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Compaq PDP-121P - Baked MOV

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    #61
    Re: Compaq PDP-121P - Baked MOV

    Originally posted by socketa View Post
    It's a long shot, but could broken stands that were going to the coil result in the power supply not getting, (and maintaining under load) enough voltage? Hence the computer shutting it down.
    Probably not.
    The DC voltages on your input caps looked good and didn't seem to drop out / dip when turning On the PSU... so I suspect this isn't the issue.
    Nonetheless, it does need to be addressed just in case. Just because I don't *think* it is a *likely* issue doesn't mean that it can't be one.

    Originally posted by socketa View Post
    Can i cut the yellow plastic tape, and reattach the wire to the coil? (hopefully there is no windings inside of it)
    Yes.
    Though there should be windings inside. This is the PPFC inductor, after all.

    Originally posted by socketa View Post
    Would it be OK to remove it (i'm guessing that it's passive PFC) and replace it with a jumper?
    Yes.

    Originally posted by sam_sam_sam View Post
    yes but the values less 1k ohm are an issue
    Why would that be an issue?
    I've seen a lot of PSUs with low-value load resistors... hence measuring much lower resistance on the 12V rail. So I don't really think the resistance alone can tell you anything.

    Originally posted by sam_sam_sam View Post
    Your reading should be very close or the same as the good one if both switching power supply are exactly the same model number and have the same exact ic chip on both boards
    ^ This.
    If the PSUs were the same, I'd definitely try to see why there was such a wild mismatch between the resistances. But...
    Originally posted by socketa View Post
    The power supplies are not the same
    That pretty much invalidates the resistance comparison... or at least as far as showing any information that may suggest a problem.

    Originally posted by socketa View Post
    i know that PSUs can have quite different values for minimum load resistors.
    Exactly!
    I suppose no harm in posting the resistances. Though when things measure above a few hundred Ohms on the output, then generally that means at least there is nothing wrong with the big power delivery components (i.e. rectifiers or their snubbers being shorted.) But it doesn't really amount to much of anything else. Open feedback resistors can essentially go completely unnoticed, due to their relatively high resistance value. So a resistance check on the voltage rail won't tell you if one went bad or not. This can only be found through careful component testing and perhaps a schematic or application diagram of the supervisor chip to understand what is going on.

    That being said... and recently trying out an experiment with a non-working PSU that helped me find the problem... maybe this troubleshooting trick could work here too:
    - Take a working ATX PSU and backfeed each rail into the non-working PSU, one at a time only, then power-up the non-working PSU too and see if it stays turned On.

    So for example, let's start with the 3.3V rail: connect the 3.3V of both PSUs together and turn On the working PSU. Verify that 3.3V is back-feeding into the non-working PSU. Then turn On the non-working PSU with a load and see if it can stay turned On without shutting down. If not, repeat this experiment for the 5V rail (after disconnecting the 3.3V rail.) Then do the 12V rail and -12V rail. Since the 5VSB is derived from a different circuit, don't do this experiment for the 5VSB. Also don't connect any of the signal lines on the two PSUs, such as PS-ON or PG. However, for the specific test of the 3.3V rail, make sure the 3.3V "return" / "sense" (if there is one) is connected on both PSUs. Without 3.3V sense, 3.3V rail could be off-value on one or both PSUs.

    If you do find that the non-working PSU stays turned On when one of these rails is connected, then there may be something wrong with the output of that rail on the PSU.

    The only tricky part with the PSU from this thread is that it has a 12.8V rail... so not sure if we should be back-feeding this one or just the regular 12V rail. But I suppose you could try both. Nothing should really smoke or burn, so long as both PSUs are decent brands (i.e. have good working protections.) I would avoid using a "cheapie" gutless wonder for this test.
    Last edited by momaka; 05-30-2021, 10:36 PM.

    Comment


      #62
      Re: Compaq PDP-121P - Baked MOV

      Originally posted by momaka View Post
      That being said... and recently trying out an experiment with a non-working PSU that helped me find the problem... maybe this troubleshooting trick could work here too:
      - Take a working ATX PSU and backfeed each rail into the non-working PSU, one at a time only, then power-up the non-working PSU too and see if it stays turned On.

      So for example, let's start with the 3.3V rail: connect the 3.3V of both PSUs together and turn On the working PSU. Verify that 3.3V is back-feeding into the non-working PSU. Then turn On the non-working PSU with a load and see if it can stay turned On without shutting down. If not, repeat this experiment for the 5V rail (after disconnecting the 3.3V rail.) Then do the 12V rail and -12V rail. Since the 5VSB is derived from a different circuit, don't do this experiment for the 5VSB. Also don't connect any of the signal lines on the two PSUs, such as PS-ON or PG. However, for the specific test of the 3.3V rail, make sure the 3.3V "return" / "sense" (if there is one) is connected on both PSUs. Without 3.3V sense, 3.3V rail could be off-value on one or both PSUs.

      If you do find that the non-working PSU stays turned On when one of these rails is connected, then there may be something wrong with the output of that rail on the PSU.
      Can you please show a hook up diagram for how you hook up the two power supply board and give more details about how to do this test correctly

      Thanks
      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


        #63
        Re: Compaq PDP-121P - Baked MOV

        I replaced the coil with a jumper - but no change, as expected

        So then i back-fed the 5V, and no change
        Then I back-fed only the 12V, and the computer started
        That was a nice surprise
        With the computer running, I checked the back-fed 12V rail, and it's the same as the 12.8V CPU rail, at 11.65V
        Could it be mere coincidence that both rails are the same voltage?

        (Wasn't exactly sure about what you meant by connecting the 3.3V sense wire on both PSUs - did you mean that sometimes the manufacturers don't bother connecting the sense wire on some power supplies?)

        So it looks like there is an issue with the 12V rail?
        Perhaps something 'funny' going on between the 12V and the 12.8V rails?
        Last edited by socketa; 06-01-2021, 10:10 PM.

        Comment


          #64
          Re: Compaq PDP-121P - Baked MOV

          ...and perhaps it's useful to note that the resistance between the 12V, and the 12.8V, rails is 12.8kohms

          Comment


            #65
            Re: Compaq PDP-121P - Baked MOV

            I've traced out some of the circuitry and identified the components
            The MOSFET isn't shorted, but maybe it's still faulty?
            Could whatever it was that blew the primary side components also have damaged this secondary side MOSFET?

            Just because i could, i removed the 4 pin CPU plug from the motherboard and powered it from another PSU, but it didn't make any difference.
            So i get the impression that the problem is with the 12V rail, not the 12.8V rail
            Attached Files
            Last edited by socketa; 06-10-2021, 01:30 AM.

            Comment


              #66
              Re: Compaq PDP-121P - Baked MOV

              I tested the STP40NF03L MOSFET with the transistor tester, and it identifies it as an "N-E-MOS"
              with addional information:
              vt = 1.8V
              Cg = 2.2nf
              RDS = 0.2ohms
              Uf = 632 mV +

              Does that look OK?
              Last edited by socketa; 06-13-2021, 07:17 PM.

              Comment


                #67
                Re: Compaq PDP-121P - Baked MOV

                with the theory that there is a problem with the 12V rail (that appears to be derived from 12.8V rail) ...
                would it be OK (i.e. not cause any damage) to disconnect the MOSFET and then try and start it up, to see if 12.8V comes up?

                Comment


                  #68
                  Re: Compaq PDP-121P - Baked MOV

                  Originally posted by socketa View Post
                  I tested the STP40NF03L MOSFET with the transistor tester, and it identifies it as an "N-E-MOS"
                  with addional information:
                  vt = 1.8V
                  Cg = 2.2nf
                  RDS = 0.2ohms
                  Uf = 632 mV +

                  Does that look OK?
                  Yes, looks pretty normal. If the TT tester was able to identify it, then that means the MOSFET is actually working.

                  Originally posted by socketa View Post
                  I've traced out some of the circuitry and identified the components
                  Looks like the MOSFET between the 12.8V and 12V rail is used either as a linear regulator or just a simple pass-through device. Check your schematic, though - I don't think that circuit can work just like that, with the Gate only connected to the Source pin via a cap. There has to be something more. In fact, if that MOSFET really is to operate as a pass-through device (or a linear regulator for that matter), then the Gate must always be higher than the Source by at least as much as the Gate threshold voltage, V_th. And for that to happen, then the Gate has to be pulled up by an external voltage source that is higher than 12V.

                  Originally posted by socketa View Post
                  with the theory that there is a problem with the 12V rail (that appears to be derived from 12.8V rail) ...
                  would it be OK (i.e. not cause any damage) to disconnect the MOSFET and then try and start it up, to see if 12.8V comes up?
                  Yes, you can try that test. The 12V rail will just not appear. That being said, I suspect the PSU will not start at all with the 12V rail missing (when you remove that MOSFET), as it probably is monitoring it, along with the 12.8V rail.

                  So when you had the PSU power up OK with the other (working) PSU feeding into it, did you have that PSU's 12V rail connected to this PSU's 12.8V rail or just the 12V rail?

                  Comment


                    #69
                    Re: Compaq PDP-121P - Baked MOV

                    Check your schematic, though - I don't think that circuit can work just like that, with the Gate only connected to the Source pin via a cap. There has to be something more.
                    - yep, thanks for pointing that out (makes sense, and i should have recognized that there was nothing, on my drawing, to turn the MOSFET on).
                    i missed a thin trace that was going from the Gate, to a 100ohm resistor, and then to pin 3 (cathode) of a KA431AZ Programmable shunt regulator, that's located on the add-on board - i'll update my drawing later
                    And i'll check the voltage at the cathode of this programmable shunt regulator.

                    So when you had the PSU power up OK with the other (working) PSU feeding into it, did you have that PSU's 12V rail connected to this PSU's 12.8V rail or just the 12V rail?
                    Just the 12V rail
                    Last edited by socketa; 06-17-2021, 12:49 AM.

                    Comment


                      #70
                      Re: Compaq PDP-121P - Baked MOV

                      I soldered the 12V rail MOSFET back onto the PCB and connected an analogue meter to it's gate terminal.
                      Then i plugged the rail cables into the motherboard and put the power cord into the wall socket
                      (i'm pretty sure that these Compaq computers start up straight away by themselves)
                      Then the voltage quickly rises to 5.5V and quickly drops to 3.5V, and stays there until i pull the wall plug out, and it quickly drops backs to zero after about 8 seconds (probably after the primarys discharge)

                      Pretty much the same result if i connect it to the other side of the 100ohm resistor
                      Attached Files

                      Comment


                        #71
                        Re: Compaq PDP-121P - Baked MOV

                        Made an error on the schematic
                        Here is the correct version

                        After connecting the rail cables to ATX, CPU, and hard drive, and then plugging in at the wall:
                        Attaching a meter to the gate of the 12V MOSFET
                        There is initially no voltage
                        After a couple of seconds (i guess after the primary's charge up and the computer automatically turns on) the PSU fan clicks, and the gate voltage rises to 5.5V, and settles at 3.5V
                        If i, instead, attach a meter to where it says on my schematic "large toroid and heatsink-mounted component", the voltage rises to 4V and drops to 0V

                        So can we say for sure that there is some problem with the creation of gate voltage?
                        Attached Files
                        Last edited by socketa; 06-17-2021, 07:30 PM.

                        Comment


                          #72
                          Re: Compaq PDP-121P - Baked MOV

                          Looks like this PSU has a crowbar circuit that's getting triggered
                          But none of the output voltages are over spec
                          The PSU runs OK with just a hard drive, but a crowbar is activated if it's plugged into any motherboard (any 5v or 12v board)
                          What would be causing the crowbar to activate?

                          Comment


                            #73
                            Re: Compaq PDP-121P - Baked MOV

                            It would be the UC3845 main supervisory chip that is shutting this down, right?
                            probably because it senses a change to an undervoltage condition at Vcc, that is reset when the power plug is removed from the wall.
                            There is a 10uf electrolytic cap that is attached to Vcc, so maybe this this a starting cap and it provides enough energy when not connected to a motherboard, but fails to provide enough energy when connected to a motherboard?
                            Maybe it's capacitance/ESR was degraded when the surge took out the fuse and thermistor?

                            Comment


                              #74
                              Re: Compaq PDP-121P - Baked MOV

                              I lost track of this post again and I just found it again and I am hoping that you figure out what is really wrong with this power supply issue is and it you can fix it or not
                              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


                                #75
                                Re: Compaq PDP-121P - Baked MOV

                                I'm getting desperate!
                                I checked the rail pins of the motherboard ATX connector, in case the motherboard is faulty, but couldn't find any shorts -
                                just 11ohms on the 3.3 rail, and 15ohms on the 5v rail, so that shouldn't trigger over-current protection, because it's not even 1A
                                Then the thought occurred to me that there may be a slight possibility that i may have a similar computer lying around, so i noticed one that has a similar front bezel - its a HP/Compaq and it has a PSU with a 12.8v rail - nice!
                                So now i will be able to narrow down where the problem is (assuming that this other PSU is working)
                                I can now test the PSU safely also, because i finally made an enclosed dim bulb tester
                                I'll post my findings
                                And it's being so long, that i forgot that i tested the start-up cap (10uf rubycon YXF, which tested good) but i might try replacing it with a lower impedance/higher ripple one (33uf Samxon GF)
                                Last edited by socketa; 11-08-2021, 02:43 AM.

                                Comment


                                  #76
                                  Re: Compaq PDP-121P - Baked MOV

                                  It's definitely a problem with the 12.8V rail on this PSU
                                  That other PSU, that i found, starts up the computer no problem

                                  When i connect a 23W bulb to the 12V, rail the bulb lights up and the PSU keeps running fine
                                  But when i connect it to the 12.8V rail, which is supposed to be able to supply 7.5A (which means 96W), the PSU shuts down and won't start again until i unground and reground the PS-ON pin
                                  But it runs OK if i use an 8W bulb instead

                                  So it seems like a protective latch is being activated that is incorrectly sensing the 12.8V rail current

                                  Does that sound right?
                                  Last edited by socketa; 11-15-2021, 05:59 PM.

                                  Comment


                                    #77
                                    Re: Compaq PDP-121P - Baked MOV

                                    on the secondary side, this only has two 14-pin chips:
                                    LM324 Opamp
                                    LM339N Comparator
                                    i don't know what these do, except that the LM339 might be used for OVP
                                    Any suggestion as to what to look for, in oredr to find out what is causing this PSU to latch?
                                    Would the fault more likely be on secondary side, since the PSU is not shutting down when i attach the 23W bulb to the 12V rail?
                                    Last edited by socketa; 11-16-2021, 04:19 PM.

                                    Comment


                                      #78
                                      Re: Compaq PDP-121P - Baked MOV

                                      That might be a possibility that something is wrong with the LM339N if it used voltage monitoring
                                      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


                                        #79
                                        Re: Compaq PDP-121P - Baked MOV

                                        i get the feeling that it's more like an incorrect voltage (that's somehow derived from the 12.8V rail) that's being applied to one of the comparator's inputs, and hence, not creating the required output
                                        Last edited by socketa; 11-16-2021, 08:10 PM.

                                        Comment


                                          #80
                                          Re: Compaq PDP-121P - Baked MOV

                                          This is also a possibility as well but the question is why
                                          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

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