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Thread: IMSAI 8080 need wiring verification on PSU section

  1. #41
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    All back the way it was for the small caps.

    The one in the right with the yellow lead connected to the negative cap terminal is actually connected to the -16V rail of the s100 bus.
    Last edited by zippysticks; June 20th, 2019 at 10:03 AM.

  2. #42

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    Quote Originally Posted by zippysticks View Post
    All back the way it was for the small caps.

    The one in the right with the yellow lead connected to the negative cap terminal is actually connected to the -16V rail of the s100 bus.
    That is correct.

    When you power up, use your voltmeter and measure on both of the small capacitors. You want to use the variac and bring the voltage to 0.5V and no higher. Make sure both are at 1/2V. The +8V caps many not have much voltage. That is OK for now.
    The variac should be at about 5V. If you have to go higher there may be a problem so don't go higher.
    If all is well, leave it like this over night.
    Dwight

  3. #43

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    Yellow -18V
    Red +18V
    White +8V
    Dwight

  4. #44
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    Yes, thanks Dwight - thats a job for the weekend as I need to make up a power cable adaptor to connect my variac first.

    I want to leave the backplane disconnected as I don't want to remove the front panel board or damage the backplane active termination components on this wunderbuss noiseguard backplane.

  5. #45

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    Good idea.
    Dwight

  6. #46
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    Slowly does it.

    Just 'forming' the capacitors with low voltages from the variac to bring them back gently into service - it's like watching paint dry.

    Thankfully no sparks, shorts or surprises so far.

    IMG_20190622_150925865.jpg

  7. #47

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    Once you've stepped the capacitor voltages up to about 2V, I've not had issues bringing them all the way up.
    Do check the capacitor polarities with the meter. The machine has been through a lot.

  8. #48
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    Machine is up to 110V but the Unloaded DC voltages (no backplane attached or cards installed) are all over the place

    8V sitting at around 6.5V
    -16V sitting at -19.5V
    +16V sitting at 14V

    The 8V line takes about 30 seconds to reach its v6.5V

  9. #49
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    After a coupe of hours of running things have settled down a little.

    -16 is at -18
    +16 is at 16
    8V is at 8V

    I think unloaded these are in range.

  10. #50

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    I'm not too happy about 6.5V and 30 seconds. Unloaded, the 8V line should be more like 9V. There is a high resistance in series with the rectifiers that shouldn't be there or the capacitors are empty cans. It might be bad rectifiers as well but if all was well you shouldn't even notice the time it takes to get to full voltage. Something is wrong.
    The rectifiers are stud mounted. You might loosen and tighten the nuts on them. Dissimilar metals can make poor connections over time.
    Dwight

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