Announcement

Collapse
No announcement yet.

Zombie Samsung UN55NU6900F dead / no standby voltage - randomly came to life after hours of troubleshooting then died

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Zombie Samsung UN55NU6900F dead / no standby voltage - randomly came to life after hours of troubleshooting then died

    This is my first attempt at troubleshooting a board like this, I know I could just buy a new board but I'm curious to figure out how this works. I've been testing different components in the evenings this week and I think the problem is with the IC failing to start up for some reason. When the board is plugged in there is no standby voltage.

    Then today it randomly sprung to life like a zombie but shortly died thereafter. I have no idea why it would have done that and its the first time I've seen it running. I discharged capacitors on the board last night, then it was sitting today. I had the board out of the chassis bench testing, poking around testing continuity across different components, then plugged it in and one of the lights for optical audio on the board came and you could hear a faint noise which had never happened. The standby voltage pin was reading 12.8V (supposed to be 11.7 but I figure it was just because I didn't have anything plugged in), which is the closest to running I've seen it. Then about 30 seconds later the light on the board went off and it went silent back to it's dead state.

    I have no idea how it could have come to life like that. The only thing I can think of is that the caps were all fully discharged and sat idle for the whole day. I had also just cleaned off RM819 (2.2ohm resistor connected to the VCC pin on the IC) with rubbing alcohol to get the silicone off of it.

    What I've tested:
    • Power cord / outlet tested ok
    • Fuse on the board tested ok
    • No visible caps / components burned
    • Diodes all appear to be working based diode test
    • Rectifier appeared to be working properly, 170V DC between +/- and diode test seemed ok
    • No components I've tested were shorted out
    • There's 120v AC coming from power cord
    • MOSFETs seem ok in diode test
    • See voltages on the IC in the table below
    Here are some readings I took from the PWM, note that some fluctuate
    FA6B20N Readings
    Pin Reading 1 (V) Reading 2 (V) Reading 3 (V) Reading 4 (V) Assumed Datasheet Function
    1 97 97 97 97 VH (High voltage input)
    2 0 0 0 0 NC (No connection)
    3 0.4 0.4 0.4 0.4 STB (Standby signal input/ output)
    4 3.8 3.8 3.8 3.8 FB (Feedback input)
    5 0.5 0.5 0.5 0.5 MODE (Operating mode setting for OCP)
    6 0.1 0.1 0.1 0.1 CA (Input power sensing capacitor)
    7 0 0 0 0 IS (Resonant current detection)
    8 0.1 0.1 0.1 0.1 VW (Winding voltage detection)
    9 0.1 0.1 0.1 0.1 CS (Soft-start and burst operation setting)
    10 10.5 10.5 10.5 10.5 VCC (Power supply)
    11 0.03 0.03 0.03 0.03 LO (Low-side gate driver output)
    12 0 0 0 0 GND (Ground)
    13 0 0 0 0 NC (No connection)
    14 3.7 3.4 0.5 0 VS (High-side floating ground)
    15 3.9 3.4 0.5 0 HO (High-side gate driver output)
    16 9 7.1 3.6 1 VB (High-side floating power supply)
    Datasheet:

    Potential issues I've noted:
    • The standby circuit reads 0V at the test pin instead of the noted 11.8V
    • Tried unplugging everything to see if the pin tested properly but that didn't work
    • RM819 connected to VCC noted above doesn't read 2.2ohms, it seems hard to get proper contact with the probes because its so small but readings range from 10Mohms to ~4ohms.
    • RM819 also doesn't beep for continuity, again its hard to place the probes because its so small, but other ceramic resistors seem to beep just fine
    • When there's power, VCC reads a stable 10.5V. The pin on RM819 connected to VCC also reads 10.5V but then the other side of RM819 is unstable. Sometimes it reads as low as 3.5V.
    • The two diodes near the standby pin don't immediately go to OL when probes are reversed in a diode test, they start around 1V then climb to 2V over about 2-3 seconds then read OL. Not sure if this is an issue or not, all other diodes immediately read OL when the probes are reversed.
    • The optocouplers gave odd readings in circuit, I've attached a picture below. Some weren't showing as open on the emitter/collector side as they should in a diode test. Some were also showing voltage readings with probes on either the anode or cathode which isn't normal. I haven't been able to test them out of circuit so I don't know if that would affect the readings in some way.
    Any ideas on what I could try next or how it randomly sprung to life?

    Board Top
    Click image for larger version  Name:	Board top.jpg Views:	0 Size:	3.27 MB ID:	3566894

    Board Back
    Click image for larger version  Name:	Board Back.jpg Views:	0 Size:	3.83 MB ID:	3566896

    Hot side close up
    Click image for larger version  Name:	hot-side.jpg Views:	0 Size:	3.78 MB ID:	3566898

    VCC Loop to VS
    Click image for larger version  Name:	VCC Loop.jpg Views:	0 Size:	986.1 KB ID:	3566907

    Optocoupler Readings
    Click image for larger version  Name:	Optocoupler Diode Test.jpg Views:	0 Size:	1.98 MB ID:	3566901

    IC - Optocoupler Continuity
    Click image for larger version

Name:	PWM - Optocoupler Continuity.jpg
Views:	113
Size:	836.6 KB
ID:	3566911
    Attached Files
    Last edited by tvguy12012012120; 02-07-2025, 05:23 PM.

    #2
    Maybe check your solder joints on bottom of board, reflow the transformer joints and DP811C, DP812C and any others that may need it.

    Comment


      #3
      They look fine from the back but I don't really novice so I wouldn't know for sure. I noticed some on the solder didn't go all the way to the top of the board one some of the joints near DP81XC (see the two blue caps CP810 and resistors RP824 and RP822), would that matter as well?

      DP811C / DP812C / Transformer solder joints
      Click image for larger version

Name:	Screenshot 2025-02-07 at 8.46.24 PM.png
Views:	84
Size:	2.89 MB
ID:	3566947

      CP810 and RP824/822 Solder Joints
      Click image for larger version

Name:	Screenshot 2025-02-07 at 8.49.49 PM.png
Views:	82
Size:	1.75 MB
ID:	3566949
      Attached Files

      Comment


        #4
        Following this! I have a few of these all in one boards and can't figure them out and haven't posted yet..

        But I also got weird octocouplers voltages. Mine would go up and down fluctuating for no reason.

        A known defect (not any of mine yet) are those 2 blue caps you have in red and that mosfet. They aren't bad on mine but supposedly they go bad so check for shorts in sure you already did.

        Diah this is the board I didn't post about, help this guy out 😁😁😁

        I got no other input except listening now

        Comment


          #5
          Ok so after countless hours in the evenings this week testing stuff and figuring out how the circuit works, it appears that RM819 the 2R2 SMD resistor connected to the VCC pin on ICM801 was the culprit (so far).

          When I initially measured resistance I was getting super high / unstable numbers so I thought my leads just weren't making contact. When it randomly started up the first time earlier this evening (mentioned in the OP), I had lightly abraded the surface of the contacts with my MM leads and noticed the resistance fell to more reasonable levels around 5-10ohms. Still higher than the 2.2ohms it is supposed to be but that must've was enough for it to briefly start up.

          Later this evening I decided to see how difficult it was to get resistance on other small SMDs on the board and readings would show up almost instantly. So I went back to RM819 and scratched the contacts with my leads again in an attempt to get better contact. As I did this, the resistance reading fell, so I kept scratching until it was close to the 2.2ohm spec it should be.

          and...

          Its alllllllllliveeee!!!!

          I was able to turn the TV on and go through a basic setup with it for about 15-20mins without any issue whatsoever. I'm assuming it'll only be a matter of time before that resistor offs itself again so I'll obviously replace it but it's satisfying to see it running.

          This is the first time I've done anything like this and it was super interesting learning about the different parts of the circuit and how they interact. I'm miles away from actually knowing exactly what is going on but at least I can now recognize different parts of a switch mode power supply board on a TV.

          One problem is that the standby/power reference pin is supposed to be 11.7V and it's reading 12.8V so that's something I'll continue to investigate. I also don't know why the optocouplers were giving weird readings in the diode tests, so I may replace those as well.

          Comment


            #6
            tvboy, the problem here is poor solder, this why Samsung put the glue on the most delicate part, the oscilator circuit and feedback circuit, you can start resolder the most important items.. transformer, power diodes/transistors.. this is my guess..

            Comment


              #7
              Originally posted by EazyBone View Post
              But I also got weird octocouplers voltages. Mine would go up and down fluctuating for no reason.

              Diah this is the board I didn't post about, help this guy out 😁😁😁
              the reason and the usual fluctuating reading when testing parts on the Primary such now the Opticcouplers with out discharging the circuit.

              Comment


                #8
                So I busted mine out same exact model, finally like I said I would.

                My resistor is 2.5 ohms like it should be. I don't get any standby light. Back lights do not turn on.

                What I get is voltage fluctuations constantly as I was saying about the octocoupler I also get the fluctuation going out to the standby button. I attached a small video to show. The main cap is at 390 volts and stay steady.

                https://youtube.com/shorts/HWlKJa2wD...CMQCAbM1PDeduF

                Diah what do you think?
                Attached Files

                Comment


                  #9
                  So I busted mine out same exact model, finally like I said I would.

                  My resistor is 2.5 ohms like it should be. I don't get any standby light. Back lights do not turn on.

                  What I get is voltage fluctuations constantly as I was saying about the octocoupler I also get the fluctuation going out to the standby button. I attached a small video to show. The main cap is at 390 volts and stay steady.

                  https://youtube.com/shorts/HWlKJa2wD...CMQCAbM1PDeduF

                  Diah what do you think?
                  Attached Files

                  Comment


                    #10
                    Anyone got an idea for me before I pack this thing away again?

                    Comment


                      #11
                      Originally posted by EazyBone View Post

                      Diah what do you think?
                      you have much like line cutting off at the primary , test all printed tracking line from end to end and the parts installed on them

                      Comment


                        #12
                        delete--- double
                        Last edited by Diah; 02-17-2025, 02:37 PM.

                        Comment


                          #13
                          LOL.... double with out reason... what kind of forum software we do have here

                          Comment


                            #14
                            .
                            Last edited by nomoresonys; 02-17-2025, 02:55 PM.

                            Comment


                              #15
                              Check FP801S, CP810, CP813, QP801CS, ICP801, RP813.

                              Comment


                                #16
                                Originally posted by Diah View Post

                                you have much like line cutting off at the primary , test all printed tracking line from end to end and the parts installed on them
                                If only it were that easy lol. Guess I'll start looking.

                                Will do Sony. 801cs is good checked that

                                Comment


                                  #17
                                  As with so many reports, these are junk, lots of times the panel is bad and can mess up a good mainboard replacement. Guess they're not worth investing very much into.

                                  Comment


                                    #18
                                    Is there 3.3v here?
                                    Attached Files

                                    Comment


                                      #19
                                      Originally posted by lotas View Post
                                      Is there 3.3v here?
                                      No if you watch the video the voltage goes all over the place. Zero to 3 and up down constantly

                                      Comment

                                      Working...
                                      X