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    Delta DPS-250AP-8 repair

    I am repairing a Delta DPS-250AP-8 power supply from a Fujitsu SFF machine. The main fuse was blown, and culprit was that one MOSFET is shorted. Standby +5V line came up nicely when I powered it via a variac.

    I ordered the MOSFET, but there are still two burned looking resistors. One is too charred, but the other looks reasonable. However, upon further look I can't get proper value off the color rings, as the colors just don't add up.
    Colors are: red, red, silver, gold, silver.

    0.22 ohms perhaps?

    Or if perhaps someone has the proper schematics, these two are labeled R8A and R8B.

    See the attached pictures. Thanks!
    Attached Files

    #2
    Re: Delta DPS-250AP-8 repair

    Given the proximity to the primary cap and APFC coil, there's a good chance those resistors may have something to do with the APFC.

    I suggest posting good top-side and bottom-side pictures in case a schematic can't be found, as we may be able to find out what those resistors do. That said, do the resistors show any resistance on your multimeter? Indeed the color code shown on them doesn't quite make sense.
    Last edited by momaka; 09-06-2017, 08:08 PM.

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      #3
      Re: Delta DPS-250AP-8 repair

      maybe more like 22 ohm.

      Comment


        #4
        Re: Delta DPS-250AP-8 repair

        Hi, thanks for the answers. I snapped few more photos, please excuse somewhat poorer quality.

        These resistors look like they are for protection for the D4 diode, as they are wired in parallel and in series with D4.
        They are both the same and both completely open, so no measurement possible. One that looks good, actually has a hole on the bottom...

        Thanks in advance
        Attached Files
        Last edited by mitchw; 09-11-2017, 07:01 AM.

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          #5
          Re: Delta DPS-250AP-8 repair

          It appears that Q1 is the switching FET for the main power supply (12V, 5V, and 3.3V rails) and Q2 is for the APFC.

          So which MOSFET blew, Q1 or Q2?

          Judging by the blown resistors R8A and R8B, I would guess that Q1 was the one blown. R8A and R8B connect the primary's positive (+) bus to one leg of the main transformer. The other leg of the main transformer is connect to the Drain of Q1 and is how the main power supply works (it's a single-transistor forward topology, or STF for short).

          Thus, check R12, as that may be blown (open-circuit) too. While at it, also check the snubber components across the main transformer's primary: that would be the two diodes next to it, along with a ceramic cap in there (it's not visible in the pictures, but I think there should be one, and I think I can see one too from the solder-side shots). All of these components are located between the main transformer and coil/inductor L2.

          That said, I wouldn't worry too much about resistors R8A and R8B for now. I think you can safely go with 0.22 Ohm resistors, at least for testing purposes. Just make sure they are flame-proof. In fact, even jumper wire may do for testing. So go with that if you don't have 0.22 Ohm resistors.

          But before substituting the new FET, check if the 5VSB comes up and that nothing else blows or overheats. To do that, simply install some jumper wire in place of D4. DO NOT install Q2 or Q1. Then connect an incandescent light bulb in place of the fuse. 40-100 Watts should do. This will limit the current if there is anything else wrong with the PSU. Here is actually how it's usually done:
          https://www.badcaps.net/forum/showpo...4&postcount=70

          If the 5VSB comes up, remove the jumper wire from D4 position, install a jumper in place of R8A (or R8B, whichever you prever, as they are both in parallel, so it doesn't matter), and install Q1, Q2, and D4 back (the whole primary heatsink with its components, that is). Keep the incandescent bulb connected across the fuse. Now plug the PSU back in the wall and try to turn it on (short PS-ON to ground). Depending on the PSU, you may or may not need to put a small 1-5 Watt light bulb on the 5V rail or 12V rail (sometimes, an HDD will work too). If it turns on, you are probably done. If not, you may have to start checking the small components around the UC3845B PWM chip, as there may be some more that are blow, including the UC3845B chip itself.
          Last edited by momaka; 09-11-2017, 10:09 AM.

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            #6
            Re: Delta DPS-250AP-8 repair

            Thanks for the exhaustive answer!!

            The one that blew was Q1.

            R12 is still okay and it is not open (measured less than 0.4 ohm).

            5VSB already came up when I powered it on at first with fuse bypassed with an 60W bulb and using a variac. If I tried to power the PSU on (ATX PS_ON to GND) it immediately whined and bulb lit brightly.

            I will replace the Q1 and see how the thing goes.

            Also snubber components all seem to be okay, no shorts, etc.
            Last edited by mitchw; 09-11-2017, 01:18 PM.

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              #7
              Re: Delta DPS-250AP-8 repair

              It's fixed!

              Replaced Q1 and both blown resistors. I used 0.22 ohm ones. Also did a quick check of the ESR on the capacitors and found all small value ones to have a very high ESR, so they all went (Ltec brand ones).

              Tried it with a dim bulb across the fuse and it worked. Replaced the fuse and called it good.

              Thanks for all the info!
              Attached Files

              Comment


                #8
                Re: Delta DPS-250AP-8 repair

                Well, I spoke too soon.

                I assembled the case and tried it in a computer. Sadly, there was a loud bang and I disconnected the mains immediately.

                This time, there is much more damage, Q1 blew again, also the IC501 (UC3845B chip) blew its head off, R12 is open, some diodes are shorted, few SMD resistors open, one SMD transistor shorted. R8A and R8B this time survived.

                I am a bit baffled why did this happen, as it worked properly for about 30 minutes (albeit with minimal load), but when tried in a computer it just went bang. Perhaps the UC3845B was somewhat damaged before and it blew now or perhaps there is something else?

                I will reconsider if the repair is still within reasonable economical boundaries.

                Comment


                  #9
                  Re: Delta DPS-250AP-8 repair

                  Well, good to hear this PSU didn't have anything else blown. Primary faults usually tent to take a lot of casualties with them. But every once in a while, we get lucky and it's only a few components.

                  Anyways, congrats on the repair!

                  Oh, and by the way, those small caps could have been the reason why the primary MOSFET blew, as sometimes they filter power to the ICs and can make them act weird when their supply voltage gets noisy. So get all those small stupid caps replaced with a known good Japanese brand.

                  *EDIT*
                  Looks like I typed my post too slowly. Sorry to hear that.

                  Yes, the UC3845B PWM IC could have had issues or some other component around it, even if the PSU worked with minimum load.

                  Once you have a PSU with primary damage fixed, it is best to replace the incandescent light bulb with a 1000 Watt space heater / hair dryer / toaster oven. The 1000 Watt rating may allow the PSU to draw enough power to test a low-end PC without shutting down (with a 40-100 Watt incandescent bulb, you won't be able to do that).
                  Last edited by momaka; 09-11-2017, 07:15 PM.

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                    #10
                    Re: Delta DPS-250AP-8 repair

                    By the way, can you point to us which SMD resistors and transistors burned?

                    I think the PSU should still be repairable. Not sure if you'd deem it economically repairable, as that will depends on how much time you have and how much you value your time. But on the other hand, it may also be a learning experience, so that's also worth considering sometimes.

                    Comment


                      #11
                      Re: Delta DPS-250AP-8 repair

                      Hi, I finally managed to find some time to replace all the blown components.

                      Most of the primary side was gone, so I replaced all the burned out resistors, etc. Also all ICs and most of the transistors. Also the +12V optocoupler was dead. I replaced the original SFH617A with a K1010 (almost the same). The only thing I left is the +5VSB IC, which seems to be working properly (TNY268PN).

                      Now I have a strange problem: I usually use a low current high voltage (0-500V) DC bench power supply to test the PSUs, and it is usually just fine for the testing purposes. It is limited to about 50mA. Most PSUs would start at about 150VDC.

                      However, the strange thing happens, when I put the DC voltage to about +310V it starts up automatically, which is not right as the PS_ON signal is still low. But if the input voltage is lower, it won't start no matter what you do to it. It seems to work okay with the high input voltage, but no means to turn it off. There is a small whine, however.

                      Any hints and directions would be appreciated I don't know whether this is a secondary side or a primary side problem...

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                        #12
                        Re: Delta DPS-250AP-8 repair

                        Check the circuit that is responsible for generating the PS_ON signal = perhaps something is shorted there, keeping the PS_ON low. Remember, when PS_ON is LOW, PSU will be signaled to turn ON. And when PS_ON is HIGH, PSU should be OFF.

                        So technically, your PSU is operating correctly given the PS_ON state. Once you raise PS_ON to 4V or more, the PSU should definitely NOT turn ON anymore.

                        Comment


                          #13
                          Re: Delta DPS-250AP-8 repair

                          I made a mistake, the PS_ON is being held high at all times.

                          Even if I short it to GND, the PSU won't start. However, there is some change in the produced whine, so it looks like it is doing something...

                          But The PSU would start automatically if I raise input DC voltage to 310V, but even then the PS_ON is held high.

                          Hope that clears it a bit

                          Comment


                            #14
                            Re: Delta DPS-250AP-8 repair

                            Going by that description, it sounds like the optoisolator that enables/disables the UC3845b (in order to turn On the PSU) is conducting when it shouldn't be.

                            I would say start from the UC3845b Vcc supply (pin 7 for the DIP-8 and SOIC-8 packages, and pin 12 for SOIC-14) and trace back to where it comes from. There should be at least 3 optoisolators in this power supply: one for 5VSB, one for enabling/disabling the UC3845 PWM controller (and possibly also the APFC controller), and one for the main PS feedback from the secondary side. I actually see four optocouplers, so the extra one is likely for 5VSB crowbar (short-circuit) protection. Either that, or something to do with enabling the APFC supply.

                            In any case, the UC3845b PWM chip should not be getting any power when the PS_ON signal is high - at least from what I have seen in many other power supplies. So it's worth checking that part out. To do that, find the optoisolator that is responsible for providing power on the Vcc pin of the UC3845 PWM chip and see if either that optoisolator is faulty (allowing current to pass when it shouldn't) or if something on the secondary side is faulty and triggering the optoisolator to conduct when it shouldn't.

                            Other than that, I don't think it matters much that you used a K1010 opto in place of the 617A. Looking at their data sheets, they both have very similar specs. The 617A has slightly better typical cutoff frequency (100 KHz vs. 80 KHz for the K1010), but I highly doubt this is the problem here.
                            Last edited by momaka; 05-25-2018, 08:01 PM.

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                              #15
                              Re: Delta DPS-250AP-8 repair

                              Mark this as closed...

                              I got another power supply from a lightning damaged computer (bad motherboard) which kinda worked, but needed new capacitors on secondary side as two were bulging.

                              So I replaced most of the capacitors, and it is now working like new.

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