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    #1

    lenovo 305-151BD display problem

    Hi all.
    Need assistance with no display on this lenovo model 305-151BD. M\B LA-B092P Rev3.0
    I can get display using external monitor but nothing on normal screen.
    I have been using rev 1 schematic to try and diagnose the problem but i need some help.
    I am getting 19v at pins 1,2and3,3v at 11 12, and 3v at pin 40. Also tested cable for continuity and seems ok. Ive been looking at pin 8 which says DISPOFF but I dont know whether it needs voltage there to enable the backlight.Only a novice and struggle with schematics so any help would be greatly appreciated.
    Attached Files
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    #2
    Re: lenovo 305-151BD display problem

    What is the voltage on pin #8?

    It is active low with the '#' suffix. This means the display will remain OFF if the pin is low = 0 volts.

    Comment

      #3
      Re: lenovo 305-151BD display problem

      Thanks for the quick reply.Doesn't look like any v at pin 8. Should I try injecting volts there.?
      Last edited by adamski1952; 12-25-2021, 12:35 PM.

      Comment

        #4
        Re: lenovo 305-151BD display problem

        No, not yet on injecting a voltage.

        This display enable signal (DIPOFF#) is linked to a zero ohm resistor - see top left of the lcd connector in the schematic.

        Check the voltage on the other side of the resistor (R123).

        Is it also 0 volts?

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          #5
          Re: lenovo 305-151BD display problem

          Goes to the resistor marked in the picture.No voltage either side.
          Attached Files

          Comment

            #6
            Re: lenovo 305-151BD display problem

            If you wish to try, remove this resistor. Keep it handy for later.

            Then apply +3v3 to the lcd pin side of this removed resistor.

            This should turn on the display with the logic high signal.

            The resistor must be removed so that the manually applied +3v3 is not challenging the logic low from the other side of this resistor. It will be a conflict to have a logic low mated with a logic high so the resistor must be removed for this test.

            Comment

              #7
              Re: lenovo 305-151BD display problem

              Will give it a try.Noticed that u15 is missing on rev3 board.

              Comment

                #8
                Re: lenovo 305-151BD display problem

                Yes, see attached. Note the '@' prefix on the U15 label - that symbol often denotes that the part is a 'NO STUFF' during the PCB assembly process.

                That is, design engineers will often allow for multiple ways to build the same logic. For this case, the 0 ohm resistor is used to bypass the need for U15.

                After you remove the 0 ohm resistor @ R123, check with a meter in resistance mode that there is nothing loading down R123 pad, pin # 2. Red meter probe on pad # 2 = lcd pin # 8. Black meter probe on ground.

                This will also confirm that resistor @ R124 is not causing this fault. That resistor should be ~100k ohms (according to the schematic).

                This is the same trace that leads to the LCD display. In summary, we want to be 100% sure that nothing on the LCD side of this broken connection is not being loaded down by some other factor.

                After this is confirmed, it is safe to apply the external +3v3 signal onto the removed R123, pad #2 to mimic what the logic board should be doing. Of course something is causing this line to be logic low but for now, it is worth a try.
                Attached Files

                Comment

                  #9
                  Re: lenovo 305-151BD display problem

                  Thanks for the help.i'll report back on wed or thur. In the meantime am I right in presuming the 3.3v comes from p1n 76 of the sio?
                  Last edited by adamski1952; 12-27-2021, 07:12 AM.

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                    #10
                    Re: lenovo 305-151BD display problem

                    Had no luck injecting the 3.3 at pin 8.Tried injecting at inverter but still same. Noticed a split second flicker on boot up so tried flashing the bios,no dice.
                    Incidentaly there doesn't appear to be even a faint display.on the screen (normally i would expect to see something using torch)although external monitor works fine.

                    Comment

                      #11
                      Re: lenovo 305-151BD display problem

                      What about pin7 INVPWM??

                      Comment

                        #12
                        Re: lenovo 305-151BD display problem

                        Nothing there either.Looks like that signal comes direct from uc1i (misread schem as uc1l) there is a resister r63 should I have 3.3v there?
                        Last edited by adamski1952; 12-30-2021, 08:01 AM.

                        Comment

                          #13
                          Re: lenovo 305-151BD display problem

                          If there is a series resistor between this pin and the driving circuit then ok to remove this resistor.

                          After the resistor is removed, apply +3v3 onto the isolated resistor pad that is connected to the lcd connector.

                          With PWM, the solid high logic level (3v3) will result in full on status to the inverter circuit. Otherwise this line will pulse on / off to dim the intensity as you select via keyboard or software.

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