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Thread: Help Identifying Resistor on Miniscribe 20mb MFM drive

  1. #1

    Question Help Identifying Resistor on Miniscribe 20mb MFM drive

    Hi All,

    My Miniscribe MFM hard drive in my IBM XT decided to bite the dust the other day randomly. There's a resistor on the control board that I watched glow red and pop before I was able to cut power to the computer. Before this happened, everything worked completely fine. I've owned the computer for almost 20 years now so thankfully the status of it wasn't a mystery before this happened.

    With the hard drive removed, the rest of the PC works fine. I'm able to boot from the floppy disk and run programs.

    I'm hoping I can just replace the resistor that failed and all will be good. (Big "if" but worth a shot). I haven't had any luck searching online and hope you guys might know what I need to order.

    On the board labelled "PCB ASSY 300322200" it is R53. Serial number is MS III E 23013.

    Here's a pic of the resistor:


    Thanks in advance!

  2. #2
    Join Date
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    Page 10

    1.5 ohms, 2W.

    (You can thank Al Kossow for this one)

  3. #3
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    Resistors that burn up like that almost certainly did so because of a short somewhere in the circuit. It's a good bet a replacement resistor will fail the same way.

  4. #4
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    That's why the schematic is useful. Of course, this is a 2W carbon comp resistor, so failure of these power resistors isn't uncommon, but good point.

  5. #5

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    Yep, carbon-comps are junk, but given where it's at in the schematic, I'd suspect something else (maybe the FET's or C44) failing...

  6. #6

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    Any way, check with an ohm meter. I suspect it to be more likely one of the driver chips that failed, U6 or U7. As for caps, one should include C43 and C45.
    The power fet looks to be a charge pump circuit for the driver chips. I think these are only for high side gate driving for the driver chips. If so, they should be lightly loaded. This points more toward U6 or U7. The diodes are relatively hefty so may still be un-shorted. If so, an ohm meter should show which side of the diode the failure is on.
    Dwight

  7. #7

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    I take it back, these are the supply voltage for the driver chips. See:
    https://www.mikrocontroller.net/atta.../PBL3717_E.pdf
    I guess these are a charge pump to kick the coils with a pulse. I can't imaging it is too much of a boost with only 22uf. Anyway that brings the fet back in as suspects.
    Of course it could be a failure of the 6801 uP.
    Dwight

  8. #8

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    Thank you very much everyone for the info. I ordered the replacement resistor and I'm currently waiting for it to arrive. I figure I'll try to replace it first and see if it works. If things still don't work, I'll start heading in with my multimeter and check the other locations mentioned.

    T-R-A, nice to see a familiar name. I was going to post this over at Uncreative Labs (I'm Erik btw) but figured it would get more traction over here as this site seems to be more active.

  9. #9

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    I hope you ordered several. There is likely going to be a lot more smoke before you done. There is nothing in the normal operation of that circuit that woul blow out that resistor. What ever it was is still there waiting to fry the next resistor.
    Dwight

  10. #10

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    Dwight,
    Thank you for your concern. I did end up order a pack of them just in case.

    I'm no where near an electrical engineer or even fully at a hobby level when it comes to circuits beyond simple tests and soldering in replacement parts. Do you think the MFM data cables being reversed could have caused any of this? The ones I have don't have the tab to make sure they're situated the correct way so you can easily install one or both upside down.

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