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PDP-8/L restoration - PSU problems.

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    #16
    Originally posted by Al Kossow View Post
    very strange. not a DEC PCB part number and almost no US manufactured components.
    The machine does not have the regular text on the front panel, it's says "READING, ENGLAND" instead of "MAYNARD, MASSACHUSETTS". Might explain something.



    More on the restoration: http://www.pdp-9.net/pdp-8-l

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      #17
      Originally posted by MattisLind View Post
      I am aware of the abs max rating. My thinking is that it might be a bad idea to expose 50 year old TTL chips to voltages at 5.7V at above which is what the PSU will output momentarily at startup. Or maybe it is a quick way to weed out marginal chips?
      Don't laugh!

      I was once told by our HVAC SME (Subject Matter Expert) that our buildings were allowed to heat up to 90F at night to expose marginal circuit packs.

      Since he had already shown that he had no idea how to work a wet/dry bulb hygrometer, much less what it was for, I just took it all with a grain of salt.
      "It's all bits on the bus, Cowboy! It's all bits on the bus!" -- Tom Beck, #1ESS Instructor, Southern Bell Opa Locka Training Center

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        #18
        I've been told that it's the thermal cycling that stresses things. Note that incandescent light bulbs usually blow when first turned on. When devices go from room temperature to powered on temperature, their component materials expand rapidly and at different rates. This stresses the points where one material is physically connected to another material and over time those connections fail. When things are powered off, they typically return to room temperature at a much slower and therefore much less stressful rate.

        You've probably heard of this lightbulb:

        https://en.wikipedia.org/wiki/Centennial_Light

        The key here is probably that it has remained powered on for most of it's working life.
        "It's all bits on the bus, Cowboy! It's all bits on the bus!" -- Tom Beck, #1ESS Instructor, Southern Bell Opa Locka Training Center

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          #19
          Originally posted by DDS View Post
          Since he had already shown that he had no idea how to work a wet/dry bulb hygrometer, much less what it was for, I just took it all with a grain of salt.
          I have a sling psychrometer at home. Doesn't everyone?
          Member of the Rhode Island Computer Museum
          http://www.ricomputermuseum.org

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            #20
            The crew at UMN Duluth had a crowbar problem with H-724 power supply in their PDP-12. Reforming the output caps for about a week, and putting a significant dummy load on the power supply reduced the voltage overshoot at power on and stopped tripping the crowbar. The power supply has been working fine for the last few weeks.
            Member of the Rhode Island Computer Museum
            http://www.ricomputermuseum.org

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              #21
              I have now tested the idea changing the zener value in the real world (well almost - with dummy loads).



              I replaced the zener with a 1.3W 4.3V zener BZX85C4V3 and the input network for the matched pair was changed into a voltage divider consisting of a 560 ohm to the 5V and a 2.4k to ground. I tested with dummy loads loading the PSU with 2.5A, 3.3A and 5 A. I got no overshot in the output voltage when ramping up the input voltage. At 2.5A the output was at 5.17V and a 5A it was at 5.08V. The crowbar triggered at around 5.7V. No false triggering occurred.

              I am happy with this solution and will put it in use in my other PDP-8/L and BA08. Possibly even the PDP-12.
              www.datormuseum.se

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                #22
                Good idea!
                Member of the Rhode Island Computer Museum
                http://www.ricomputermuseum.org

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                  #23
                  Originally posted by MattisLind View Post
                  I have now tested the idea changing the zener value in the real world (well almost - with dummy loads).

                  I replaced the zener with a 1.3W 4.3V zener BZX85C4V3 and the input network for the matched pair was changed into a voltage divider consisting of a 560 ohm to the 5V and a 2.4k to ground. I tested with dummy loads loading the PSU with 2.5A, 3.3A and 5 A. I got no overshot in the output voltage when ramping up the input voltage. At 2.5A the output was at 5.17V and a 5A it was at 5.08V. The crowbar triggered at around 5.7V. No false triggering occurred.

                  I am happy with this solution and will put it in use in my other PDP-8/L and BA08. Possibly even the PDP-12.
                  It would be interesting to see the waveforms when powering up the actual hardware. It is possible the overshoot doesn't occur in reality due to inrush current. It's possible your change will slow the slew rate of the supply when powering up the computer, but I can't think of any reason this would be bad.You might see higher currents due to the long transition through the undefined state.

                  CW

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