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3743Altair Floppy Drive Troubleshooting/Repair

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  • Rick Bensene
    Apr 17, 2014
      Hello, folks,

      I have an two original Altair 8" floppy drives for my Altair 8800
      The system as an original MITS floppy controller.

      One drive works perfectly.
      The other one, though, has problems. Every read attempt results in I/O
      The drive is a Pertec FD-400.

      I've cleaned the head carefully. I have checked all of the connectors
      and cables and all is well there.

      The drive select logic on the buffer board is working properly.
      The spindle drive spins the floppy just fine, and the stepper motor
      moves the head in and out smoothly. The head load solenoid properly
      loads the head and pressure pad when commanded.
      The Track 0 switch is working as it should. The index sensor is working

      I can look at WRITE DATA going into the drive when the system is doing a
      DSKINI (using Disk Extended BASIC), and the data is going out to the
      drive just fine is clearly visible on the oscilloscope. I have taken a
      disk that has been initialized on this drive with DSKINI, and I can see
      that it has been initialized -- there are no files on the disk, and I
      can put files on the disk and read them back just fine. So, it appears
      that the write circuitry is working fine.

      However, when I issue read requests to the drive, the READ DATA does not
      appear to be coming out of the drive. I have checked all of the
      circuitry in the buffer board to make sure that there wasn't a problem
      there, and all seems good.

      It appears that there's something actually wrong in the read circuitry
      in the drive itself.
      I've downloaded the Pertec FD-400 drive manual from bitsavers (I can't
      thank Al Kossow enough for this amazing resource), and the read
      circuitry looks pretty simple, just three IC's and a few diodes and
      transistors, so I guess the next step is to start probing the drive
      itself to see if there's a problem there.

      I haven't scoped anything in the drive yet. I was hoping to avoid that.
      I just hope that the problem isn't with the head itself, as that'll be
      almost impossible to find a replacement for.

      Just out of curiosity, and to perhaps save some of my time in exchange
      for paying someone to fix the drive, I sent a query to M. Farris &
      Assoc. to see if they could fix the drive. They wrote back and said
      that they no longer support the Pertec FD-400.

      I'd really like to get two drives running on the system, as it makes
      things a lot easier for disk backups and provides more usable space.

      Does anyone have any ideas short of digging into the drive circuitry?
      Does anyone know of a place that will still repair these old FD-400

      Thanks for any advice,

      Rick Bensene
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