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Imsai 8080 power supply problem

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  • valley8080
    I received a working IMSAI 8080 recently but after unpacking it found it did not work. Checked main board voltages, even removed all boards, including front
    Message 1 of 3 , Dec 15, 2010
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      I received a working IMSAI 8080 recently but after unpacking it found it did not work. Checked main board voltages, even removed all boards, including front panel, and found voltages on the main board were +3v, +12v, and -23v. Power supply is a PS28 with both 95K caps in place. Nothing visually wrong in the PS section. Scope traces at the transformer secondary may show some clipping. Looks like there is a DC bias of around -7v (add this to each voltage and they are close to the published spec). House wiring is grounded (checked this). Scope traces at MB looks smooth (just off). Any ideas?
    • Bill Degnan
      ... did not work. Checked main board voltages, even removed all boards, including front panel, and found voltages on the main board were +3v, +12v, and -23v.
      Message 2 of 3 , Dec 15, 2010
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        >
        > I received a working IMSAI 8080 recently but after unpacking it found it
        did not work. Checked main board voltages, even removed all boards,
        including front panel, and found voltages on the main board were +3v, +12v,
        and -23v. Power supply is a PS28 with both 95K caps in place. Nothing
        visually wrong in the PS section. Scope traces at the transformer secondary
        may show some clipping. Looks like there is a DC bias of around -7v (add
        this to each voltage and they are close to the published spec). House
        wiring is grounded (checked this). Scope traces at MB looks smooth (just
        off). Any ideas?
        >
        >

        works! powers up!

        love it.

        Bring it to my office in Wilmington and we can take a look at it, compare
        with an actually working IMSAI. I assume you have the original
        PW/backplane, etc.

        Who are you and where are you.

        Bill
      • Dan Roganti
        ... These reading on the main board do not look healthy - provided you did unplug everything from the backplane. That +3v is -not- good. The +12v reading is
        Message 3 of 3 , Dec 15, 2010
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          On Wed, Dec 15, 2010 at 9:21 AM, valley8080 <jkranyak@...> wrote:
          I received a working IMSAI 8080 recently but after unpacking it found it did not work. Checked main board voltages, even removed all boards, including front panel, and found voltages on the main board were +3v, +12v, and -23v.
           
          These reading on the main board do not look healthy - provided you did unplug everything from the backplane. That +3v is -not- good. The +12v reading is very low. Also the -23v is very high, although it will still provide enough power to the Neg 12v regulators on the cards. Are you using a Digital multimeter or an Analog multimeter - cause these tend to become uncalibrated and give erroneous reading after time.
           
          The Main Power supply is supposed to be +8v, +16v and -16V. The +8v rail should not be any lower than that and, givien tolerances, about 1v more. The +16V, -16v power rails should be in the same ball park too, given tolerances, maybe 2v more.  this is without any load whatsoever on the power supply - no cards, front panel, etc.
           
          Power supply is a PS28 with both 95K caps in place. Nothing visually wrong in the PS section. Scope traces at the transformer secondary may show some clipping. Looks like there is a DC bias of around -7v (add this to each voltage and they are close to the published spec). House wiring is grounded (checked this). Scope traces at MB looks smooth (just off). Any ideas?
           
           
          You'll have to start investigating the +3v reading -first- because this is supposed to supply power to the Logic via the 5V regulators on every S-100 card, including the Front Panel. Without this, you will have -squat- at any attempt to bootup this system. This low voltage could be attributed to a leaking filter cap, or even a leaky diode in the bridge rectifier . Start looking there.
           
          =Dan
           
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