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    testing an LCD macbook A1286

    Hello

    Simple question, I have a 2915b a1286 macbook pro 2011 that is showing image but no backlight, sorry this stupid question but, in order to check if led bklight is working on the screen can inject 50v to pins 39/40 and see it illuminated? how do I turn on backlight just for testing purposes if all circuits were in good condition no shorts, thnks in advance
    Last edited by marce002; 05-21-2022, 07:06 AM.
    Marcelo J

    #2
    Re: testing an LCD macbook A1286

    Never tested it this way before but can you check the usual?

    Vin to the backlight IC.

    Is backlight IC being enabled?

    Fuse is ok?

    Otherwise, the backlight is a string of LEDs but we need to watch the current else will kill the backlight panel.

    Best to try to fix the root cause on the logic board.

    Comment


      #3
      Re: testing an LCD macbook A1286

      Hello mon ... this is a non client testing device... it has no backlight driver chip 8550 at all... pads vroken so no way to operate with bck driver but image is there... just wanted to iluminate the display from external power to pins 38 39 40 with 50v ... did not ligth up though... maybe those LED_RETURN_x lines need to be active too? not really. as they deal with ic and ic not present but those lines are in the lvds connector too
      https://imguh.com/image/t3Ugz
      Last edited by marce002; 05-21-2022, 08:05 AM.
      Marcelo J

      Comment


        #4
        Re: testing an LCD macbook A1286

        Yes, the return lines are required to be connected and each such return line = one strip of the backlight inside the panel.

        Here is my 2 bit suggestion - review the 8550 datasheet and note the current it can supply. Current limit your power supply to a similar value or the lowest setting to start.

        Then select 25v-35v on the power supply - again current limited for the output.

        Then proceed as you have noted. You do not want too much current otherwise you will overdrive the backlight strip and burn it out.

        Increase the current slowly if you do not see the backlight - effectively increasing the intensity.

        Post your results with pics but use the forum's attachment feature so this thread can be viewed in the future. It is a cool experiment and myself and others will want to see your results.

        Comment


          #5
          Re: testing an LCD macbook A1286

          Mon it might need to deal with PWM Brightness Control Proprietary Phase Shift to make it work?
          Marcelo J

          Comment


            #6
            Re: testing an LCD macbook A1286

            Perhaps but in the end - the backlight is only a back to back string of leds. If you proceed with your idea, it will work but just do not over drive the strip. PWM will allow for the led to be on with a duty cycle so that they are not always working hard to keep the light on. For simple testing, you will be fine to use the lowest current and dial it up till the strip turns on.

            Comment


              #7
              Re: testing an LCD macbook A1286

              thks mon .. to be clear my power supply at 30v around 400mah starting injecting in pins 39 40 and what to inject in the other 6 pins? 2 bit solution you meant 2 of them with identical 30v?
              Marcelo J

              Comment


                #8
                Re: testing an LCD macbook A1286

                See attached.

                Suggest to start @ 25V and 250mA on your DC power supply.

                Test with only a single return line from the LCD display = GROUND it on your power supply.

                As per attached, each return line is the CATHODE (-ve) leg of the LED string.

                Then increase the current to increase the intensity but stay under the same range as the LP8550 backlight IC.

                For this test, you should only require 2 leads - for now, do not worry about the actual LVDS signals. Just want to see if the backlight can turn on which it should.

                Once one leg is confirmed to be working, move to a different return line = will light up another part of the backlight and so on...
                Attached Files

                Comment


                  #9
                  Re: testing an LCD macbook A1286

                  ok i will test and report back please remember no IC driver chip is present in the board because tracks are damaged. ok?
                  Marcelo J

                  Comment


                    #10
                    Re: testing an LCD macbook A1286

                    I mean no ic U9701 at all...cant try ???
                    have only its pads (some are bad but one is GND redundant and other is NC ...so is ok) ... If I solder pad corresponding to "led return #1" which will be my ground power supply? then the + to the pin #39 40?
                    Last edited by marce002; 05-21-2022, 01:00 PM.
                    Marcelo J

                    Comment


                      #11
                      Re: testing an LCD macbook A1286

                      Correct. This entire test is based on the missing backlight IC.

                      You will be using your external DC power supply and performing the job of the missing backlight IC. The difference is that your power supply will be considered to be 100% duty cycle = always full on. Where the backlight would have an adjustable PWM = variable intensity using the keyboard controls.

                      You can solder a wire to the backlight pin on the LVDS connector on the logic board = + for your DC power supply.

                      Then for the first return on the LVDS connector = - for your DC power supply.

                      This is much like a light bulb but has a polarity since they are LEDs.

                      Comment


                        #12
                        Re: testing an LCD macbook A1286

                        Nothing... maybe pwm "not being seen"... used 25v and up to 800mah (working screen off course) and nothing at all... 0.0000mah... used pin 25 (in the 8550) and all you ve stated...any hint?
                        Marcelo J

                        Comment


                          #13
                          Re: testing an LCD macbook A1286

                          With 0mA current draw, the circuit is not complete.

                          On the same note, do not dial up the current so high else you will permanently kill the backlight strip. Stay within the range of the LP8550 datasheet since we know that works in the real world for this display.

                          The display is known good for the LCD graphics but also known good for the backlight ?

                          See attached and test again. It should work - personally have never tested it but it should function. You may have to increase the power supply voltage but if you do, again, keep the current down to not overshoot.

                          PS: The schematic / pin #s for the LCD connector were taken from the 820-2915-B schematic. Please confirm the same is present for the schematic you are referencing before proceeding.
                          Attached Files

                          Comment


                            #14
                            Re: testing an LCD macbook A1286

                            ok ill try that way volt up and currwnt constant........ yes 2915b and of course yes it is a good screen (both backlight and image) i will report back ... thks
                            Marcelo J

                            Comment


                              #15
                              Re: testing an LCD macbook A1286

                              Yes!! worked at 29v and just 2mah was enough to see a nice white screen!!

                              thnks i will try more lines tomorrow... thks mon





                              Attached Files
                              Marcelo J

                              Comment


                                #16
                                Re: testing an LCD macbook A1286

                                Mon , now that backlight it is working this way, where would be posible to grab 30v in the entire macbook 2915b board, so I can leave my external power supply and make it shine by itself (ideal if ON when corresponding signal enabled like real driver)...? any hint where to get from or join two independant sources? or maybe adapt a step up? what would you do
                                Last edited by marce002; 05-22-2022, 08:23 AM.
                                Marcelo J

                                Comment


                                  #17
                                  Re: testing an LCD macbook A1286

                                  This is a typical backlight circuit where Apple / Foxconn decided to use the LP8550 IC for this purpose. Ideally you would want to use the same again but if your logic board is in bad shape then you will have to consider an external circuit board to act in the same manner.

                                  Can you post pix of where the backlight IC and circuit is to review the condition of the repair ?

                                  A headache will be how to squeeze in a small PCB for the backlight inside the casing for this laptop.

                                  The laptop on its own does not produce the higher (boosted) voltages. That is the sole function of the LP8550 device.

                                  We design such products every day but not sure of the need. Essentially, another PCB with the parts, including the LP8550 would be required. Then this PCB will need to be wired carefully back onto the logic board with the Vin + I2C / SMBUS + return line pads. Only then, the logic board will be fooled into thinking it is chatting with the original LP8550 and not your external PCB. This way, the keyboard intensity functions, etc. will continue to function.

                                  Are you trying to replicate the original backlight circuit or just want it to work, even if the backlight board is outside the case ?

                                  Comment


                                    #18
                                    Re: testing an LCD macbook A1286

                                    Thnks Mon yes it is so bad board that is not worth it, maybe will adapt an step up fixed to 30v just because it is a nice machine, with all pins led response grounded, and use like this.... very clear all your responses to thks again!! case closed haha
                                    Marcelo J

                                    Comment


                                      #19
                                      Re: testing an LCD macbook A1286

                                      Something like this should work - agree that if you can salvage the unit - you should:

                                      https://www.aliexpress.com/item/1005...80102209%21sea

                                      Comment


                                        #20
                                        Re: testing an LCD macbook A1286

                                        yes Mon thaks ended up buying this one and worked fine

                                        Regulador De Voltaje Step Up Xl6009 Dc Dc 5v-35v

                                        https://articulo.mercadolibre.com.ar...-dc-5v-35v-_JM
                                        Marcelo J

                                        Comment

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