Samsung UE75MU7009TXZG Endless Power Cycle

[hill01]

Samsung UE75MU7009TXZG Endless Power Cycle

Hi everyone and thanks in advance for any help I might get here

My Samsung TV is about 4 years old. The exact model number is UE75MU7009TXZG.
About half a year ago I started seeing occasional green frames pop up while watching movies. I mean literally green frames. Not the image turning green, but the entire screen turning green for a split second. This happened maybe once or twice during a 2 hour movie. So no big deal, but clearly something was not quite right.

Since a few days, the TV won't turn on anymore. I removed the back-cover to take a look at the internals, in the hope of spotting one of these bad capacitors that Samsung seems to be quite famous for.





Unfortunately all the capacitors seem to be in good order. While the TV was open, I powered it up, just to see what would happen. Here is the exact sequence of events:

1) Connect the power cord -> The little red LED at the lower right hand corner of the TV turns on (meaning the TV is in stand-by)

2) Press the power-on button (either on the TV or on the remote) -> something on the power board makes a "click" sound and on the Backlight Control Board three blue LEDs turn on.

3) For about 5-10 seconds, nothing happens. Then the power board emits a "click" sound, the blue LEDs on the Backlight Control Board turn off. Immediately afterwards there is another "click" from the power board and the blue LEDs on the Backlight Control Board are on again. This cycle repeats for as long as the TV is powered.

While the TV is powered up, there is no image showing up on the screen. The backlight LEDs also never turn on.


I tried disconnecting various cables to see if any of them have an impact on the behavior of the TV.

When I unplug the 20 pin cable that is connected to socket CNM803 (on the power board), this has quite an impact:

1) Connect the power cord -> power board makes "click" and all the bright white backlight LEDs come to live. No more power-cycling, the TV just stays in this state until I disconnect the power cord.

The voltages on CNM803 seem ok. According to the printed table next to the connector, there should be a bunch of 12 V and 18 V lines. Multimeter measurements confirm that these voltages are present (12.8 V and 18.0 V), regardless of CNM803 being connected or disconnected. In case CNM803 is connected and the TV is in stand-by, the voltage of the 12 V line is reduced to around 8.5 V, which is in line with the stand-by value listed in another table on the power board. As soon as the TV goes from "stand-by" to "on", the voltage goes up from 8.5 V to 12.8 V.

Thermal imaging of the power board reveals no particular hot-spots. The VRMs that are attached to the heatsinks are going up to around 30 degrees C, and one resistor lights up a bit as well (not as hot as the VRMs though). Otherwise the board and the components stay close to room temperature.

Some additional information regarding the TV:

* The TV was attached to the wall with a VESA mount bracket for almost its entire life
* The TV was connected to a master/slave power distribution unit. It was a slave (the AVR was the master).
* The TV was basically a glorified monitor: usually it would just display whatever video signal it gets from the AVR. Maybe 1% was spent watching Netflix or Youtube directly through the TV.
* As mentioned in the introduction, completely green frames started to be inserted at random while watching movies. It is unclear if this was caused by something in the TV slowly breaking or if it was a problem of the AVR or the media center.

What would be the next debugging step?

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Much probably a problem in the panel distribution boards, try disconnect the flat cables that goes to those one at a time, now the problem is if the tcon is included in main board, on how to find the point of insertion of power in the tcon, if there is a fuse must be identified and done some tests with ir camera..

[hill01]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Thanks for the suggestion. I disconnected the four flat cables:



The image on the left shows the state of the board when all four cables are CONNECTED (three LEDs light up).

The image on the right shows the state of the baord when NONE of the cables are connected. In this case only one of the LEDs lights up.

In both cases the TV goes through an endless loop of power-cycles.

What else can I try?

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

you can first remove the panel distribution boards bezel and test all diodes and capacitor for shorts, test also the resistors

[hill01]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

That seems to be hundreds of components. I removed the heatspreader and inspected the whole board with the IR camera. The main CPU thingy and a couple of other smaller ICs light up (to around 30-40 degrees C), everything else remains at ambient temperature.

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

You has not understood, panel distribution boards can't have hundreds of components, those are little board glued to all wide long the screen with small flat cables, them are under a metal frame, sorry i told bezel before.. you have the luck of a thermall cam, but you'll use if you'll not find a single short, i'll told to you how...

[hill01]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Oh I see! Sorry indeed I misunderstood, I thought you were talking about one of the big 20 cm X 20 cm (ish) boards. Ok I'll see if I can pry those tiny boards out from their hiding holes and check them!

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Those board can't be removed, them are glued to the screen fpc.

[hill01]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Thanks indeed it is easier to remove the cover on top of them. Cannot really spot anything wrong with them either though. Thermal only shows heat around the cables that go to the display.

Honestly I'm close to giving up

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Noo.. you are a record man as giving up on tv.. IR camera must not be used so, which is your skill level?Have you ever repaired something? I hope this giant tv doesn't end in a recycling isle but on sale for another passionate repairer or a repair center..

[hill01]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Thanks for the support! For me the frustrating aspect is that I do not understand the internal workings of the TV enough, and that I do know if I'm even looking in the right place. Any compontent on any of these three big boards could be broken, plus of course anything on those tiny boards hidden under the bezel.

And the only place I can put the TV down on is the couch - which mean that is blocked while I am trying to fix.

My repair skills are probably neither the best nor the worst. I can solder to a decent degree, use a multimeter and oscilloscope, clearly I have access to an IR camera (don't ask how), so yeah I'd say probably a bit above average. Thing is, the TV scares and overwhelmes me a bit because it has so many failure modes: a million things could be wrong with it, an depending on different sources you find very different answers. For most people it is just blown caps on the power board (which is rather easy to fix). Then there are some guids that start replacing ICs on this display control board (which honestly I would say is a bit out of my skill level first to diagnose and second to actually pull off due to the tiny pitch of the IC connections)...

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

So you have already asked help on another place, ok but here you are in the right place, i was here to entirely guide you, the people say you that are electrolitics capacitors are not well updated on tv faults, those are the past, i wanted to understand if you are able to normally test capacitors, diodes, resistors.. i tell you that a very common fault on these Samsung are the diode packs that are 10 pins small components in qfn format so without true pins. those are to be tested in a certain way..

[sam_sam_sam]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Samsung are the diode packs that are 10 pins small components in qfn format so without true pins. those are to be tested in a certain way..

Can you go into the details on how you test the diode pack I would like to know for my own information

Thanks

[Ancient1]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Hi

I am using the single image ,: full view of exposed backside of the TV.
You have 3 boards.
Left (green)= Main
Right (yellow) = PSU
Bottom (middle , green) = T-con ( Timing Control )

TV start-up proccess :
* Stby V is always present : Front LED works. Hot Side Main Cap ( Big one) is charged ( usually ~310V). Stby is usualy 3.3V but can be 5V.
* Pressing ON button signals the CPU on Main to send PSU-ON signal through the Cable line/pin to the PSU . This it does by feeding the STby V to PSU-ON pin.
* PSU clicks : The Relay ( in some designs ) clicks and passes the Power to the Higher Voltages production sub-circuits . Working power is sent to Main, T-con and Backlight
* CPU starts booting i.e running the firmware : Logo , than TV functions.

Troubleshooting at this basic stage means isolating parts in order to find where the issue is generated. This is done by disconnections and Powering up.

Your issue : Boot Cycle.
This happens when the PSU goes into protection , shuts down but retries since it the ON signal is still present.

From what you describe : When Main is Disconnected, You get stable activity, Including high V (?, nah) . If you follow the sequence I described , you'll see that STby = Front LED would still work , but higher V won't be present since the PSU-ON signal comes from Main.

The question is : where is the cause of Boot cycle located.
The answer lays in Auto-Starting just the PSU ( Main and T-con DISCONNECTED) by feeding , using the Cable other end , Stby V/pin to PSU-On Pin. I.E mimicking the Mainboard action. If the PSU is fine you should see the High V and stable. This means PSU is OK and the Cause is on Main. Or, ofc , PSU sub circuit is faulty and causes the Boot Cycle.

Good Luck

[Davi.p]

Re: Samsung UE75MU7009TXZG Endless Power Cycle

Those chips are with 5 pins per side, the center pins are the gnd, so test in diode mode between the others 8 and gnd , both ways, if you find a short or a reading different by infinite then it is broken, to the gnd pins you can have connected a pair of bigger resisters that could be bad..