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DELL P2417H no BL no image

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    DELL P2417H no BL no image

    Hi,
    I have this monitor that turns on but doesn't have BL and it seems no image too.

    Isolating the PSU, I get voltages on the mainboard connector (red+orange is ground):
    Green+yellow: 18.9V
    Black+brown: 0V
    The BL connector gets 18.7V

    I checked the caps visually and with an ESR meter and seem fine.

    Also the panel is ok because I hooked it up to a spare PSU and mainboard from another 24' monitor and works well.

    If it helps D701 is MBRF10200CT. I am looking at a faulty transformer here?
    Attached Files

    #2
    Re: DELL P2417H no BL no image

    If the transformer was bad you would not have 18.7 volts, It could be that the main board is not sending the BL on signal, Since there is 18.7 volts being supplied to the main board, I suspect there are other regulator ic's on the main board, I would check their voltages.
    Looking at main board pictures, there are a couple fuses, TH121, TH751 & Th752 on the main board
    Last edited by R_J; 09-26-2021, 07:04 PM.

    Comment


      #3
      Re: DELL P2417H no BL no image

      I am uploading a picture of the mainboard.
      The pins of black and grey are connected directly on the main chip U1

      Update: I cleaned the mainboard with alcohol and resoldered the chip and I have 3V no in black and grey BL_ON. But the backlights still won't go on. I checked again with a flashlight and can see image now, but to be honest I can't tell 100% in the beginning if it already had one.
      Anyway, at least now we know it's the PSU?

      I am uploading another image closer to the circuit
      Attached Files
      Last edited by Sraros; 09-27-2021, 03:14 PM.

      Comment


        #4
        Re: DELL P2417H no BL no image

        As I am still learning on these repairs, I am just wondering.. if I have the right voltage to the LED bar 18.7V and the monitor is operating well with the other boards which they too supply 19V to the LED bar.. Then is it a matter of Amps then not reaching the right amount?
        Can I measure just by changing the port of the MM and in parallel between ground and hot?

        And another thing, which I don't know if it has anything to do with it at all:
        the legs of the transformer on the secondary circuit go back and forth between 0 and a few Volts (2-3V) but on the diode the output is steady 19V. Same on the primary side on one of the legs jumps like crazy the voltage between 0 and 70V

        Comment


          #5
          Re: DELL P2417H no BL no image

          Originally posted by Sraros View Post
          And another thing, which I don’t know if it has anything to do with it at all:
          the legs of the transformer on the secondary circuit go back and forth between 0 and a few Volts (2-3V) but on the diode the output is steady 19V. Same on the primary side on one of the legs jumps like crazy the voltage between 0 and 70V
          Okay, DO NOT measure voltages directly on ANY transformer pins while the PSU is operating - especially on the high-voltage/primary side. This is a good way to damage your multimeter and possibly damage the PSU as well.

          The fact that you are getting 19V DC means the PSU is working.

          The backlights take 19V from the PSU, but it doesn't go straight into the LED strips. Rather, MOSFET Q801, inductor L801, and diode D801 form a boost circuit that then boosts the 19V DC to a higher voltage needed to drive the LED strips. Actually, the voltage of this boost circuit is not exact. Rather, the boost circuit is used to control how much current goes into the LED strips.

          That being said, if you have verified that the LED backlight works with the boards from another monitor, then the issue is likely somewhere in that boost circuit on the PSU board. More than likely, Q801 is NOT bad, because if it was, it would sink down the 19V DC rail, and that would make the monitor appear to have no power at all... which is not the case here. However, do check diode D801 for short circuit, and also check all of the other small SMD components. In particular, see if any of the small SMD resistors in the circuit read 10% higher value than what is printed on their body. And for SMD ceramic capacitors, see if any show short-circuit or very-low resistance in that circuit. Again, this is for the components of the circuit around IC801. This is your backlight driver IC... i.e. the one you see in this picture:
          https://www.badcaps.net/forum/attach...1&d=1632777272

          Comment


            #6
            Re: DELL P2417H no BL no image

            I didn’t know this for the transformers! I will never forget now...

            Ok so I checked all the resistors and caps on this area around IC801, Q801 and all seem in order.
            Q801 is PA410BD Vgs(th) is 1.3V min and I have on the gate 0.03V and at source 0V.
            Diode D801 tested on diode mode with PSU off ok and PSU ON forward voltage drop 0.5V
            The voltages on the pins of the IC801 that lead to G-S pins on Q801 are the same.

            Btw I found another PSU board also from a non working DELL monitor 22’, almost identical to the 24’ and has exact same problem.

            I had a general idea how this part of the board works, for the BL but your explanation was great, so thanks!
            Last edited by Sraros; 09-30-2021, 03:43 PM.

            Comment


              #7
              Re: DELL P2417H no BL no image

              No problems!

              Keep us posted what you find. Given that you found another PSU board with the same issue, perhaps there is a common problem with these boards?

              Comment


                #8
                Re: DELL P2417H no BL no image

                I don’t know what else to look for.. I checked again everything and can’t find sth faulty, so maybe it’s the IC itself? I measured the pins of the ic801 and they go:
                V1(hole): 0.02 V2: 3.24 V3: 0.25 V4: 0 V5: 0.06 V6: 0 V7: 2.4 V8: 0.25
                V9: 0.22 V10: 0.25 V11: 19.02 V12: 0.3 V13: 0 V14: 0.02 V15: 11.4 V16:0.25

                But unfortunately I couldn’t find a datasheet for MP3375 which is IC801

                I found another datasheet from MP3398 though which looks quite similar

                https://www.monolithicpower.com/en/d...n/sku/MP3398A/
                Last edited by Sraros; 10-10-2021, 05:20 PM.

                Comment


                  #9
                  Re: DELL P2417H no BL no image

                  I have ordered the ICs to change them in both PSUs. I will post when I replace them

                  Comment


                    #10
                    Re: DELL P2417H no BL no image

                    The local repair shop had 8 f these but the issue was vertical and on some cases horizontal lines on right lower 2/3 of the screen. He asked me to search online but I couldn't find any solution.
                    Last edited by raptor22; 10-13-2021, 09:29 PM.

                    Comment


                      #11
                      Re: DELL P2417H no BL no image

                      SOLVED!
                      I replaced the backlight IC MP3375 on one of the two faulty power boards and now the monitor works just fine
                      So thanks everybody for the advices and guidance!
                      I will replace tomorrow on the 2nd PSU the same IC to see if there is a pattern here with these boards.

                      Comment


                        #12
                        Re: DELL P2417H no BL no image

                        replaced the IC on the 2nd board but nothing. I changed nit twice in case one was faulty but same result.
                        EN pin has 3,22V which means it turns on.
                        Vin 19V, Vcc 6,2V but the gate PIN doesn’t output voltage.
                        The only “abnormal” is PWM pin is 6,18V but then the datasheet is also from another chip so I dont know if this has anything to do with it.

                        Comment


                          #13
                          Re: DELL P2417H no BL no image

                          Originally posted by Sraros View Post
                          SOLVED!
                          I replaced the backlight IC MP3375 on one of the two faulty power boards and now the monitor works just fine
                          Hey, great to hear you got at least one of the monitors working again.

                          But as for the 2nd one that doesn't want to work after the IC swap... did you try swapping its board for the one from the now-working 1st monitor? If no, try that first, just to rule out the backlight LEDs are OK or that there isn't something else wrong, like perhaps driving signal from the logic board. If that checks out all good, then perhaps there are bad components around the IC that you maybe missed? Or maybe the replacement IC was bad? (If it's from AliExpress or some other China market, I wouldn't be too surprised if it's a recycled part or possibly a low-quality knock-off... or legit part, but salvaged for cheap from a reject bin - hence, a small chance some ICs not working.)
                          Also, double-check your soldering. I've missed soldering a pin on an IC like that before and kept wondering what the issue was until I looked at it more carefully.

                          Comment


                            #14
                            Re: DELL P2417H no BL no image

                            I ordered a batch of 5 from Aliexpress. 1 went to the working board.
                            2 of them to the board that still doesn’t power on.
                            Hooked to the same panel so LEDs are working fine.
                            No bad resistors nearby. Values are the same as working one.
                            Soldering is also fine. When sth doesn’t work as I supposed so, I check with a cheap USB microscope and try also continuity check.

                            I will give another go with the rest two chips.
                            But what still gets me thinking is this with PWM. The datasheet says:
                            “If PWM dimming is not required, pulling this pin to high voltage(1.25V<Vpwm<5V)”
                            So if it is required then must be less than 1.25V
                            Mine is at 6.18V so in any case looks wrong.
                            Then last thing yoh said about driving signal. Is this sth I must have an oscilloscope to measure? Or you mean the voltage coming from the main to the chip. Because I do see after a couple of minutes the chip going off. The board stops sending the 3.22V to the chip into EN pin and then it shuts off.

                            Comment


                              #15
                              Re: DELL P2417H no BL no image

                              Originally posted by Sraros View Post
                              But what still gets me thinking is this with PWM. The datasheet says:
                              “If PWM dimming is not required, pulling this pin to high voltage(1.25V<Vpwm<5V)”
                              So if it is required then must be less than 1.25V
                              Mine is at 6.18V so in any case looks wrong.
                              Well, it appears that pin 3 (PWM) of the IC is not connected to anything... so it will likely have a floating voltage on it. Since your Vcc = 6.2V and pin 3 is floating, it's probably normal that it's at 6.18V. Of course, if you want, you can just inject 5V into that pin through a 1-KOhm resistor, just to be compliant with the datasheet.

                              And no, PWM dimming is likely not used here, since you can see that pin 2 from connector CN701 goes to pin 7 (ADIM) of the MP3375 IC. That means IC801 should be getting an analog dimming signal from the main/logic board. According to the MP3398 datasheet, though, the analog dimming voltage should be somewhere between 0.4V and 1.5V for backlight brightness of 0 to 100%. So just check what DC voltage you get on this pin. If you have an oscilloscope, you can also check if the voltage is PWM or not, since MP3398 datasheet says you can also used this pin for PWM backlight dimming (which doesn't make much sense, since it seems pin 3 can be used for the same function... but whatever.) On that note, check that resistor R818 (looks like "01b"... so probably a 1-KOhm) is not open and that the resistor next to it (R819?? looks like "332", so 3.3-KOhm) is also not open.

                              Another thing I noticed is that the area around resistor R801 ("01A" - so 100 Ohms) looks a little darkened. Make sure this resistor is not open or high resistance either.

                              While at it, check R811 (270 Ohms) - it should measure at or very close to 270 Ohms. If it measures lower, ceramic cap C812 may be bad. If in doubt, remove and check C812.

                              Same goes for R812 (looks like 240 KOhm) - this is the resistor that sets the oscillating frequency of the IC. If R812 is bad, IC will appear dead and not work.

                              Next, check R814 and the ceramic cap next to it (C803?). Like the above, the value of R814 should be pretty spot-on to what is printed on the resistor. If it isn't, remove it and check it out of circuit. If it checks out OK, check the ceramic cap (C803?) out of circuit as well. R814 sets the current the backlight should use, so if it is bad, the current may become set too low and not allow the backlight to work.

                              Also check resistors R821 and R822 - these set the over-voltage protection of the LED driver circuit.

                              Finally, resistors R803 and R804 - these both appear to be 0.3-Ohm resistors and they are in parallel. So unfortunately, you may not be able to measure them accurately in circuit. But if they have gone even slightly high on resistance, then IC801 will think the MOSFET is drawing too much current and stop the backlight. So it may be just a good idea to replace them if you can. On that note, make sure R805 ("01b") is not open. This resistor reads the voltage across R803 and R804

                              And that's all I can think of to check. If all of this checks out fine, it may be just time to "shotgun"-swap parts. I personally dislike doing this. But sometimes, it may be the quicker way to get things working.

                              Comment


                                #16
                                Re: DELL P2417H no BL no image

                                I found an identical monitor with bad panel so I will cross check 1 by 1 the PSU around the BL chip and return with findings

                                Comment

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