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Thread: Testing and Repairing early Apple II (Silver) power supplies

  1. #1
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    Default Testing and Repairing early Apple II (Silver) power supplies

    So I am restoring an Rev-0 Apple II supply for someone.

    I was told it wasn't working and It was missing the power connector, so I replaced that. I opened it up (it already had screws, I know older supplies were riveted). Visually everything looked good, nothing leaking or bulging. I checked the fuse.

    OK basics done before I dig into real troubleshooting.

    So I figured I'd plug it in to test. However I know these early silver supplies need a load resistor or a bulb. The later ones you can simply plug in and check the voltages at the cable. I obviously don't want to risk an Apple II+ board if the supply is bad, but it's been a while since I repaired my own Rev-0 supply that I don't recall if all the voltages need a load, just the +5 or both the +5 and +12. Before I rig something up, I figured I'd ask. and trust me it will be rigged up, I don't have any spare PCB side connectors (I think I will order a few for the future and make a real tester).


    So which voltages on these older supplies need a load resistor for me to test?

    Also since the early supplies don't have a bleeder resistor in them, what do you guys do besides wait a week after turning them on to start changing out parts on the board.

    Thanks,
    Corey

  2. #2

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    Hi Corey
    Just the +5V should be enough. It is to ensure that the transformer isn't seeing zero load. Most flyback type have issues without special extra circuits or a load to keep them from overshooting.
    Surprisingly, you can use a 12V bulb for 5V. Incandescent bulbs tend to be like constant current devices but are slow to react.
    Dwight

  3. #3
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    Should have just emailed you directly...

    I do have a 12V bulb setup I have been working on for my OP-80, I'll just repurpose it for now to be a test load.

  4. #4
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    One more question... Can I use the bulb to bleed down the caps... I've only used real bleeders in the actual circuit.

  5. #5

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    Most are too low a resistance for the high voltage side. It is better to have some permanent resistors attached all the time. Especially on those with voltage doublers. If discharged in the wrong order, you can get a nasty bite. Most units come with a built in resistor but these are famous for going open as they don't bother the performance of the supply when they fail. Target a time constant of about 5 seconds and that they are of enough wattage to easily take continuous use. Just tack them on and leave them on while working on the supply. By the time you pull the AC cord and look to see where you want to get at things, they will be safe enough.
    I do recommend disabling the AC side and start off using bench supplies to connect to the rectifier filter caps on the secondaries. This allows one to easily check the feedback path on the lower price supplies that do not include a separate transformer to run the regulator circuits. Most of these need an early spike through the transformer to bootstrap themselves into operation. May faults will keep them from starting and will be particularly difficult to analyze, since it all happens in the first milliseconds of powering up.
    Dwight

  6. #6
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    Thanks... I think the supply is actually working now that I put a bulb load on it, so I may not need to actually troubleshoot it. I guess the power connector was the "broken" part of the supply and that is fixed now. However the strange part is the load needed to be on the +12V side for the silver supply. I went and checked a non-aztec gold supply (there were a few) and it needed it on the +5V.

    I plan on testing it now with a II plus before plugging it into it's matching Rev-0 Apple II board.

    Thanks,
    Cheers,
    Corey

  7. #7

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    Quote Originally Posted by Corey986 View Post
    Thanks... I think the supply is actually working now that I put a bulb load on it, so I may not need to actually troubleshoot it. I guess the power connector was the "broken" part of the supply and that is fixed now. However the strange part is the load needed to be on the +12V side for the silver supply. I went and checked a non-aztec gold supply (there were a few) and it needed it on the +5V.

    I plan on testing it now with a II plus before plugging it into it's matching Rev-0 Apple II board.

    Thanks,
    Cheers,
    Corey
    That is interesting. Most smaller supplies use the 5V for regulation and the +-12V is done by winding count ( or in some cases an analog regulator ). They do often run the regulator circuit from the +12V line but that needn't be tightly regulated. With the 5V using the largest amount of current, it makes sense to keep it loaded.
    Hope to see you soon.
    Dwight

    Dwight

  8. #8
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    The over current protection is on +12, with no load it will be at its maximum and any load on the regulated +5 will drive the supply harder and increase +12.
    The over current protection relies on the fact the 12+ line will increase as +5 is loaded, to much load and 12V gets high enough to trip the SCR.

  9. #9

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    Quote Originally Posted by David_M View Post
    The over current protection is on +12, with no load it will be at its maximum and any load on the regulated +5 will drive the supply harder and increase +12.
    The over current protection relies on the fact the 12+ line will increase as +5 is loaded, to much load and 12V gets high enough to trip the SCR.
    Good information to know. So the +12 is controlled by the windings as is typically done on these smaller supplies. It is a clever way to look for an over current. I'd never have thought of that. It would also catch a runaway regulation as well. Two birds with one stone.
    Dwight

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