Differences in Mac Plus

Discussion in 'Apple Collectors' started by Ih8reno, Sep 22, 2015.

  1. Ih8reno macrumors 65816


    Aug 10, 2012
    I have in my possession 2 macs plus with varying model numbers. I was wondering if anyone knows what differences if any they have. I ask because the one I recently purchased (M001A) will not work with my external drive but the one I got last year (M5880C) works with it just fine.
  2. MacTech68, Sep 23, 2015
    Last edited: Sep 23, 2015

    MacTech68 macrumors 68020


    Mar 16, 2008
    Australia, Perth
    The only things I can think of are:

    Poor Voltage spec when the external drive is attached. - Check/Verify/Adjust.

    Miss-matched ROMs or older ROMs (shouldn't be in a MacPlus but - you never know what somebody else has done)

    Miss-Matched Floppy drive in external case.

    I don't know about the external drive BUT internally, there are 3 models of 800K drive and are used internally thus:
    Series Number.................................Cable Needed
    MFD-5lW-03 (Red on Silver Label)......Yellow Stripe (or Red Stripe with one cable on the ribbon cut open)
    MFD-51W (Black on Silver Label)........Red Stripe
    51W-10 (Black on Silver Label)...........Red Stripe

    Finally, the most likely cause (back in the day) was a failed floppy port "Filter" - There are two involved. One filters the Mouse Port & Floppy, the other Floppy only.

    In the picture below, the left Filter does the Mouse and the ENABLE signal for the External Floppy Drive. It also does the Write and PWM for BOTH external & Internal drives, using the same signal path for BOTH.

    The right Filter does other signals for BOTH drives using the same signal paths.

    In the circuit diagram, the filters are noted as U8A and U9A in ellipses.

    I'd suggest checking everything else before trying to; a) FIND and b) REPLACE the filters.

    A few manufacturers were used, but the easiest to find might be the Bourns 4120R-601-250/201

    Anyhow, those are just my 'guesses'. ;)

    Mac Plus Logic Board 820-0174-C Floppy Filters.jpg
    Mac Plus 050-0159-A Schematic Floppy Port Filter.gif

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