Announcement

Collapse
No announcement yet.

Diagnose PS BN44-0467A

Collapse
X
Collapse
 
  • Page of 3
  • Filter
  • Time
  • Show
Clear All
new posts
Previous 1 2 3 template Next
    #1

    Diagnose PS BN44-0467A

    Hi everyone!
    I have a small 19" TV set that started flashing the front led while also dimly flashing the BL, like trying to start but it was power cycling

    I opened it and found nothing clearly broken or visibly burnt, no visible bad caps.

    I removed the PS to MAIN harness and PS to BL plug plug and nothing changed (these are the two only wires getting out of the PS). So my guess is that the problem MUST be in the PS board

    I checked DC at the main capacitor and found a correct rectified main voltage

    I went to the primary section in the main transformer and found no AC voltage on inputs, just "leftovers" of 0-5v at times

    This kind of leads me to believe that the issue must be in the swtching section that drives the primay, right?

    I can't spend much money in such a small / cheap TV set by just going ahead and changing the chopper IC (GM0765R), transistors, diodes and capacitors, so I would truly appreciate some comments from experience on what to test first

    THANKS
    Attached Files
    Tags: None
    #2
    Re: Diagnose PS BN44-0467A

    Show a photo of the reverse side.

    Comment

      #3
      Re: Diagnose PS BN44-0467A

      Originally posted by kasfamily View Post
      Show a photo of the reverse side.
      See attached.
      Thanks
      Attached Files

      Comment

        #4
        Re: Diagnose PS BN44-0467A

        Using HOT ground (- of CP801s) check the voltage on pin 1, 3, 4 & 6. Post the results.

        I would also resolder the connections on ICM801S and the transformer if they look cracked, and replace CS812 vcc capacitor
        Last edited by R_J; 11-13-2022, 12:08 PM.

        Comment

          #5
          Re: Diagnose PS BN44-0467A

          Here it is:
          1: 321v
          3: Oh wait I just blew the fuse while touching lead 1&2... Will get this tomorrow
          4:
          6:

          Comment

            #6
            Re: Diagnose PS BN44-0467A

            Originally posted by R_J View Post
            Using HOT ground (- of CP801s) check the voltage on pin 1, 3, 4 & 6. Post the results.

            I would also resolder the connections on ICM801S and the transformer if they look cracked, and replace CS812 vcc capacitor
            Ok last update on this
            I ended up ordering a new chopper IC (GM0765R) that took all this time to get home. Replaced it today and got the same effect

            I redid all the transformer solder points and again the same

            Replaced CS812 with a test one (larger) and got the same effect (longer time between flashings as this test capacitor was bigger in uF). I put the original back in

            Measured voltages on IC
            1: 320V (Im in Spain, 220v AC mains)
            3: 8-12v fluctuating
            4: 150-250 mV fluctuating
            6: 230-300 V fluctuating

            Im starting to be a little lost...

            Thanks!

            Comment

              #7
              Re: Diagnose PS BN44-0467A

              It looks like the ic is trying to start but the run vcc is not there, Chcek that RS821 and DS812 are good.
              Check if you are getting any voltage in the secondary side, like CM864 or CM8882, I would also check all the diodes in the secondary to be sure none are shorted

              Comment

                #8
                Re: Diagnose PS BN44-0467A

                Agree, could be a short on the secondary side.
                Need to get A5V running first, this should be standby voltage.
                Also CM856 in the picture looks suspicious.

                Comment

                  #9
                  Re: Diagnose PS BN44-0467A

                  This is an always on power supply, there is no separate standby voltage. The power supply operates in a low power burst mode in standby.

                  Comment

                    #10
                    Re: Diagnose PS BN44-0467A

                    Originally posted by R_J View Post
                    It looks like the ic is trying to start but the run vcc is not there, Chcek that RS821 and DS812 are good.
                    RS821 should be a jumper (0ohm) and that's what it measures
                    DS812 gives 0,544v forward, infinite backwards (seems good to me)

                    Originally posted by R_J View Post
                    Check if you are getting any voltage in the secondary side, like CM864 or CM8882, I would also check all the diodes in the secondary to be sure none are shorted
                    I checked all diodes and still while on the board none shows shorted

                    CM864: steady 5.2v
                    CM882: LO v then pulses of >5v (too short to measure)

                    QM851 shows only steady 27 Ohm (while on the board) between pins 1&3

                    Ok....DM887 was shorted, in and out of the board. I would swear I had chcked all the semiconductors for shorts initially, but this one is bad

                    Just by removing it from the board until I find a replacement recovers steady power on some of the capacitor that previously were at 0v or pulsing so this is promising.

                    Going to find a quick replacement for it in a second in my drawer and will get back to you guys

                    Thanks

                    Comment

                      #11
                      Re: Diagnose PS BN44-0467A

                      Ok this changed a lot but still didn't fix it. TV shows BL on all the time but stdby led flashes several times and restarts the cycle. No real image on TV during all this. No signs of taking any input from remote too.
                      Will get voltages from PS later on today
                      Thanks!

                      Comment

                        #12
                        Re: Diagnose PS BN44-0467A

                        Ok I just checked again.

                        The TV actually turns on normally and works. But when you turn it off, the BL keeps ON (actually goes to full bright), and that is when I though the TV was on but unresponsive before.

                        All voltages look good

                        As you can see in the picture I swapped that diode that now sits (almost) vertically (taken from another board). Also that N-mosfet on top-left corner was having 30ohm between G&S but I unsoldered it and it doens't have that resistance while OFF the board. Neither the board shows that resistance when the mosfet is not on it...weird.

                        Anyway, what can be keeping the backlight ON all the time?

                        Thanks!
                        Attached Files

                        Comment

                          #13
                          Re: Diagnose PS BN44-0467A

                          Do the B5v and B13v turn off when the tv is turned off? also check the state of the P_ON pin I suspect the power supply board is not going into standby mode.
                          Check pin 6 B_ON pin on CNM803 This should be the backlight on signal from the main board, it should change when the tv is turned on or off.
                          Last edited by R_J; 11-27-2022, 01:15 PM.

                          Comment

                            #14
                            Re: Diagnose PS BN44-0467A

                            Originally posted by R_J View Post
                            Do the B5v and B13v turn off when the tv is turned off? also check the state of the P_ON pin I suspect the power supply board is not going into standby mode.
                            Check pin 6 B_ON pin on CNM803 This should be the backlight on signal from the main board, it should change when the tv is turned on or off.

                            Thanks for your support R_J. These are the voltages:
                            PIN TV_ON TV_OFF
                            B5V 5.2V 5.2V
                            B13V 12.5V 12V
                            P_ON 3.5V 0V
                            B_ON 5V 0V

                            What do you think?
                            Thanks

                            Comment

                              #15
                              Re: Diagnose PS BN44-0467A

                              The control signals from the main are correct so the problem is in the control of the power supply.
                              Check that QM851 is not shorted) QM851 is the B13v switch, The emitter has A13V and the collector supplies the B13v (when the B13v is on it also turns on QM852 which supplies the A5V). So you need to trace the base circuit of QM851 back to the P_ON signal. If you trace the line it goes to JP850? it will likely control a couple small transistors QM872 and eventually QM851
                              Last edited by R_J; 11-27-2022, 02:14 PM.

                              Comment

                                #16
                                Re: Diagnose PS BN44-0467A

                                By the way, are you following the specific schematic for this board? Because I haven't been able to find it. Only BN44-0468 which is ALIKE but not the same, works for some areas of the board but not all.
                                Thanks, I will trace that now

                                Comment

                                  #17
                                  Re: Diagnose PS BN44-0467A

                                  I am just using the pictures of the board. I see that the BN44-00468a is similar, it uses a pfc circuit which your board does not but the B5v and B13v outputs are likely similar in operation.

                                  Comment

                                    #18
                                    Re: Diagnose PS BN44-0467A

                                    Yes, that's the one I was using but I see differences, and not sure how the B_ON signal is driven to the BL IC
                                    Do both PS_ON and BL_ON need to be HIGH for the Backlight to be turned on? It's very hard to trace the signals on the board even with a magnifier or zooming in with my cell phone... There's sometimes black ink that hides some connections
                                    Anyway I've gone to all Q's one by one and I see no shortcuts between pins
                                    But I still see LOW Z between pins in QM851 (the large mosfet that drives B13V) but when I took it off the board (with no replacement, just empty holes) the BL was still doing the same if this helps

                                    Comment

                                      #19
                                      Re: Diagnose PS BN44-0467A

                                      I just removed again mosfet QM851 and it shows LOW Z in some pins (14 ohm in one pair and 5 ohm in other 2 pins)
                                      I turn TV on without this mosfet and the BL still powers up all the time. I get no image as the main board is not getting B13V but this mosfet, being shorted or not, seems not to be related to permanent Backlight

                                      Comment

                                        #20
                                        Re: Diagnose PS BN44-0467A

                                        LOW Z (impedance)? What is the resistance between the points you are talking about, ie: emitter to collector, emitter to base
                                        Without QM851 installed you should not have B5V
                                        If you are trying this without the main board connected you will have backlights due to pullup resistors in the circuit

                                        Comment

                                        Previous 1 2 3 template Next
                                        Working...
                                        X