Samsung UN55MU8000 - Backlight But No Picture Problem

[diif]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

With the power off, measure the resistance to ground of all the coils (square devices with 100, 220, 1R5 marked on them).

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

With the power off, measure the resistance to ground of all the coils (square devices with 100, 220, 1R5 marked on them).

Dear @diif

Attached are the measurements of all the coils on the board. I measured them with the board off the chassis on my bench.

I have no experience with coils whatsoever and had to look at a few references on their function and what to expect when testing them.

The measurements with a "???" were coils which fluctuated heavily when reading them. They would move from the MΩ to the kΩ range and go back up again just to drop back down. I have no idea what I that was about so if you could give me some insight I would greatly appreciate it. No coil measured open but one measured shorted (to coil I believe).

Let me know what you think and thank you in advance!

[diif]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

The coils indicate different power rails, checking their resistance to ground is a quick way to see where the problem might be if looking for a short.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

The coils indicate different power rails, checking their resistance to ground is a quick way to see where the problem might be if looking for a short.

Thank you for that! I looked at the datasheet of the 220 coil and it said thatbits max resistance would be 0.5 so it may not be shorted like i initially thought. What do these readings on each coil/rail indicate to you?

What should my next step be?

Thank you again

[diif]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

If I'm looking for a short I check the resistance to ground of all of the power rails. The easiest place to measure is the coils.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

If I'm looking for a short I check the resistance to ground of all of the power rails. The easiest place to measure is the coils.

How do my coil readings look to you @diif?

[lotas]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

The coils (inductors) themselves do not need to be measured, they will always show a short circuit, it is necessary to measure between one end (any) of the inductor relative to ground (GND).

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

The coils (inductors) themselves do not need to be measured, they will always show a short circuit, it is necessary to measure between one end (any) of the inductor relative to ground (GND).

Thanks for the reply @lotas.
Yes, that's exactly what I did.
However Im not sure how to interpret the measurements I got (see last pictured posted).

[lotas]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

And you connected the One Connect box after you insulated the contacts on the cable (in my opinion there will be no image in this series without One Connect).

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

And you connected the One Connect box after you insulated the contacts on the cable (in my opinion there will be no image in this series without One Connect).

Yes I connected the One Connect box to the TV.
As you surmised, without the OCB the TV will not boot as normal.
It will go to a warning screen with an image of a TV and OCB prompting you to connect the two with the cable.
But there WILL be an image even though the TV will not respond to any commands or perform any other functions without it.

See attached example images.

In my case, with or without the OCB I do not get an image nor voltages at key test points. With the OCB connected; the TV is able to boot up and I can hear the menu selection sounds while scrolling through the OSD with the remote. But again no image.

Back to the readings of the coils... do they tell you anything regarding faulty rails or shorted components?

Thank you in advance

[diif]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

What resistance do you get when you touch the probes together?

[mmaven806]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Ok, q211 is good. Pin 4 is the gate, so no problems there.
Q2002, based on your voltages readings, isn't the one I thought it was, so disregard it.
VIN_12V and HVIN_12 are netlabels, so to speak, you won't see them spelled out anywhere, sorry for the confusion.

Now that I have your board on my projector I can make out a Q400 just to the right of AVDD_17V. Is it a "4435"?
If so, ohm between q211 drain and q400 source.
If it doesn't ring good (0 ohm's), then check the fuse near the lower left corner of q211.
If it does ring good, what voltage do you have at q400 drain?

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

What resistance do you get when you touch the probes together?

Naturally the reading will be 0.1-0.2Ω but as I mentioned before the max resistance of that certain coil is about 0.6Ω. Im not sure what the normal resistance would be so I wasn't sure if that coil would be considered bad if it read 0.2Ω or shorted according to my MM.

Ok, q211 is good. Pin 4 is the gate, so no problems there.
Q2002, based on your voltages readings, isn't the one I thought it was, so disregard it.
VIN_12V and HVIN_12 are netlabels, so to speak, you won't see them spelled out anywhere, sorry for the confusion.

Now that I have your board on my projector I can make out a Q400 just to the right of AVDD_17V. Is it a "4435"?
If so, ohm between q211 drain and q400 source.
If it doesn't ring good (0 ohm's), then check the fuse near the lower left corner of q211.
If it does ring good, what voltage do you have at q400 drain?

Thank you for this and pardon my ignorance referring and looking at the Gate (G) as Ground. Complete idiot moment on my part.

Q400 is indeed a 4435. Its G4435SS from Diodes Incorporated.
I measured the resistance between the Source of Q400 and the Drain of Q211 and it was good (0.2Ω).
I proceeded to check the Y-coded fuse and its was good (showed continuity).
I then plugged everything up and turned the TV on measuring the voltages of Q400:
Drain reads almost no voltage (0.1v).
Source read 12.75v.
Gate also reads 12.75v. (NOTE: The voltage at the gate of Q211 is 6.35v)

Source and Gate do not read shorted with the MM.
Drain to Gate beeps momentarily on the MM but doesn't show any other signs continuity afterwards.

Please let me know what you think

[mmaven806]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

KYBOSH,
I tried a pm but it went south. I'll try again later when I'm at the office(desktop).

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

To make some clear view: the main board is good, you have no panel tensions because panel is shorted and it leads to protection...! To do: check 3rd and 6th photo, on the side that disconnected lets the tv boot, remove all those 3 tri-diode packs or to do it complex test each diode in ohm before and see if one is shorted, the top pins are cathodes and are connected to clocks, the bottom one are anodes connected to gnd.. when removed all or the bad one, boot the tv and see..

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

To make some clear view: the main board is good, you have no panel tensions because panel is shorted and it leads to protection...! To do: check 3rd and 6th photo, on the side that disconnected lets the tv boot, remove all those 3 tri-diode packs or to do it complex test each diode in ohm before and see if one is shorted, the top pins are cathodes and are connected to clocks, the bottom one are anodes connected to gnd.. when removed all or the bad one, boot the tv and see..

Thank you very much for this lead @Davi.p!

Im looking at the photos you referred to and I want to verify the components you are suggesting I test (see attached pic). The 3 diodes you pointed out aren't tr-pack (like a SOT23) but rather they are 6-pin diode array with an E5 marking.

I cant find anything online to identify what this is but I found a similar looking one with a K5 marking and it was identified as a 5 V ESD protection diode array made by Philips NXP Nexperia.

Also please remember that the TV set boots up with 2 traces taped off (the first 2 traces on the right side FFC). I even hear menu the menu selection sound as I scroll with the remote. I am just not seeing any picture. This is the same case whether I have no, just 1 or both FFCs connected. The TV isn't in protection mode (the type where the TV won't start up at all) but rather isn't sending any power greater than 3.3v to the panel.

Is it possible for there to be still another short(s) in the panel but the TV will still start up?

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

i wrote tri-diode, i meant three-diode (3-diode) pack sorry, english-italian mix :-)

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

i wrote tri-diode, i meant three-diode (3-diode) pack sorry, english-italian mix :-)

No your English is very very good!
I think I read too much into the name as I hadn't heard it before.
So just to clarify, you want me to check the 3 diode arrays labeled "E5" to see if they are good?

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

yes of course, tester in ohm, check each top pins respect to the bottom one (gnd) and compare them

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

yes of course, tester in ohm, check each top pins respect to the bottom one (gnd) and compare them

So I went through and tested each pin for resistance with respects to ground.
Ive attached photos with the values. On one side all of the diode arrays are OPEN on the top pins while on the other side only half of them are.

Since this is my first time delving into UHD panel board repair I am unfamiliar with what I should expect (what's good vs what's bad) so I hope you guys can school me a bit on this.


I then went back and tried the tape off method methodically one each FCC (still leaving the 2 traces covered which I know causes the TV to go into a boot loop). I started off with only 8-10 tracks exposed then cut the tape to expose 8-10 more tracks as I tried over and over to get a picture. I did this for both FCC. Never got a hint of a picture.

Also not getting any voltages on VGH or any other panel related test point on the mainboard with or without the FFCs connected. I think there should be at least a voltage reading at the test points but I dont know enough about this system to know if power is only activated by the panel under certain conditions... even at the test points.

Any guidance would be greatly appreciated!