AT PSU rebuild

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  • pdavid
    Badcaps Veteran
    • Jan 2011
    • 279
    • Hungary

    #1

    AT PSU rebuild

    I noticed that barney is looking for a good AT psu so I decicded to rebuild a generic crapy one I have laying around, just to see if it works out. Might be good for some old Pentium retro rig.
    I replaced the EMI filter components, added a 4A graetz bridge, E13007 primary switchers, Samxon 470/200 primary caps, EI35 transformer, MBR3045 for 5 volt and SBL1660 for the 12 volt rail. Added the missing pi-filter coils. Yageo SY and Arcotronics ESX caps.
    Had to replace two small 4.7uf and 22uf caps, I put Nippon KME there.
    ...also I fited bigger heatsinks and a temperature controlled fan circuit. I'll put the pcb into a grey anodized Skyhawk psu case. It has holes for both the AC switch and the periphelial cables. This desing has an LM494 pwm chip an a few transistors for the regulation. I'll add a 220k trimmer to where just two resistors where soldered underside the pcb for voltage adjustment.
    I hope my modifications worth it and this thing will live
    Also cabe gauge will be awg18 for the periphelial and the AT style motherboard connectors.
    Attached Files
    Last edited by pdavid; 03-03-2012, 05:51 AM.
  • PeteS in CA
    Badcaps Legend
    • Aug 2005
    • 3579
    • USA, Unsure of Planet

    #2
    Re: AT PSU rebuild

    The big uncertainties may be whether that new transformer is sufficiently similar to the old, smaller, one, and whether those output caps are good enough but not too good (which might mess up the loop compensation). If that's a new output inductor, there may be compatibility issues there as well. But for adding robustness, I think you've accomplished that.
    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

    • Th3_uN1Qu3
      Believe in
      • Jul 2010
      • 6031
      • Romania

      #3
      Re: AT PSU rebuild

      For the inductor i wouldn't worry - i've tested a bunch of output inductors of various sizes from cheap halfbridge PSUs and all had the same values.
      Originally posted by PeteS in CA
      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
      A working TV? How boring!

      Comment

      • PeteS in CA
        Badcaps Legend
        • Aug 2005
        • 3579
        • USA, Unsure of Planet

        #4
        Re: AT PSU rebuild

        I remember seeing your post to that effect. The P/S pd rebuilt is nearly 14 years old (the date code on the PCB is "9816", 16th week of 1998). I was just wondering whether output inductors have been more or less identical for that long.
        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

        • pdavid
          Badcaps Veteran
          • Jan 2011
          • 279
          • Hungary

          #5
          Re: AT PSU rebuild

          Originally posted by PeteS in CA
          I remember seeing your post to that effect. The P/S pd rebuilt is nearly 14 years old (the date code on the PCB is "9816", 16th week of 1998). I was just wondering whether output inductors have been more or less identical for that long.
          The supply is an ancient UTT model. The output inductor has the same size as the original. The only differance is that the two 5V windings are a bit thicker. Right now I'm looking for white and blue wires for the AT motherboard connector... though the originals would be fine as well for those.

          Comment

          • Th3_uN1Qu3
            Believe in
            • Jul 2010
            • 6031
            • Romania

            #6
            Re: AT PSU rebuild

            Originally posted by PeteS in CA
            I was just wondering whether output inductors have been more or less identical for that long.
            Considering that the Chinese keep using the same early 90s designs for their cheap PSUs, there's a good chance they've been like that from the get go.
            Originally posted by PeteS in CA
            Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
            A working TV? How boring!

            Comment

            • PeteS in CA
              Badcaps Legend
              • Aug 2005
              • 3579
              • USA, Unsure of Planet

              #7
              Re: AT PSU rebuild

              The places where P/Ss are built has been shifting over the past 3 decades or so. In the late 70s, Boschert was building production P/Ss in Sunnyvale, CA (Silicon Valley). I'm sure they weren't the only company building P/Ss in the US. Production has since migrated to (then) lower labor cost places like Mexico, Japan and Taiwan, and then to even lower cost places like Thailand, India and China. Putting it in perspective, the target price for power supplies in 1980 was ~$1 per watt. IOW, the cost to the computer manufacturer for a 250W P/S would have been $200-$250; in 2000, Delta Products sold 250W P/Ss to Compaq or Dell for $13 or $14 or to Apple (who had some unique aspects in what looked like a standard ATX P/S) for $14 or $15. And competitors were under-selling Delta on price (but for, then, lesser quality). So when things like inductor inductance values/winding ratios, cores, and transformer ratios and pin-outs became more or less identical is uncertain to me.
              Last edited by PeteS in CA; 03-04-2012, 10:42 AM.
              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

              • pdavid
                Badcaps Veteran
                • Jan 2011
                • 279
                • Hungary

                #8
                Re: AT PSU rebuild

                It's finished! Sort of... I've turned it on, voltages were good (except for -5 and -12, these were low) and I noticed that the fan spins faster and faster. The primary switchers were cool, but the secondary heatsink got so hot I could see the heat. The thermometer read 79C! Also there was an audable buzz coming from the trafo. So I put back an EI-35 transformer and it runs cool now, without load. Maybe I'll take it to work to measure ripple and stuff.
                Attached Files
                Last edited by pdavid; 03-05-2012, 01:34 PM.

                Comment

                • PeteS in CA
                  Badcaps Legend
                  • Aug 2005
                  • 3579
                  • USA, Unsure of Planet

                  #9
                  Re: AT PSU rebuild

                  Remember that the -5V and -12 may be regulated by 79XX 3-terminal regulators, which are 5% devices. Or if it's a reeeeeeally cheap piece of @#$% those two outputs might not even be regulated.
                  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

                  • PeteS in CA
                    Badcaps Legend
                    • Aug 2005
                    • 3579
                    • USA, Unsure of Planet

                    #10
                    Re: AT PSU rebuild

                    Looking again at the pic, I don't see a 7905 or a 7912.
                    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

                    • pdavid
                      Badcaps Veteran
                      • Jan 2011
                      • 279
                      • Hungary

                      #11
                      Re: AT PSU rebuild

                      Originally posted by PeteS in CA
                      Looking again at the pic, I don't see a 7905 or a 7912.
                      Hooked up to a mobo and few hdd the voltages are good. Even the negative ones. Still, this psu is a low end at design and parts. A have a Sparkle and a Seventeam unit, now those are well built.

                      Comment

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