iMac screen problem - PSU schottky's failing?

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • Crystaleyes
    Badcaps Veteran
    • Sep 2021
    • 493
    • Brazil

    #1

    iMac screen problem - PSU schottky's failing?

    Hi all,

    have another iMac with flickering then black screen issue.

    It is a 21.5" 2010 A1311 model.

    When switched on, the screen flickers before going black. With a torch you can see the image is there, as are all four diagnostic LED's.

    Measuring the PSU output only gives around 10.5v with the cable harness connected, although it measures 12.06v unconnected. This leads me to suspect something the power supply.

    I saw that some people had found a shorted mosfet on the output however these seem fine, when tested for shorts on the board.
    There are two power-Schottky rectifiers which although the datasheet gives the Forward Voltage at 0.57v, are only reading around 0.19v across all four.

    So my question is, "could these low Schottky readings cause the PSU output to drop under load?


    Thanks in advance
  • Crystaleyes
    Badcaps Veteran
    • Sep 2021
    • 493
    • Brazil

    #2
    Re: iMac screen problem - PSU schottky's failing?

    Just to add, that the inverter board is receiving the 3.3v ENA and PWM signals

    And the Schottky's are these;

    https://pdf1.alldatasheet.com/datash...TPS3045CT.html
    Last edited by Crystaleyes; 03-31-2022, 07:21 AM.

    Comment

    • Crystaleyes
      Badcaps Veteran
      • Sep 2021
      • 493
      • Brazil

      #3
      Re: iMac screen problem - PSU schottky's failing?

      Just in case anyone ever has the same question, the Schottky's are fine.

      Testing a brand new one gives around 0.175v across each leg, so the problem lays elsewhere.

      Comment

      • R_J
        Badcaps Legend
        • Jun 2012
        • 9535
        • Canada

        #4
        Re: iMac screen problem - PSU schottky's failing?

        Check the inverter board, but I suspect you have a bad led in the panel

        Comment

        • Crystaleyes
          Badcaps Veteran
          • Sep 2021
          • 493
          • Brazil

          #5
          Re: iMac screen problem - PSU Schottky's failing?

          Turns out the problem is the PSU.

          This iMac has just been gathering dust whilst I had other projects and it wasn't clear where the problem lay, which potentially, meant a headache to locate the issue.

          Fast forward a few months and by luck, I was asked to replace the logic board in an identical iMac, which gave the opportunity to swap over any parts necessary to identify the source of the problem.

          Installing the known good PSU resolved the flickering display, so I set about having a closer look at the bad PSU and after testing a few things, think I may have found the problem, I floopin hope so anyway as this is the last transistor to be tested on the thru-hole side of the pcb.

          Buried under a heatsink is a 15N60C3 MOSFET, and this is giving strange readings. On a T7 component tester is comes up as a N-Channel Mosfet, yet when performing the DMM test with -ve on Source and +ve on Drain, and touching Gate, instead of shorting, it starts with maybe 40Ω and just keeps rising to OL. Similar behaviour in Diode mode.

          This Fet is out of circuit and being tested on the bench, so unless I am about to learn some funky Fet behaviour which was unknown to me, then I would say that this Fet is fecked. I mean, Mosfets shouldn't have a rising impedance across the Drain and Source should they?

          It would also explain why the display deteriorates after being switched on.

          It'll be great if this is the only problem. I haven't yet had the most success with SMPS's.
          Last edited by Crystaleyes; 07-03-2022, 04:30 PM.

          Comment

          Related Topics

          Collapse

          • mmccomputer
            problem with replacing lcd screen
            by mmccomputer
            I have an HP model : 14-cf0011nf and I want to change its display screen (it has an original 14" full HD TN 30 pins panel) which works without problem but I want to put another screen with an IPS panel, the problem is that I tried several screens but there is no display except with the original screen, I never had this problem before I changed hundreds without problems but this PC is not 'accepted than its original screen
            otherwise on the motherboard it displays edp and I don't know, do the panels I tried only work with the LVDS protocol?
            If anyone has any information on the subject...
            12-15-2024, 03:44 PM
          • jimjoe123
            LCD TV Green/Pink screen problem
            by jimjoe123
            I have a Dick Smith 37" LCD TV, Model GE6804. I am using it as PC extended monitor only.

            Problem is while watching tv, I hear a semi loud click(like a electrical breaker tripping due to overload, just sound like it but no actual electrical overload), then screen is covered with green or pink. I can still see the original picture image but overlayed by green or pink. See attached images. Pink.jpg is what I see, and Normal.jpg is before green or pink screen, What I captured was pink screen, but about 80% of time it's green and 20% of time it's pink.

            Then each time...
            02-23-2020, 01:29 AM
          • Chrissygizmo
            Sony KD-55XG9505 lines on screen problem
            by Chrissygizmo
            Hello again everyone,

            This is my latest mission, to try getting this Sony tv back to normal working order…

            I've only just got it today, when you power it up everything is as should be, in fact it works perfectly for a few minutes then the screen starts getting horizontal lines on & eventually the full screen goes back, I will upload photos of the problem & also some photos of the board etc. You might notice that I'm busy trying the tape method, I'm not sure exactly where the clock lines are so I'm just guessing & hoping for the best lol . I've already tried...
            02-19-2025, 11:31 AM
          • smic717394
            iphone XS turns off when press on screen.
            by smic717394
            Hi,

            I have a iphone XS that came in for a screen replacement, the screen was badly cracked, and inside missing scues, on the back of the screen still the blue protector, the screen was a LCD. So we replaced the screen and all good, installed the screen and we noticed that pressing the screen with a bit of force but not excessive, the phone dies. After plugging the charger it turns back on, but again if you press it it dies. I open the phone again, and I can't see anything major, with the phone open, if I press on the logic board, battery, charging board, or even putting pressure on...
            07-25-2024, 10:16 AM
          • mferna14
            MacBook Air A2681 battery problem after Screen Replacement
            by mferna14
            Hi, my dear friends., Maybe this might be a stupid question or something I missed or do not know. I have 2 MacBook Air A2681 (Indentical color & EMC #), both LCD screens broken. Before ordering the LCD Screens I confirmed both worked with their respective batteries & AC Adapters, & they did 100%. The LCD Screens came in today & I replaced LCD for the 1st A2681. (Removed Battery Terminal cable for both Laptops before installation of LCDs). Perfect Screen & Booted up fine. But it did not work with battery. I checked the battery connector & it was seated in well/perfectly....
            12-06-2024, 08:10 PM
          • Loading...
          • No more items.
          Working...