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Basic question: How to use board test points for filters during backlight repairs?

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  • Basic question: How to use board test points for filters during backlight repairs?

    iPhone 6, no backlight. Customer went to sleep, put the phone to charge on his night stand, next morning: No backlight.
    Diode mode readings on connector tells me OL on PP_LCM_BL_ANODE_CONN, which is connected to FL2024, so I assume I must replace the filter to get the phone back to work.

    Anyway, when I click over that filter on the schematic, I can see there is a test point on the board I can use to tell if it's working or not, but I don't know how to do that.

    I will probably replace the filter anyway, but I would really like to know how test points work.


  • #2
    They are just spots on the line like a pad that doesn't happen to have a component on it. You could put your probe on a test point the same as any other spot to see if the line overall is short to ground. If you know where in the line the test point is, i.e. is it between the connector and filter? between filter and diode? between diode and coil? etc....then in theory you could use it as a little bit of an easier spot to get a probe in--for example, if the test point was between the filter and the diode---then you could measure from connector to test point AS IF you were measuring from connector to leg of diode to ask the question "do I have continuity across this filter"

    I think for most people you end up finding your 'favorite spot' to take measurements for each common line. I always measure anode line in the connector because it tells me at once if the filter is bad (OL) or the line is short (less than 0.5 in diode mode). The test point isn't more useful to me so I ignore it. For iPhone 6s, the test point is useful because it is on the connector side of the filter and I can put a filter on the test point for cases with broken pads in the regular filter position by the connector.


    • #3
      The iPhone 6 has a signature failure for backlight circuit where one of the 3 capacitors on that circuit gets shorted out, becomes a wire when it's not supposed to be a wire.

      Your History says that could be the problem, However, you said you get OL on PP_LCM_BL_ANODE_CONN... I think you are measuring it incorrectly - If you use Red on connector and Black on Ground you will get OL. I suspect this from history a Filter doesn't go bad without tech damage or outside source like water.

      In diode mode reading, you must put the Red probe on ground and black probe on the points you want to test. Backlight circuit on the iPhone 6 diode GOOD mode reading is 0.548 this reading is from a FLuke 115, you will get different numbers depending what model Multimeter you use However it should be close in this range, you can verify your good number by using a known good board to help you get good numbers that you know for sure for your multimeter.

      From the history of the signature failure on this model I suspect it has a short on PP_LCM_BL_ANODE if there is a short in diode mode, you will get 0.000 if it's a full short in Continuity mobe it will Beep.

      in the case, if short on PP_LCM_BL_ANODE.... C1530, C1531 or C1505 .. one of these guys most likely has a short or has a partial short. Usually, this issue will need a full circuit rebuild to insure integrity since it is near the Backlight driver that is underfilled and also its close to underfilled CPU. The micro soldering technique is important if this rebuild needs to be done, the diode also may be bad and also the Coil next to it.


      • epumer
        epumer commented
        Editing a comment
        Great answer! very very useful for me (I'm starting with microsoldering).
        I had previously discarded shorts to ground on C1530, C1531 and C1505. Diode (D1501) was also ok. Visual inspections didn't give me any clue at first, until I got OL on the anode line and went directly to the filter which - by the way, didn't look bad either. Anyway, I replaced the filter and the phone is working OK again.

        Thank you very much for your answer because there is a lot of useful information for a newby like me!