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Thread: Help Identifying Resistor on Miniscribe 20mb MFM drive

  1. #11

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    Quote Originally Posted by slack View Post
    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.
    I'm not sure. I'd need to analyze the circuits much more. If power was applied where it shouldn't have been then it could have. Still, we did give you a list of likely suspects. Using an ohm meter is not all that difficult. As I recall, power comes in on another connector. Wth the wide cable being just signals, it is possible that if the cable was reversed, it could have put the drive in some illegal state.
    Dwight

  2. #12

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    It is almost certain that the resistor had the power supply across it, in other words feeding a dead short, to glow and burn up like that, otherwise something else would have likely heated up too, even if it had a resistance of an Ohm or so.

    Given as Dwight points out that this resistor feeds the IC. This type of IC has an H bridge output stage, it could short out. Probably if is was just in a usual state with 2 of the 4 bridge transistors conducting to power the motor coil, the motor's DC resistance would have limited the current and the resistor not burned out.

    The problem with the H bridge configuration, is that the two transistors on each side are connected directly in series across the power supply so if for any reason at all they both simultaneously get into a conducting state their junctions melt and get destroyed, if its too long in this condition. But it is not supposed to happen of course and the drive circuits that feed the bridge transistors inside the IC are supposed to never allow this to happen. So in theory at least it should not do this no matter haw abnormal any control signals presented to the IC are. However in practice, bridge output IC's like this do fail.

    Before installing the new resistor, check for a short across the supply terminals of the 3717/2 IC or any capacitors on that voltage feed that might be shorted . It possible that the short could have "disappeared" if two of the transistors in the bridge were conducting in and abnormal state and were not damaged by the event, but that doesn't seem too likely.

  3. #13

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    I'd have ordered a couple 3717 myself. They seem to be common on ebay. Like you say, it is almost impossible for the resistor to burn out on its own unless he shorted the line with a wire.
    Dwight

  4. #14

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    I didn't get a chance to check this thread again before I soldered in the new resistor. With the new resistor in place, the computer booted right up into DOS from the hdd.

    I ran the computer for ~30 minutes and the replacement resistor was warm, but not hot where I couldn't hold my finger on it. I didn't notice any drive abnormalities while I was using the computer. It seemed to save/load/etc fine. The following day I did the same exercise and the computer again booted right into DOS.

    So I suppose it might have just been the resistors time to give up the ghost? I would think the drive would have acted strange if something else was amiss while I was using it, no?

    That said, I should have some time after work tomorrow or the next day to check for shorts with my multimeter and report back.

  5. #15
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    Well, carbon composite power resistors have a peculiar failure mode--and this may be evidence of one.

    As a composition resistor ages, even if it's not used, the resistance rises. Said resistance rises faster if the resistor is run hot.

    So, you could well be victim to effect of the higher the resistance, the hotter it gets and so on, particularly if driving a motor winding. (Non-moving rotors draw more current than moving ones. Which is why low AC line voltage is murder on induction motors.)

    That said, it could also be a transient failure, such as a stuck positioner or a failing component.

    Should the resistor run warm? Absolutely--that's why it's a 2W unit.

    At any rate, nice to see that your drive is working again.

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