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Thread: IMSAI 8080 Power supply issues - i think...

  1. #41
    Join Date
    Jun 2012
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    Livermore, CA
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    Working on IMSAI #2. It is in better physical condition then #1.
    I have pulled out all the cards except the front panel. That one has the power switch soldered to the PSU board.
    I have been slowing bring up the power over the last two days. Im at 120vac now. I am getting different readings.
    See picture
    The voltage on the larger wire connections on the bus-board is 6.8vdc. on the two sets of smaller wire connections I see 34.8vdc. Many of the LED's on the front panel are lit. except the one over the power switch. Also the large transformer in the center is considerably warmer then the one in #1 when it was at this voltage. It is not hot, but noticeable warmer. None of the capacitors are warm. The fan is spinning.
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  2. #42

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    Are you seeing both a + and a - 34.8V?
    Dwight

  3. #43
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    Jun 2012
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    Yes, I get the same number + or - if I switch probes to the leads.
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  4. #44

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    No, there are two separate supplies. One is + relative to ground and the other is negative relative to ground. It is not just swapping the leads. It is measuring two different places. It sounds like you are missing a ground connection.
    Dwight
    Last edited by Dwight Elvey; November 24th, 2019 at 09:26 PM.

  5. #45
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    Jun 2012
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    This is where i measured. I get the high voltage with each pair of wires.
    Is there another location I should check?
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  6. #46

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    You should be making all measurements relative to ground, not across these two wires.
    Ground is the large black wire on the other side of the board.
    I can't tell for sure but I believe the small black wire is negative 18V and the small white wire is positive 18V.
    The large white wire is +8V and should also be measured to the large black wire on the other side of the board, not the small black wire near it.
    Dwight

  7. #47
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    Jun 2012
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    Thank you.
    Those numbers are much better. ( I am still the leaner, you are the master..)
    I get 17.6 to 17.7 vdc + or - on the small wires, 6.6 to 6.8 vdc with the two larger wires. These seem a bit low, but there is some draw from the front panel. AM I good with these voltages?
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  8. #48

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    You'll have problems with that voltage for that rail. Most regulators are rated at 2V drop as a minimum. Old regulators don't work well with less. You need to put a scope on that rail to see if what we are looking at is a bad filter capacitor or something else. The front panel isn't that much of a load.
    The +- rails will be fine.
    Dwight

  9. #49
    Join Date
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    Dutchess County, New York, USA
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    Quote Originally Posted by Dwight Elvey View Post
    You'll have problems with that voltage for that rail. Most regulators are rated at 2V drop as a minimum. Old regulators don't work well with less. You need to put a scope on that rail to see if what we are looking at is a bad filter capacitor or something else. The front panel isn't that much of a load.
    The +- rails will be fine.
    Dwight
    +1 on Dwight's reply. Unloaded most old S-100 linear power supplies should be in the +8.5 vdc +/- 0.5vdc more or less range and 16v to 18v for the other two. So the "+8vdc" supply has a problem with either a bad bridge rectifier or bad cap's. Look for AC voltage on the output and less is better.
    Crazy old guy with a basement full of Pentium 1 laptops and parts

  10. #50

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    Quote Originally Posted by DeltaDon View Post
    +1 on Dwight's reply. Unloaded most old S-100 linear power supplies should be in the +8.5 vdc +/- 0.5vdc more or less range and 16v to 18v for the other two. So the "+8vdc" supply has a problem with either a bad bridge rectifier or bad cap's. Look for AC voltage on the output and less is better.
    Bad diodes will usually show up as 60Hz ripple while a bad capacitor will be 120Hz ripple. You need a scope to see this.
    Dwight

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