Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

[KYBOSH]

Came across a 75" QLED TV with restart-backlight no boot and it was headed to the dumpster. Asked the owner to let me have a crack at it. They said the TV went out during or soon after a thunderstorm. I noted this but didnt out too much stock into it as it could have been anything.

VERY big TV and i was worried about the display cracking during the moved as well as the repair but it was alright as i try very hard to handle with care.

In the video (zip file) you can see' as the TV tries to start up, tbere are a lot of gaps in the backlight so I immediately suspect this is a backlight issue and order a new set.

When they arrived I tested the new backlights which were sourced from another TV with my backlight tester and noted the voltage and amperage draw (~115v and 44mV) and there were the same across all 4 LED strips as each strip is identical to the other.

I carefully removed the display and filters and pulled the old backlights off. To fulfill my curiosity right then and there i tested them to see which one was bad but they all read the same as the replacements i just got (115/44).

I then plugged the TV up to the mains with just the pwer supply and the backlights connected and they fired up and stayed lit. I lut the original backlights back on and reran the test and had the same result. No backlight issue. I then connected the mainboard and speaker (without the display) and i got the TV to boot up with sound.

So this is a display problem. This TV uses only 1 large lLVDS ribbon cable so i started taping off sections to try to isolate the short and i found it but this did not get the TV to display a picture.

Looking over the panel boards connected to the display and disconnected one on a whim (not actually expecting anything to happen) but to my surprise i got picture on all parts of the screen except the panel board i disconnected was.

I started probing some caps and resistors with my multimeter and saw some caps were reading as shorted in diode mode. I tested the panel board on the opposite side of the TV (there are 4 total) and the corresponding caps did not read as shorted.

My questionn is how likely is the source of the problem a SMD or two vs a short wirhin the panel itself?

I understsmd very little of how to troubleshoot these LCD panel boards. I tried my hand at it once several months ago and never got any where although i have/had no idea if the panel was beyond repair or not. And certainly did not have a partial picture on the display encouraging me that i was on the right track.

Please have a look at the pics and video and pls let me know what you think!

Thank you in advance

[KYBOSH]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

I'll post a diagram of the panel board showing which SMDs are reading abnormal as soon as i can.

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

Inspect them very closely, sometimes you can see a hairline crack or such, if not seeing anything obvious, retest them in ohm mode, if one reads lower than the rest, start with it. I would remove one at a time, test it off the board if it reads short it's probably bad, retest the pads it came off of, does it read short, also retest the others in circuit that read short, see if they still do because if one is shorted it can make the rest read short even if they are good because of how they are connected.

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

There's other ways of testing, like seeing if one is hotter than the rest, using a milliohm meter, maybe a flir thermal camera.

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

They're not too hard to remove with just a soldering iron, can practice on a scrap board, just add a little solder to each end like so: https://www.youtube.com/watch?v=8JM4oCpWnjU

[KYBOSH]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

Inspect them very closely, sometimes you can see a hairline crack or such, if not seeing anything obvious, retest them in ohm mode, if one reads lower than the rest, start with it. I would remove one at a time, test it off the board if it reads short it's probably bad, retest the pads it came off of, does it read short, also retest the others in circuit that read short, see if they still do because if one is shorted it can make the rest read short even if they are good because of how they are connected.

There's other ways of testing, like seeing if one is hotter than the rest, using a milliohm meter, maybe a flir thermal camera.

They're not too hard to remove with just a soldering iron, can practice on a scrap board, just add a little solder to each end like so: https://www.youtube.com/watch?v=8JM4oCpWnjU

Thank you for these tips!
The next item on my to-get list is a thermal camera.
Makes hunting down shorts so much easier I am seeing.

Removing them 1x1 should not be a problem. I just got a tube of low-temp solder i've been meaning to try out and I think this maybe just the occassion for it. Desoldering on such a thin panel board while its standing up maybe a challenge even with hot air. I hate the fact that the caps are so close to the film but I will do my best to be careful and mask them off.

I'll be back later with measurements to see the best place to start.

Thanks again!

[KYBOSH]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

@nomoresonys

I think you hit the nail on the head.
I went back and probed to record which SMD was showing short/open. When I was done I realized it was only the 4 biggest SMDs (C101, C201, C301 and C401). Taking your advice I started removing each 1x1 and testing them out of circuit. The SMD would test fine but the empty pads were still showing shorted on both side. I moved on to the next cap. Same thing. Then the next... same thing. It was only when I remove to the last cap (C101) did the shorts stop. Sure enough that cap was defective. Not reading any capacitance and shorted.

The remaining good caps read a capacitance of 22.5, 22.5 and 23.7µf. The closest multi layer ceramic cap I have measures 20-21 µf and I admit they are probably low quality as I got them as part of a sample kit off of Amazon. My idea was to replace all four caps but I may just replace C101 as the ones I would replace the rest with are probably inferior. If I am not mistaken these act as filter caps and so using a cap with a slightly lower or greater capacitance should be okay. Another option I thought of, but dont know if it would be a good idea, is to use one of the good quality electrolytic capacitors I have laying around. They are 22µf.

Any advice?
After I get this part figured out and this board buttoned up I can see if I can get a full picture on the display.

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

I would probably go with the new ones from amazon, I doubt a couple of ohms difference would matter, probably don't use the electrolytic. My reasoning is why did a relatively newer tv already have one shorted out, guess it could have just been a bad one in the bunch, with samsung it's hard to tell, looks like their quality control has vanished.

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

Thank you for these tips!
The next item on my to-get list is a thermal camera.
Makes hunting down shorts so much easier I am seeing.

Removing them 1x1 should not be a problem. I just got a tube of low-temp solder i've been meaning to try out and I think this maybe just the occassion for it. Desoldering on such a thin panel board while its standing up maybe a challenge even with hot air. I hate the fact that the caps are so close to the film but I will do my best to be careful and mask them off.

I'll be back later with measurements to see the best place to start.

Thanks again!

Great point about masking the area off, easy to burn something up if it is not.

[EazyBone]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

I must say, I've had many suiciding Samsungs and you damn lucky son of a gun lol. I always test the panel boards and get no where. But you get a short that potentially fixes lol. I'm happy to see this and following in case I run into future something here.

I want to hear the results!

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

It is kind of rare to find just one shorted cap lately, I too find them very seldom.

[KYBOSH]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

I must say, I've had many suiciding Samsungs and you damn lucky son of a gun lol. I always test the panel boards and get no where. But you get a short that potentially fixes lol. I'm happy to see this and following in case I run into future something here.

I want to hear the results!

It is kind of rare to find just one shorted cap lately, I too find them very seldom.

You asked for it... utter and complete success!

After my cup of coffee this morning I had a little time so I went in and did the deed. First I had to reuse the old caps bcos they were sized 0805 and not 1206 as I originally thought. The 1206 are just too big and would not tack to the solder pads when I heated them with hot air. The crappy SMD kit I had did not have the right caps in 0805 so I put the original 3 back on and use one 1206 with a few strands of wire to bridge the gap between the solder pad and the end of the capacitor. It worked out just fine thankfully. I dont have or regularly use kapton tape so I just used some aluminum foil to mask the film and protect it from the hot air.

After double checking that I had no solder bridges and there was nothing reading as shorted I connected all the ribbon cables and fired it up. A sweet, beautiful and most importantly COMPLETE picture appeared. I was very happy. Thank you very very much for your guidance on this @nomoresonys (and the Badcaps community in general) . This was an excellent exercise and the experience was exactly what I was after here.

@EazyBone You are right about never having success with panel troubleshooting. I think this is the first time I have had success with such a thing. I think it was my hoping and praying that the short was not within the panel that had me looking at every other possibility.
Ive seen how those internal panel shorts are sorted out and that's just not something that I think I can do.

On to the next one!

[nomoresonys]

Re: Samsung QN75Q70TAF - Problem ON or WITHIN the Display??

Nice work, that's a beautiful save on that monster.