses,  aaarggg   
https://north.pl/karta/ebu62394910-plyta-glowna-do-telewizora,706-HV-2666.html

Thanks a lot for your comments !