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    iMac A1311 PSU repair questions

    Hi all.

    I have an ongoing issue with a 2010 A1311 21.5" iMac. It starts up, chimes, lights up and boots to the OS, however the screen starts to lightly flicker, steadily getting worse before switching off altogether. Measuring the voltages reveals that the 12v rail drops to around 10.5v once the Mac is switched on.

    Until yesterday, I wasn't sure if the problem lay with the PSU, the backlight inverter or the logic board.
    Yesterday, however, I borrowed a good PSU and the iMac works fine.

    Hence my opening a new thread within this troublshooting section.

    Searching online, I could find only one other person with this problem, on the iFixit site. He then managed to trace the fault back to a shorted Mosfet on the output of his PSU and resolved the problem.

    This is where I am looking for some advice.

    It seems reasonable to assume considering the identical symptoms and model iMacs that the problem should be similar, or at least related.

    It has become clear to me that I don't have particularly good diagnosis skils when it comes to good or bad mosfets. Mosfets are not something I've much experience with but this is how we learn.
    The fella in iFixit had an easily identifiable short. The PSU here doesn't.

    I have pulled three of the five fets on the board. All three read a mosfets with a Chinese T7 component tester. Two of them switch on and off fine, when using the DMM in diode mode. One doesn't (although it does show as N channel Fet with the T7).

    So my first question is, "which test is more reliable? Whether the Fet turns on and off with a DMM, or the Chinese tester?"

    The Fet in question is a P15N60C3 and Drain/Source either shows 'open circuit' or the reading steadily climbs from just above zero (maybe 20) rising up to the 'OL'
    Datasheet is here;
    https://pdf1.alldatasheet.com/datash...PI15N60C3.html
    Last edited by Crystaleyes; 07-04-2022, 05:19 PM.

    #2
    Re: iMac A1311 PSU repair questions

    The thing that made me realise how little I understand Fet testing was that yesterday, whilst testing a brand new 20N60 Fet, that it wouldn't turn on or off at all with DMM's, whilst appearing to test fine on the T7.

    Therefore, another questions is, "why do some Fets turn on and off with a DMM in diode, whilst others don't, and does that not make the DMM test essentially useless?"

    Comment


      #3
      Re: iMac A1311 PSU repair questions

      DMM diode checkers source around 2V, which is too low to turn on a MOSFET. Depending on the type, MOSFETs need ~3V (for logic level types) to ~5V, Gate-to-Source, to start to turn on, and even then not fully on.

      Bipolar transistors turn on and are controlled by Base-Emitter current. MOSFETs turn on and are controlled by Gate-Source voltage. To a DMM a Base-Emitter junction looks like (and is) a diode junction; a Gate-Source junction looks like an open circuit.
      PeteS in CA

      Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
      ****************************
      To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
      ****************************

      Comment


        #4
        Re: iMac A1311 PSU repair questions

        Originally posted by PeteS in CA View Post
        DMM diode checkers source around 2V, which is too low to turn on a MOSFET. Depending on the type, MOSFETs need ~3V (for logic level types) to ~5V, Gate-to-Source, to start to turn on, and even then not fully on.

        Bipolar transistors turn on and are controlled by Base-Emitter current. MOSFETs turn on and are controlled by Gate-Source voltage. To a DMM a Base-Emitter junction looks like (and is) a diode junction; a Gate-Source junction looks like an open circuit.
        Thank you.

        I fully get it regarding the BJT being current controlled, versus the Fet being voltage controlled, just hadn't considered the DMM having a diode-test voltage too low to trigger the Fet.

        Every day's a school day, as they say...

        I was actully just looking at the schematic of a 5v DIY rig to properly test Fets.


        Back to the PSU, if the Fet has not failed, yet the symptoms are identical to a failed mosfet, can anyone suggest where next to look?

        Or shall I just buy another PSU?

        Comment


          #5
          Re: iMac A1311 PSU repair questions

          Originally posted by Crystaleyes View Post
          Thank you.


          Or shall I just buy another PSU?
          As a final word, I threw in the towel, bought a used PSU for a tenner, and now the iMac lives again!

          Comment


            #6
            Re: iMac A1311 PSU repair questions

            Hi all.

            Rather than start a new thread, I'm just gonna resurrect this one, as now that the iMac is working I was thinking of using the replacement, functioning PSU as a reference to discover which component on the original PSU had failed.


            To recap on the problem...

            The PSU cannot maintain the 12v supply once it is switched on. It remains at around 10.5v, which is insufficient for the backlight inverter board to remain stable, and ultimately to remain 'on'.
            As mentioned, there was one fella on a forum with identical symptoms who resolved the problem by replacing an output FET.

            Feel free to correct me if mistaken, however my understanding of SMPS design is that this kind of problem will be in the secondary side, and most likely in the feedback circuit, which would obviously include Mosfets.

            As it was a while ago now that I last touched this board, I don't recall everything, but I know that I pulled and tested the electrolytics, the two STPS 3045 Schottkys, the two 90T03P N-Channel mosfets on the secondary, and the 15N60C from the primary (just for luck)..
            Haven't touched anything on the SMD side, but can provide photos.


            So, does anyone have any suggestions as to where to look first?


            Edit:

            Comparing the two boards visually, the only clear difference on the top side is that the lettering of one of the 90T30P's is noticeably more faint than the other three (on the two boards), yet without pulling it again, it seems to test the same as the others. I wonder if it only fails under load?
            Looking on the SMD side, it is clear that the 'new' board has also been repaired at some point.
            Attached Files
            Last edited by Crystaleyes; 09-05-2022, 05:27 PM.

            Comment


              #7
              Re: iMac A1311 PSU repair questions

              Here is the SMD side detail
              Attached Files

              Comment


                #8
                Re: iMac A1311 PSU repair questions

                Gonna swap the Mosfet over from the working supply and see what happens...

                Comment


                  #9
                  Re: iMac A1311 PSU repair questions

                  Considering the overwhelming interest in this thread (I've honestly done my best to read all the replies and suggestions) it's only right that I keep everyone updated in order that you can all get a good nights sleep....

                  Can't remember who it was who suggested swapping the components but that was that was pure genius. Thank you.


                  So...

                  ::: after swapping the 'forward-most' 90T03P, the broken PSU now works!!

                  Whoopee-fuckin-doo!!

                  Just to fuck with my head however,..

                  The replacement 'good' PSU (from where I took the FET and inserted the 'broken' FET) now also fuckin works!!!!! WTF???!!!???!!!

                  Who knows?.. 'Rhetorical Cheese' PM'd me to clarify that only Russia could have done this. Especially Putin...
                  but it could also be;
                  Climate change? Donald Trump? Bad solder joint? Gremlins? Extra-Terrestrials? Mossad? WEF?


                  The original 'broken' PSU came with the iMac which my wife bought, so if anyone comes across this problem in the future (flickering screen on startup which eventually turns black)...

                  Just get your hammer out and smash the fucker to pieces


                  ... Or just replace one or both of the 90T03P FETs on the secondary.


                  Hopefully this will help someone in the future.



                  Can't thank everyone enough for all the help and support.
                  There is no way, this could have been resolved without you.

                  XXXXXXXXXXXXXXXXXXXX
                  Last edited by Crystaleyes; 09-07-2022, 09:29 PM.

                  Comment


                    #10
                    Re: iMac A1311 PSU repair questions

                    maybe the solder joint was bad?
                    they can be bad but look good sometimes.
                    this busy thread is because not many people understand psu's with syncronous rectifiers and even less own mac's

                    Comment


                      #11
                      Re: iMac A1311 PSU repair questions

                      Originally posted by stj View Post
                      maybe the solder joint was bad?
                      they can be bad but look good sometimes.

                      Hi fella.
                      Obviously I am just an amateur in electronics repair, however this has given me invaluable experience, which no amount of qualifications can cover.
                      For example, I had already pulled and tested this FET (and others) a while back, but because it didn't test as a short, I failed to re-test the PSU again after replacing it - just got a friend to send a replacement from Blighty.
                      And as you quite rightly state, it can 'look good - but be bad'.. and THAT is something which I shall now carry with me as I continue forward. Just because the Mac PSU's 'look' all posh and perfect, they can still suffer a simple poor joint.

                      It is always a nice feeling when we resolve a problem, especially when it is hidden in such a complexed piece of kit, SMPS repair is something that I would really like to focus on, so onwards and upwards.



                      this busy thread is because not many people understand psu's with syncronous rectifiers and even less own mac's
                      I'll bear that in mind and never utter the words, "synchronous rectifier" unless I want to talk to myself - Lol

                      They say that the first sign of madness is talking to yourself... I was beginning to worry that the second sign was talking to yourself in a forum.
                      Last edited by Crystaleyes; 09-08-2022, 07:14 AM.

                      Comment


                        #12
                        Re: iMac A1311 PSU repair questions

                        when we used to repair crt tv's all day,
                        we used to always resolder the transformers in them.
                        the high frequency going through them effected the soldering on the pins

                        Comment


                          #13
                          Re: iMac A1311 PSU repair questions

                          This is all essential info for me.
                          My work experience has for many years been in electrical installation and/or inspection and was originally in relay-logic control circuitry, but always where we tighten all the connections with screws, so this problem is not so hidden.
                          As in, a loose connection in a 200+v system will tend to arc and cause obvious burning.

                          I 'am' getting there with electronic repair, even if it is still only when there are free moments during the week.

                          The info which you lot freely share on this forum is gold-dust and I'm not alone in being truly grateful for that, so hat off to all you professional tech's who actually understand the deeper levels of this art.

                          Comment


                            #14
                            Re: iMac A1311 PSU repair questions

                            I will add something that STJ said also if you see a lot of flux around a soldering joint that is probably a bad soldering joint and it would be advisable that you put a soldering iron to that joint and see how much flux residue comes from that joint because I seen this too many times before and this is not something to over look I have seen this more often than not on switching power supply transformers and three pin devices that have heavier pins on the device
                            9 PC LCD Monitor
                            6 LCD Flat Screen TV
                            30 Desk Top Switching Power Supply
                            10 Battery Charger Switching Power Supply for Power Tool
                            6 18v Lithium Battery Power Boards for Tool Battery Packs
                            1 XBox 360 Switching Power Supply and M Board
                            25 Servo Drives 220/460 3 Phase
                            6 De-soldering Station Switching Power Supply 1 Power Supply
                            1 Dell Mother Board
                            15 Computer Power Supply
                            1 HP Printer Supply & Control Board * lighting finished it *


                            These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                            1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                            2 Hem Saw Computer Stack Board

                            All of these had CAPs POOF
                            All of the mosfet that are taken out by bad caps

                            Comment


                              #15
                              Re: iMac A1311 PSU repair questions

                              Originally posted by sam_sam_sam View Post
                              ... this is not something to over look I have seen this more often than not on switching power supply transformers and three pin devices that have heavier pins on the device

                              Nice one, and thank you.

                              This great stuff, as I have to admit I have never 'actively' hunted bad joints. In that, unless they were 'clearly' poor, then if testing the components hadn't revealed a problem, then I had just discarded or stripped whatever the board was and put it down to my lack of electronics knowledge.

                              So with that in mind, there is actually different, older iMac PSU in the parts box, on which I had tested various components yet failed to find any fault... and had completely overlooked bad joints.

                              Seems like a good time to have another look at it, with my newly attained wider spectrum of focus.
                              Last edited by Crystaleyes; 09-12-2022, 04:14 PM.

                              Comment


                                #16
                                Re: iMac A1311 PSU repair questions

                                HEllo,

                                finally the post i was looking for. I have the same issue with my a1311 i5. when i start the imac the screen starts flickering then sometimes it turns of. When i change the power supply it works fine. So the problem is there.

                                my power supply looks different from yours. i do not have " 90T03P" , instead i have " 85T03GP9 "

                                can you tell me if does are in charge of supplying power to the LCD ?


                                Should i try to replace them ?





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