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Thread: Fixing a PC09

  1. #1
    Join Date
    Jul 2014
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    Default Fixing a PC09

    The PC09 paper tape punch on the RICM's PDP-9 has been broken for about 30 years. The Timing Generator coil that senses the crankshaft rotation is open, so no PUN SYNC signal, so no punching. Because we have been able to use a flaky GNT punch, and Mattis has kindly punched tapes for us, we have ignored it for a long time.

    Saturday I borrowed a coil winder from the local Wireless Radio Museum and unwound the coil. It was an incredibly tedious process to get the very fine wire off the coil because of the adhesive tape residue, and to minimize the number of times that the wire broke. Fortunately the coil winder has a counter on it that counts to 100. Several hours and 4400 turns later we were done. Guessing at an average coil diameter of 3/8" that means about 432 feet of wire.

    The wire measures 0.0025" in diameter, so that should be 42.5 AWG. Remington Industries sells this size magnet wire through their eBay page. I bought the smallest spool they offer, which contains 6594 feet, for about $11. That means that I can have about 10 practice sessions rewinding the coil and still have enough left for a working coil. I noticed that the PC04 schematics show the coil as having 685 Ohms resistance. The Electrisola WWW page lists the resistance of 42.5 AWG wire as 1793 Ohms per 1000 feet. That calculates to about 384 feet of wire, close to the wild guess of 432 feet.

    I will get some Mylar or Kapton tape to hold the first turn of the wire to the spool, and to hold the larger diameter pigtails to the magnet wire. The wire insulation is Polyester, so apparently a soldering iron will burn it off. On the coil winder one turn of the crank makes one turn of the coil. It would take 4400 crank turns to rewind the coil. I will try replacing the crank with a variable speed electric drill to reduce the hours required to rewind the coil.

    All comments and suggestions gratefully accepted.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  2. #2
    Join Date
    Mar 2004
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    Glenview IL and Buchanan MI
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    Sounds like the perfect job for a summer intern. Too bad it's autumn.

  3. #3
    Join Date
    Jul 2014
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    Rhode Island
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    Default

    Quote Originally Posted by jackrubin View Post
    Sounds like the perfect job for a summer intern. Too bad it's autumn.
    You are right, the summer interns are back in school.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  4. #4
    Join Date
    Dec 2013
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    Near Milwaukee Wisconsin
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    I've re wound a few coils in my time, but never anything that small. Did you find the open? Many times they are very near the end turns. I have used super glue to hold the initial turns in place. If you do use a motor to drive the turns, go slow and keep a slight tension on the conductor. You may find this a two man job. A sudden torque of any amount will break that small wire. Good luck Mike

  5. #5
    Join Date
    Jul 2014
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    Rhode Island
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    Quote Originally Posted by Mike_Z View Post
    Did you find the open?
    The open was where the wire went down the side of the bobbin and made a 90 degree turn to start the winding. I might run the wire down the side and intersect the bobbin at a tangent so there is less stress on the wire.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

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