Yet another Samsung bootloop - QE50Q60T

[efidan]

Hello,

I've ended up with a Samsung QE50Q60T with the classic bootloop problem. I've read plenty tape solutions on these forums but never seen one that made me progress to an image. Perhaps the model number makes a difference.

I've tried the usual disconnect LVDS and disconnect left/right methods and TV boots with no image on both. What amazed me though, if I tape the pin that sends 7V VCOM voltage then the TV starts up normal but due to missing VCOM voltage the image is whitewashed.

I've disconnected left/right VCOM from each other with another sticky tape in between and applied 7V external power to either side and it just made the TV go to bootloop mode again. I was expecting at least one half the work normal if the fault was with the VCOM rail. I was wrong.

I'm certainly baffled, what is so special about the VCOM rail that makes the TV get upset? It is not consuming much power either so clearly not a short but I cannot figure out what else does that rail drive to initiate a feedback to main board. I wish I could just disable the feedback and just power VCOM.

Any ideas?

Thanks.

[lotas]

Yes, and VGH on the left side is lower than on the right....

[lotas]

Take a look at this topic, the problem is most likely related to the protective diode assemblies on the panel buffer strips....
https://www.badcaps.net/forum/troubl...ue#post3168521
https://www.badcaps.net/forum/troubl...500-restarting

[efidan]

Thank you for the quick response.

I've seen both posts already. I've checked with thermal camera, nothing goes over 30 degrees.

[lotas]

You first need to determine which half of the panel the problem is on, then use a multimeter to check the resistance on these diode assemblies (1,2,4,5 pins) relative to ground (3 pin)...
To determine which side is not working, you need to install a jumper on the motherboard between the control points FB_TRDY_1 and FB_TRDY_3 (feedback).

[nomoresonys]

I have a question that's probably dumb but I'll ask it, what tvs do you need to put on the jumper, just the ones that have FB_TRDY_1 and FB_TRDY_3 marked on the mainboard probably. More over I guess I was wondering what year model it started with.

[efidan]

There are no marked FB_TRDY_1 or 3 on the board but I put a jumper on the points highlighted on picture 1 (remember seeing them referred to as feedback test points). Unfortunately made no difference. It is just continuous loop with no difference on left/right of panel.

I only have two diode assemblies and they are both on the left hand side (pictured). Second one reads 1,2,4,5 correctly as a diode but the first one only reads 2,4,5 and reads nothing for pin 1. Does it worth putting a standard diode between 3 and 1 to allow current flow or am I just jumping to a conclusion too early?

[lotas]

Yes, these are these points, you just have a different panel (from a different manufacturer), this panel does not have them and you have one lvds cable, you should only have a U-shaped cable connecting the two halves, so disconnect it from one either side, take a photo of both planks.

[efidan]

My photography skills are not any better but here you go

[lotas]

Check these test points on both sides for resistance, relative to the ground on the bar itself.

[efidan]

	Left	Right
LC2	1.1M	1.1M
LC1	1.1M	1.1M
VGH	4.8K	4.8K
VSS	1K	1K
CK8	1.5M	1.5M
CK7	3.4M	3.4M
CK6	3.4M	3.4M
CK5	3.4M	3.4M
CK4	3.4M	3.4M
CK3	3.4M	3.4M
CK2	3.4M	3.4M
CK1	3.4M	3.4M
CLR	4M	3.8M
STV	4M	3.8M

Left and right are identical. Anything else to check?

[lotas]

These measurements were carried out with the U-loop turned off (that is, the left and right strips were disconnected from each other)?

[efidan]

Apologies, you are right. Below are the measurements without strip and without connection to mainboard. >200M ones start counting from 50M and goes over what my multimeter can read. Perhaps a digital multimeter cannot read them correctly.

	Left	Right
LC2	-	-
LC1	-	-
VGH	5K	>200M
VSS	1K	6M
CK8	>200M	1.3M
CK7	>200M	>200M
CK6	>200M	>200M
CK5	>200M	>200M
CK4	>200M	>200M
CK3	>200M	>200M
CK2	>200M	>200M
CK1	>200M	>200M
CLR	-	-
STV		-

Should I suspecrt CK8 - Right?

[lotas]

Yes, and VGH on the left side is lower than on the right....

[efidan]

Thank you; I was doing my measurements wrong all along and feeling like a fool now!

I assume CK8 jumper on the right and VGH jumper on the left to be taken off to stop the TV from rebooting but VGH worries me more. Hopefully I have something on screen as a result 🤲

[capthookb]

Hey, efidan and lotas.
I have the same tv model. When it first started rebooting, there was image on the screen. It was displaying "Samsung QLED" logo (or something like that), then it would work normally for 1-2 seconds and it would reboot.
At some time it started rebooting and the image was gone as well. I tried the tape method and the backlight is on, but with no image. I tried removing the U shaped ribbon that connects the two lvds without the tape and the backlight would stay on, remote working, sound working etc. But no image.

I thought the motherboard had to be replaced and bought a used one from ebay. But the same situation exists with the replaced board as well.
In case you have any progress towards having picture as well, besides backlight, please share here. Thanks

[Takeatry]

I have a QN50Q with the same boot loop issue. I started a post but it seems to have run its course. I narrowed down the problem to the right side panel looking from the back of the tv. I have been reading and reading many post regarding the issue and came across this. If I understand the testing procedure correctly I remove the ribbon from the main board to the panel and in my case the flexible jumper ribbon from the right side panel. Checking resistance on all the test points by the clock signals on both sides. My meter only goes to 50M ohms. I get the 5k ohms on the VGH point on the left and 1k ohms on VSS. All other points go to >50. On the right panel every test point goes to >50, although a few of the clock signals bounce under 50M but settle out >50. Assuming my method is correct what should I do next.
thanks

[Davi.p]

be patient and don't trash bin it, maybe i have the answer you want in few days, the part i ordered is arrived today (at the collection point), tomorrow may i get it, i hope the two other users have not given up on their..

[Takeatry]

That would be great. As I was going through multiple test on the panel I measured the voltage on all the test point on the right panel in the same clock signal area. The VGH was 25v and VSS was 6v all the clock test points also measured 6+v. Did this occur on the unit you are working on?

[Davi.p]

The voltages does not apply, it's far far from the problem, i can't anticipate a thing, this evening i have set up a small Panasonic smart tv that i got for free long time ago, it was with lines appearing in after many minutes, but now it is working fully ok since a lot of minutes, without doing repairs, there is approximately a temperature of 20 degrees, i don't know ho to do now, i wait..

[Takeatry]

A new development. I was doing some voltage measurements at the tests-out 0,1 and 2 by the cofs from both panels to see if there were differences. I found none. So I shut it down for a while. When I turned it back on the loop was gone. Unfortunately so was the picture. The back lights were on I measured the output of the PS and still steady 13v. Did the standard removing the ps cable then the ribbon from the main board. Back lights came on stayed on. Reinstalled the jumper on main board removed the ribbon between the panels. No picture. Did the soft and hard resets no picture. Reconnected the speakers and got perfect audio.
The only voltage measurement I ever did in the MB until yesterday were what I believe are capacitors in the first 2 pictures below. 1st picture in the past measured the right side on the cap by the ribbon cable while it was cycling on and off was 15v. In picture 2 the cap circled was 5v. After the loop stopped the reading at cap by the cable is 20mV. Second photo one is still 5v. Also in the second photo is the arrow SMD/DIP pointing at 8 connections. The top 2 are 12.9 v the next 2 down are 3.3v. The bottom 4 are .5mV. I also measured the contact points on the board before I put the jumper wire on. Both 3.3v
I am guessing the cap by the ribbon cable is somehow connected to the panels. The voltages on the panels where I can measure it is exactly the same 20mV.
Not sure if I somehow damaged the MB or something else happened. Are there other checks of the board I can test? To see what there is only 20mV getting to the panel.