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    DELL P2210 - SMPS +5V reads ~+3.5V

    Hi,
    I've a DELL P2210 LCD that doesn't power on. Power led is off too. It seems completely dead.
    I replaced all the capacitors but no change. Original capacitors however seemed OK.
    Fuse is OK and the bridge rectifier seems OK too. When the power cord is plugged I read ~+3,5V instead of +5V on the connector towards the logic board. I'm stuck! Any suggestions?
    I've a DMM and a digital scope. What else may I check?
    Thanks!

    PS: Service manual is in attachment.
    Attached Files

    #2
    Re: DELL P2210 - SMPS +5V reads ~+3.5V

    Q890 is not turning on.

    When Q890 is off, the 5VU voltage is set by resistors R861 and R860.

    V = 2.5V x (R861 + R860) / R860 = 3.5V

    When Q890 is on, R890 shunts R860 and the supply voltage then becomes 5.27V.

    Comment


      #3
      Re: DELL P2210 - SMPS +5V reads ~+3.5V

      did you replace any small caps in the psu, or just the output ones?

      Comment


        #4
        Re: DELL P2210 - SMPS +5V reads ~+3.5V

        Originally posted by fzabkar View Post
        Q890 is not turning on.

        When Q890 is off, the 5VU voltage is set by resistors R861 and R860.

        V = 2.5V x (R861 + R860) / R860 = 3.5V

        When Q890 is on, R890 shunts R860 and the supply voltage then becomes 5.27V.
        I'm afraid the problem is before Q890 because I tested voltage output from the transformer T850-01 (T850 is not present on my board) and I didn't found any voltage.
        I measured +16V output on R888 resistor, +12V output on R877 and everything reads 0V.
        If the problem is Q890 I would have +16V and +12V output, am I wrong?
        Could it be Q850 that failed?
        If Q850 fails I would still have an output of +3,5V?
        Thanks for your answers!

        @stj: On the board remains only two other little electrolytics that I didn't change C46 (4,7uF) and C878 (47uF 50V).

        Comment


          #5
          Re: DELL P2210 - SMPS +5V reads ~+3.5V

          C878 is part of a regenerated supply. It commonly causes startup problems in TV/monitor PSUs. That would be my first choice when shotgunning the PSU. I can't see what C46 does, though.

          That said, if 3.5V is present on 5VU, then the chopper circuit on the primary side of T850 must be working. I'd be looking for a problem in the 16V secondary. The 12V output requires that the 16V output comes up first, and it is then switched on by the Audio_On/Off signal from the logic. I'd be measuring the voltages at C865/C862 and C884 (470uF).

          Q890 is also switched on by a different signal from the logic, so I doubt that it is faulty. I suspect that 3.5V may represent a power saving mode.

          Comment


            #6
            Re: DELL P2210 - SMPS +5V reads ~+3.5V

            Originally posted by fzabkar View Post
            C878 is part of a regenerated supply. It commonly causes startup problems in TV/monitor PSUs. That would be my first choice when shotgunning the PSU. I can't see what C46 does, though.
            I finally changed both but nothing changes.

            Originally posted by fzabkar View Post
            That said, if 3.5V is present on 5VU, then the chopper circuit on the primary side of T850 must be working. I'd be looking for a problem in the 16V secondary. The 12V output requires that the 16V output comes up first, and it is then switched on by the Audio_On/Off signal from the logic.
            I'm not sure about it. It seems that they follow separates paths with different output from T850.

            Originally posted by fzabkar View Post
            I'd be measuring the voltages at C865/C862 and C884 (470uF).
            C865/C862: 0V
            C884: ~+4.3V (interesting!)

            Originally posted by fzabkar View Post
            Q890 is also switched on by a different signal from the logic, so I doubt that it is faulty. I suspect that 3.5V may represent a power saving mode.
            I don't think it is a power saving mode because the service manual clearly states at page 17 that "When power cord plug into AC socket, Power provides 24V and DC_5V. DC_5V is main voltage for panel and Regulator U101." so I think that I would always have a 5VU to the logic board.

            Comment


              #7
              Re: DELL P2210 - SMPS +5V reads ~+3.5V

              The 5V/12V/16V supplies all share a common winding. That said, I would have expected that the 16V supply would have been sitting at around 11V (= 16 x 3.5/5.0).

              The circuit diagram for the E190SF monitor is similar in some respects to the P2210. Pages 10 and 55 show that the mystery switching signal for Q890 is "P-saving_on/off". The circuit diagrams are too blurry to read, though.

              E190SF service manual:


              P2210 service manual:


              EDIT: Have you checked the load resistances on the 12V and 16V rails?
              Last edited by fzabkar; 09-02-2015, 03:18 PM.

              Comment


                #8
                Re: DELL P2210 - SMPS +5V reads ~+3.5V

                I think I found it. D854 is shorted. I desoldered one of the lead and +16V and +12V are there! However I still didn't tested the display because I don't have a replacement for the diode and I'm afraid that working with only D855 could cause more damage.

                I will post final test result as soon as I receive a replacement for D854.

                One strange thing is that R853 that is marked as 0.2 Ohm reads only 0.1 Ohm. I wonder if it is OK or if I must replace it too.

                @fzabkar: Thank you very much for your precious help! Without it I probably would haven't find the problem.

                Comment


                  #9
                  Re: DELL P2210 - SMPS +5V reads ~+3.5V

                  I would replace D854 and D855 as a matched pair. If you use diodes from different batches, then one may work harder than the other. That is, the current may not be shared evenly. (IMHO connecting diodes in parallel is bad practice.)

                  As for R853, it would be unusual for a resistor to fail with a lower resistance. They usually go open. I don't understand what is going on there.

                  Comment


                    #10
                    Re: DELL P2210 - SMPS +5V reads ~+3.5V

                    "One strange thing is that R853 that is marked as 0.2 Ohm reads only 0.1 Ohm. I wonder if it is OK or if I must replace it too." What resistance does your meter show when you short the probe tips together? >0.1 Ohms?
                    Never stop learning
                    Basic LCD TV and Monitor troubleshooting guides.
                    http://www.badcaps.net/forum/showthr...956#post305956

                    Voltage Regulator (LDO) testing:
                    http://www.badcaps.net/forum/showthr...999#post300999

                    Inverter testing using old CFL:
                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                    http://s807.photobucket.com/user/budm/library/

                    TV Factory reset codes listing:
                    http://www.badcaps.net/forum/showthread.php?t=24809

                    Comment


                      #11
                      Re: DELL P2210 - SMPS +5V reads ~+3.5V

                      Originally posted by fzabkar View Post
                      I would replace D854 and D855 as a matched pair. If you use diodes from different batches, then one may work harder than the other. That is, the current may not be shared evenly. (IMHO connecting diodes in parallel is bad practice.)
                      I agree.
                      But how avoid diodes in parallel? In my case, could it be a solution and a better design to replace 2 Shottkys of 20V 5A with only one of 20V 10A?

                      Originally posted by budm
                      "One strange thing is that R853 that is marked as 0.2 Ohm reads only 0.1 Ohm. I wonder if it is OK or if I must replace it too." What resistance does your meter show when you short the probe tips together? >0.1 Ohms?
                      Shorted probes reads 0,6 Ohms. So I think R853 measure is correct, am I wrong?
                      Last edited by o3365566; 09-04-2015, 03:15 AM.

                      Comment


                        #12
                        Re: DELL P2210 - SMPS +5V reads ~+3.5V

                        If the resistance of probe leads show 0.6 Ohms, then the resistance reading of the resistor cannot be less than the probe leads resistance, the minimum reading should be = to 0.6 Ohms + the resistor resistance value. It has to be measurement error.
                        Never stop learning
                        Basic LCD TV and Monitor troubleshooting guides.
                        http://www.badcaps.net/forum/showthr...956#post305956

                        Voltage Regulator (LDO) testing:
                        http://www.badcaps.net/forum/showthr...999#post300999

                        Inverter testing using old CFL:
                        http://www.badcaps.net/forum/showthr...er+testing+cfl

                        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                        http://s807.photobucket.com/user/budm/library/

                        TV Factory reset codes listing:
                        http://www.badcaps.net/forum/showthread.php?t=24809

                        Comment


                          #13
                          Re: DELL P2210 - SMPS +5V reads ~+3.5V

                          Some meters have a function that automatically deducts the probe resistance from component measurements. Perhaps the OP is using one of these.

                          As for the parallel diodes, I have a Sansui amp in which I replaced a pair of 1A recifiers with a single 3A rectifier (one of the 1A diodes was shorted). I also had a case similar to the OP's in a Toshiba LCD TV where one of a pair of 5A Schottky rectifiers was shorted. I debated whether to replace them with a single 10A diode but decided that, since the TV was not mine to do with as I pleased, it would be better to stick with the original design.

                          Comment


                            #14
                            Re: DELL P2210 - SMPS +5V reads ~+3.5V

                            Originally posted by budm View Post
                            If the resistance of probe leads show 0.6 Ohms, then the resistance reading of the resistor cannot be less than the probe leads resistance, the minimum reading should be = to 0.6 Ohms + the resistor resistance value. It has to be measurement error.
                            Originally posted by fzabkar View Post
                            Some meters have a function that automatically deducts the probe resistance from component measurements. Perhaps the OP is using one of these.
                            Any way to check this?

                            Originally posted by fzabkar View Post
                            As for the parallel diodes, I have a Sansui amp in which I replaced a pair of 1A recifiers with a single 3A rectifier (one of the 1A diodes was shorted). I also had a case similar to the OP's in a Toshiba LCD TV where one of a pair of 5A Schottky rectifiers was shorted. I debated whether to replace them with a single 10A diode but decided that, since the TV was not mine to do with as I pleased, it would be better to stick with the original design.
                            And what about voltages? In the SR520 family there are diodes with reverse voltages till 100V. I wonder if it would be better to replace a 20V with a 60V or 100V. Normally the price is the same.
                            Are there other specifications to look for?

                            Comment


                              #15
                              Re: DELL P2210 - SMPS +5V reads ~+3.5V

                              I doubt that your meter has that automatic function to deduct the lead probe resistance, actually is called ZEROing for the probe leads resistance, for it to work you will have to short the probe leads together so the meter will know what the resistance of the probes are. the meter will have ZERO function so when you touch the two probes together, hit the ZERO button so the meter will now show 000 Ohms on the display.
                              I doubt that the resistor is bad, did you remove the resistor and test it off the board, if the caps are not completely discharge, any left over Voltage can cause error in resistance reading.

                              http://www.fluke.com/fluke/m2en/supp...imeter-FAQ.htm

                              Q: When I short my test leads together in the resistance mode, it does not read zero. Why not?
                              A: The meter is measuring the resistance of the test leads, which for a new set of test leads will typically be in the 0.1 to 0.3 ohm range. The test lead error needs to be subtracted from the resistance readings. Some of the more feature rich meters will have a Relative delta (use the actual delta symbol in the final article) mode pushbutton that will subtract the lead error for you.


                              Most meter for home use does not have that function. What is the make and model of your meter?
                              Last edited by budm; 09-04-2015, 04:29 PM.
                              Never stop learning
                              Basic LCD TV and Monitor troubleshooting guides.
                              http://www.badcaps.net/forum/showthr...956#post305956

                              Voltage Regulator (LDO) testing:
                              http://www.badcaps.net/forum/showthr...999#post300999

                              Inverter testing using old CFL:
                              http://www.badcaps.net/forum/showthr...er+testing+cfl

                              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                              http://s807.photobucket.com/user/budm/library/

                              TV Factory reset codes listing:
                              http://www.badcaps.net/forum/showthread.php?t=24809

                              Comment


                                #16
                                Re: DELL P2210 - SMPS +5V reads ~+3.5V

                                The "probe resistance" function is something that would have to be done manually by pressing a certain button, so if you don't recall doing it, then your meter must not be one of these.

                                Here is one such meter:
                                http://lygte-info.dk/review/Review%2...T61E%20UK.html

                                "Pressing this switch will store the current reading and show all further readings relative to the stored value. The most obvious place to use this function is for ohms: Short the probes and press the REL switch, the meter will now subtract the probe resistance from all further readings."

                                As for diode voltages, I'd go for the higher one. I doubt that the original diode would be a 20V part, though. The margin would be too low. Notice that the diode on the 5V rail is rated for 100V. My other preferred specs would be high speed and low forward voltage drop.

                                Edit: I just noticed "budm" had already answered your question while I was typing mine. Sorry.
                                Last edited by fzabkar; 09-04-2015, 04:30 PM.

                                Comment


                                  #17
                                  Re: DELL P2210 - SMPS +5V reads ~+3.5V

                                  Thanks for the answers. My meter (a Finest 201) definitely doesn't have zeroing but as budm said. I don't think R853 failed.

                                  The diode to replace is marked SR520 and datasheet says it's a 20V. Anyway I ordered a 200V one for replacement to be sure...

                                  And now some news. A colleague gave me another DELL P2210 which doesn't power on. I opened it and surprise: the power board it's completely different!
                                  Daughter-boards are replaced by smd components and schematics doesn't apply anymore. I started investigating and I have ~+2,65V instead of +5VU.
                                  I checked the diodes and the bridge rectifier and everything seems ok. Recapped the board and nothing changed.
                                  The strange behavior I noticed is that a discrete rectifier diode (equivalent to D854/D855 in the schematics) is shorted when measured in-circuit and is OK out-of-circuit.

                                  So I think there is something else that short the path when in-circuit and I'd like to isolate the diode to check if I have correct voltage drop on it (I think it would be something like +16V). May I connect the lifted cathode of the diode directly to the GND to measure it?

                                  Thanks in advance!

                                  Comment


                                    #18
                                    Re: DELL P2210 - SMPS +5V reads ~+3.5V

                                    This time the short appears to be in the load. The circuit is being completed by the transformer secondary.

                                    Comment


                                      #19
                                      Re: DELL P2210 - SMPS +5V reads ~+3.5V

                                      Originally posted by fzabkar View Post
                                      This time the short appears to be in the load. The circuit is being completed by the transformer secondary.
                                      I think too.
                                      I tried to isolate the inverter and correct voltages were back but I grilled a resistor that started to smoke...

                                      Would it be a good idea to desolder some pins on the inverter transformer to find the shorted component?
                                      Any other suggestion for troubleshooting this kind of problem?

                                      Comment


                                        #20
                                        Re: DELL P2210 - SMPS +5V reads ~+3.5V

                                        ISTM that the dual MOSFET at U2 (P6006HVG) could be shorted. I would resolder all the pins of transformer T1 and check for shorts on the HV side.

                                        Comment

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