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Thread: Dead PSU diagnostics

  1. #1
    Join Date
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    Default Dead PSU diagnostics

    So it seems that on my second Morrow MD3P the psu is not putting anything out. I've checked with my DMM with switch on and yeah.. zip. Fuse is intact and passes resistance test.

    I'm wondering if something is shorted and preventing it from firing up.. but given that we are dealing with live AC I'm a bit cautious.. anyone have any thoughts on how to safely diagnose this thing?

  2. #2
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    You're not going to like this, but to diagnose a power supply, you have to suss out how it works and what's causing the failure. Not a simple job in the case SMPSUs. I suppose you can check the individual components, such as diodes and capacitors, but that's a big job.

  3. #3
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    Falter,
    Sure, Get yourself a 60 or 75 watt bulb and wire that in series with the Line that is feeding the HOT to the Power Supply.
    Most Supplies require a Load on the +5 VDC line to regulate. An old floppy will fix that problem. Some also require the
    Power Good signal to be satisfied before they bring up the supply. When you turn it on if the Bulb shines bright you have
    a short or near short in the PS. You should be able to check the Fuse, Bridge, and associated components on the Input
    side, if you have a schematic. Can you locate a schematic of the Morrow MD3P Power Supply.

    Larry

  4. #4

    Default

    Post some pictures of it.

  5. #5

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    Quote Originally Posted by falter View Post
    So it seems that on my second Morrow MD3P the psu is not putting anything out. I've checked with my DMM with switch on and yeah.. zip. Fuse is intact and passes resistance test.

    I'm wondering if something is shorted and preventing it from firing up.. but given that we are dealing with live AC I'm a bit cautious.. anyone have any thoughts on how to safely diagnose this thing?
    Most primary side failures would blow the fuse, so it's either a secondary short circuit or a regulation failure or almost anything on the oscillator/regulator IC. You must locate a schematic or reverse engineer one yourself if you aren't familiar with how these PSUs work.
    If you can hear a "ticking" sound, it's probably the oscillator attempting to start and fail on either overcurrent or other anomalies like high ESR on secondary side electrolytics (the most common failures on secondary side).
    Frank IZ8DWF

  6. #6
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    Okay here are some pictures of the PSU:

    https://drive.google.com/drive/folde...dy?usp=sharing

    Now, having gotten the PSU PCB out of its 'cage' for the first time, I can see now what the seller might have been talking about when he mentioned it having a blown fuse. He wasn't talking about the glass fuse - he was talking about that rectangular white block with KCMK-125X silkscreened below it. The numbers on it are too faded to make out, but you can clearly see burn marks around the base of it on both sides of the board, and on one side of the PCB one of the legs of that part is totally loose. I don't know if it came loose on its own and thus might have been sparking or if the part failed and then came loose.

    Any thoughts on what it is and whether it could be our culprit?

  7. #7
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    R4 doesn't look too good

  8. #8

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    The block you are talking about is a power resistor, from the picture i can see it's
    rated 5W but i can't tell the value. It's a load for the secondary side of the
    psu. Read its value and check with a multimeter if it's gone open.
    Also, as pointed from Al Kossow, R4 seems suspect. But can't tell from the picture,
    could be just dust or dirty.
    If you can't deal with live measurement, i would first check, with multimeter in diode
    check position, if there's no interruption between ac input and capacitors on primary
    side. Since there'e a bridge rectifier in between you should swap polarity of probes
    if you got open circuit.

  9. #9

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    Looking better at the picture i realized that the power resistor it's in the hot
    side of the psu to be a load, so i think it could be a current sense for the main
    switcher (the transistor heatsinked above it in the photo).
    If it's gone open it would prevent the power supply to start, check it with DMM.

  10. Default

    Sorry, I just now looked at the pcb picture and CLEARLY see that the solder joint
    of the power resistor is no good at all. Clear well all the dirty and resolder the
    loose leg properly.

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