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Repairing Bestec TFX0250D5W

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    Re: Repairing Bestec TFX0250D5W

    Originally posted by lewisp69 View Post
    BTW, tried to do some testing while waiting for the MOVs to come in and blew the 5v stand-by control chip, U2 (TNY276PN). Ordered some of these too. I had replaced the bridge rectifiers diodes and fuse and the chip blew as soon as I plugged the PS into the outlet. I hope it was already damaged and I don't end up blowing up the new chip.
    Yes, sounds like it was damaged already from before. Of course, before putting in new TNY276 chip, check its surrounding components for faults. Otherwise, this could repeat.

    Also, to actually test the PSU in a safer way (or at least the 5VSB circuit and the main PSU without any load), you can put a incandescent (or halogen) light bulb (40-100 Watts typical) in series with the PSU's input, like this:
    The above method should prevent the new stndby IC from blowing up, as well as the fuse and anything else on the primary side.

    First, just hook the PSU with the series bulb and see if the 5VSB comes up correctly. If it does, you can try shorting the PS_ON (typically green wire) to ground to turn on the PSU - but do so without any load (or very light load, such as a PC fan or two or an old useless HDD... or 12V car bulbs.)

    The incandescent (dim) bulb "trick" works with many PSUs (switching and linear)... though there are a few exceptions, like for example, switching PSUs with APFC circuit. In the case of the Bestec in this thread - it's an older design SMPS without APFC circuit, so dim bulb trick will be OK with it.
    Last edited by momaka; 10-19-2021, 07:15 PM.


      Re: Repairing Bestec TFX0250D5W

      Momaka, Thanks for your help with this PSU. The lamp worked wonderfully, although turns out I didn't have any issues when the PSU came to life. I ended up replacing the two MOV, using parts you suggested, the fuse, bridge rectifier and TNY276PG. Overall I'd say the PSU took the lightning strike pretty well. Momma "O" will be tickled pink to get her computer back .


        Re: Repairing Bestec TFX0250D5W

        Hi Everyone,

        I also have this PSU with me. It turns on but the output voltages are slightly low on outputs.

        11.15v at place of 12v

        4.6v at place of 5v

        3.1 at place of 3.3v

        it turns on but can't boot the CPU it's powering up.


          Re: Repairing Bestec TFX0250D5W

          Originally posted by momaka View Post
          No worries about not being able to attach pictures (though if the forum is rejecting them, it may be because they are too big in terms of size or resolution - must be scaled to 3000x3000 max. or less.)

          The placement of those MOVs is pretty standard for older PSUs like this: one in parallel to each input 200V mains capacitor. The MOVs protect these caps from over-voltage due to surge or whatever other external factor.

          That being said, since both of your PSU's MOVs blew, you might want to check first if something else didn't go bad. Most likely the fuse on the input has gone bad, and possibly the NTC thermistor as well. Bridge rectifier can sometimes go bad from that as well. If you have replaced those parts already and nothing else appears to be bad, you can actually test the PSU without the MOVs, just to verify if the PSU is working at all after whatever caused these MOVs to blow. The MOVs are there only for surge protection, so they aren't actually necessary for the operation of the PSU. Of course, they are good to have in the even of over-voltage / surge to prevent more damage to the rest of the circuit. On the negative side, they do sometimes happen to fail by themselves from old age. That's because over many years, they probably have seen many "micro" surges. Every time one of these MOVs has had to operate (clamp) a surge voltage, a little part inside it gets destroyed and makes the MOV more likely to conduct next time. So overtime, that can make them fail by themselves, so to speak.

          On the other hand, since both MOVs blew at the same time, you may also want to check your house's wiring. In particular, check the AC voltage at the plug where the PSU was plugged in and monitor it for a while - especially when a high-power device comes On in your house. If living in a house with dual-phase 120V AC supply (most single homes and townhouses in the US), an open/bad Neutral can cause some plugs in your house to go much above 120V AC, while letting others sink way below that. If you turn on a high-power device in your house and some lights flicker or go dim while other seem to go lighter (well, I guess this may not apply so much anymore, now that people have switched to LEDs and CCFL light bulbs), that's definitely a sign of your house's Neutral connection opening or going bad... and if so, you ought to call your electric supply company right away, as this is actually an electrical fire hazard.

          And finally, for replacement MOVs... you really just need ones that are rated for 200V DC. Looking at some other PSUs, these part numbers should work too:
          TVR 10241
          There are many other too, but these are the ones I can recall seeing recently in a PSU.

          After replacing the MOVs, make sure to install heatshrink tubing on them for shrapnel control (in case they blow again.) Same if original fuse was blown and you replaced it with a glass type. Glass fuses should be heatshrinked to contain glass fragments if the fuse blows.
          Hi, your inputs are welcome to my problem with this PSU


            Re: Repairing Bestec TFX0250D5W

            D20, D21, D21A, D13. These four locations we want to know Mfg details like which part goes to which location? because we don't have BOM of this product. pls help us anyone resolve our query.


              Re: Repairing Bestec TFX0250D5W

              I have one of these power supplies. The supply doesn't want to power up. 5vsb seems OK. I have a pwer supply tester. Sometimes when I connect the tester in while AC cord connected the supply will power up. Seems the power on circuit is not working correcly. The computer boots normally with a different bestec supply but wiring harness too short to mount ps in computer. Any help would be appreciated.


                Re: Repairing Bestec TFX0250D5W

                OK, found that PS_ON is only 2.8v when connected to main board. See that comes from the vertical board.


                  Re: Repairing Bestec TFX0250D5W

                  Pics from your psu, please.


                    Re: Repairing Bestec TFX0250D5W

                    Originally posted by JHPArizona View Post
                    I have one of these power supplies. The supply doesn't want to power up. 5vsb seems OK.
                    Did you measure the output voltage with a multimeter to see if it's giving proper 5V? If yes, try putting a load on it, like a 12V 20 Watt car headlamp or MR16 halogen bulb. If the 5VSB croaks and does not output 5V with this load, suspect the output caps on the 5VSB are bad. Possibly the small "startup" cap for the TNY IC too.
                    If 5VSB is OK with load, the rest of the output caps could be bad too. Either replace them all or test with a "cheap" cap/ESR/component tester. At least that would be a good starting point. Remember, caps don't have to look bad to actually be bad, particularly when it comes to lower-tier non-Japanese brands like this Bestec has.