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Thread: Repair of MPI 51M

  1. #31

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    Unless you need to move the disk from machine to machine, you can just take a disk that you think was well written and use your scope to get the best read. If single sided, make sure the pad on the back side id clean and even pressure across the head or you'll be adjusting for a bad pad.
    Make sure everything else is working well, like the speed is correct under load.
    The schematic on bitsavers shows two pots. The R23 looked to be the write bias current adjust. Do look at where the leads go before adjusting anything. The speed goes to a oneshot. The current adjust goes to the transistor pack DIP.
    Dwight

  2. #32
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    Page 77 of this manual calls it the "trim control".

    Maybe something was different in the 1A or Executive? I've looked the drive PCB up and down - there is only that one adjustable resistor on it at R23.

  3. #33
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    Quote Originally Posted by Chuck(G) View Post
    Alignment disks are prepared on special equipment and are becoming very dear (no, you can't have mine).

    Sometimes, it's possible to keep trying to read a track on a known-good disk and tweak the alignment until it succeeds.

    In any case, regardless of alignment, you should be able to format a disk and pip some files over to it.
    And I would assume given that the manual calls for the specific disk with certain programs Osborne used on it that I can't use just any 5.25" alignment disk anyway?

    This will make it pretty tough I think for me to diagnose. Too many moving targets. And I still have to figure out if something is wrong with the PCB.

  4. #34
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    There were (that's the right term, right?) two types of alignment disks. One doesn't involve a computer at all--you set the disk spinning and step to predefined calibration points with an oscilloscope hooked into the analog read channel, adjusting alignment for a symmetrical waveform.

    The other way does involve a computer, using what's called a "digital" alignment disk. Again, on specified tracks, a number of sector IDs are recorded at various offsets from track center. You use a program that continuously reads the track IDs. At some point, you'll get a result of the same number of missing IDs offset either way from track center, so you know that you're spot on.

    There are other tracks on both types for reading track azimuth errors, but usually, there's not much you can do about those without throwing the whole affair out of kilter.

  5. #35

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    You can make a simple test setup cable with some double pole double throw switches. Use the computer to step to a known track and then take over the select, drive motor and head load with the switches. You can the observe the drives analog signal at you leisure.
    Dwight

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