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    #1

    PB70 AT power supply

    I have an AT power supply (custom for a specific computer model) and it does not power on. I have had some experience repairing power supplies so normally it's not a big deal. First thing I noticed is the fuse is not blown, however it does not output any power. I have checked for shorts on the secondary side, checked all the transistors, including the main switching transistor, all the diodes, the optocoupler and photothyristor seem to work. I get voltage, about 300vdc, if I remember, at the primary side bridge rectifier. I have even checked all the resistors and they are at the rated value. From what I can tell, the main transistor must not be switching at all. Though if I keep a meter on the output, and power it on for a few seconds then turn it off, after about 15 seconds or so it jumps from 0v dc to 0.2v dc, which makes me think that there may be a collapsing field in the transformer that may be generating some current, though I don't know for sure. I did a full recap on it since some minor leakage was showing, even though the leaking capacitors didn't show any ESR issues yet.

    Any help will be appreciated, as I have been banging my head on this issue for most of this week.

    Thanks
    Attached Files
    Last edited by Zorix; 06-21-2018, 10:44 AM.
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    #2
    Re: PB70 AT power supply

    Did you also replace those small value caps like c23 and the blue one with the silicone on it? also if there are similar caps in the primary

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      #3
      Re: PB70 AT power supply

      I replaced all electrolytics except for the bulk filtering ones. The blue one as well, though they actually had a good ESR rating and capacitance was spot on. Thanks.

      Comment

        #4
        Re: PB70 AT power supply

        So the standby voltage is ok, and power on is ok. If the main switching transistor is ok, check if there are any low value resistors off of it that might be open, also what is the drive ic that drives the switching transistor?
        Last edited by R_J; 06-21-2018, 11:25 AM.

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          #5
          Re: PB70 AT power supply

          Well there is no standby since this is AT, so it's manually switched on at the primary input side. I should clarify I used one of those 3 pin component testers to check the transistors and it identified all the parts correctly and around 700 or so for Vf on the transistors. Though it detected the photothyristor as a transistor with or without turning on the LED side. However I also manually tested that using continuity through the anode and cathode and it turns on when the LED is turned on. I don't know for sure if it's completely good, don't really have experience measuring those types of devices.

          From what I can tell it uses a self exited oscillator to start the switching through the transformer, and then uses a combination of transistors and that photothyristor to drive the switching transistor. None of the resistors have been out of spec that I have found.

          My thoughts right now is that either the tester isn't picking up a bad transistor, a bad photothyristor coupler, or maybe bad transformer. I've ordered a ring tester for that though. Unless I'm missing something I don't know what else it could be.

          I thought about checking the transistor base for PWM control signals but I haven't figured out if that's safe to do with my oscilloscope yet. I do have an isolation transformer I can wire to the power supply for testing though. My Tektronix 464 states that a 500v ac peak to peak with ac coupling is the maximum input.

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            #6
            Re: PB70 AT power supply

            I see two opto's usually one is for feedback/regulation whats the other one for?, Are you trying this with the pc or just standalone? it might require a load if I remember, I think I used to connect a hard drive before they might power up, it's been a while.
            Last edited by R_J; 06-21-2018, 12:19 PM.

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              #7
              Re: PB70 AT power supply

              One is the optocoupler, 4 pin, and the other is the photothyristor coupler (s12md1v - ), 6 pin. It doesn't require a load, I do have another one of the same model in another machine and it outputs without any load.

              Comment

                #8
                Re: PB70 AT power supply

                those brown elnas leak all over in those packard bell units.
                every one of them have needed major cleanup work.
                i maintain some industrial stuff that used these packard bell pizza box pc's.
                i have had these units sent to me 20 at a time.
                every one has had severe leak damage a few so bad the case was rusty.

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                  #9
                  Re: PB70 AT power supply

                  For sure, I did another one a while back, the whole board was covered in wet dust from the caps, though it cleaned up fine and after replacement of the capacitors it came back without a problem. This one has been the hardest repair attempt I have tried. I have never had something clearly not show signs of a failure, yet still wont start up. I'm still thinking its the photothyristor not working properly. Once I know the transformer is working properly, I will replace that and then start replacing all the other semiconductors because that's all that remains. New caps, all resistors are within spec.

                  I did find some diodes that don't have a known type on it. One says DF 8 1D but I can't match that to anything.

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