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Thread: IMSAI 8080 need wiring verification on PSU section

  1. #51
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    Caps could well be empty. I'm not entirely convinced the transformer is ok either. Do you know what AC voltages I should see on the secondaries ?

  2. #52
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    I have taken a picture of the rectifier board (below)

    Both of the 8V power diodes seem to be fine - at least they pass the diode check on my meter.
    I have cleaned up the rectifier diodes and heatsink, added some fresh heatsink compound and replaced.

    There is not a lot to this circuit it being a simple 2 diode full wave rectifier so either the transformer is not generating enough or the caps are bad. Given the very slow ramp up to the target voltage I'm thinking the latter.

    IMG_20190623_153329438.jpg

  3. #53

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    Some of the solder joints look bad.
    For 9V DC with a two diode full wave rectification the AC rms voltage would be 6.8VAC.
    The two diode setup would use a center tapped transformer. I doubt the 18V rails use more than 1 diode each but I can't see all of it.
    With no loads, the nothing should get hot. The transformer may get warm.
    You want to load the rails to see what the problem might be. A good handy load is an auto marker light or brake light. The brass based ones are easier to solder to.
    You do know that you have to remove the diodes from the heat sink to do a meter check. Otherwise the one parallels the other through the transformer. One can be completely open while measuring in circuit.
    Clean up the solder joints.
    With loads, load as far away as you can, like at the back plane. Look for bad solder joints and bad crimps.

  4. #54
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    Thanks
    Yes the diodes were checked while removed from the heatsink.

    There are 4 diodes in total for the 16V lines - it looks like two diodes for each feed - the pair of white link wires in the centre of the board take the output to the smoothing caps yellow and red feeds from the top of the board.

    This machine has the upgraded Tranex 4-3751 30A transformer fitted.

    The soldering does look a little messy so I'l clean it up although I have checked continuity of the 8V section end to end.

  5. #55
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    Anyone else have a 30A Tranex 4-3751 in their machines ?
    What voltages do you see on the unloaded outputs ?

  6. #56

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    Hi Zippysticks

    Your PSU looks similar to the one in my S100 box. I have two fat diodes like those plus a bridge rectifier and three fat capacitors. Annoyingly the transformer has a label on it that just says "Standard Transformer" and it doesn't look like a manufacturer's logo. I could take measurements but I'm not sure how it's wired as the whole thing is mounted on a PCB including the fat caps. Would have to pull it completely out and reverse engineer. I sure wish I had a variac. I just connected it to the mains, checked the rails were OK, powered down and plugged the boards in. Had a few tants go pop which I replaced, then it started working. So I applaud your careful approach... keep it up, I'm watching the thread avidly.

    Hey, here's a thought. You might like to use my chassis to test your boards?

    Cheers
    JonB

  7. #57
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    Thanks Jon - I actually have a pair of N* Horizons I could test the boards in. I may end up borrowing the supply from one of those to get this IMSAI working.

    Thanks for the offer though.

    My Transformer is clearly a Tranex as its stamped on the top and I can workout from the IMSA PSU manual what the connections are for as it refers to my transformer.

    My problem is that I just can't work out why the AC isn't being properly rectified to DC. It would appear that I have 8V AC centre-tapped feeds going to a pair of mr1121 diodes configured in parallel as a full wave rectifier but still appear to have AC coming out the other end.

    In the more modern PSU's the 8v is fed through a standard 4 diode bridge rectifier device.

  8. #58

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    With no load, you should be able to see the output voltage stabilize with even a 100uf capacitor. If the voltage comes up and stabilizes right away, the large caps are dry. If there is another problem it will look the same.
    Dwight

  9. #59
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    Zippysticks,

    My IMSAI looks slightly different so I didn't jump in at the beginning. I can take some pics if it helps but I seem to be missing a cap.

    IMSAI PSU-sm.jpg

  10. #60

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    Your's is the same, just some of the wires are different colors and someone added an extra capacitor to the 8V, on his.
    Dwight

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