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Thread: REMEX Paper Tape reader

  1. #1
    Join Date
    Dec 2013
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    Default REMEX Paper Tape reader

    Today was the first day I turned on my PDP8E, since spring. OS/8 booted up, but for some reason I could not get any FORTRAN IV programs to compile correctly. Seems each time the PDP8E is turned on after a long summer vacation, something happens to it. Anyway, since OS/8 loaded and most of the functions seem to work, most of the circuits must be okay. I loaded a MAINDEC D0AB which tests instructions and it stopped immediately. Function 7 of MQ register failed, writing to AC from MQ. Turns out bit 2 of the MQ register has a stuck bit. I pulled the M8300 E17 chip and replaced it and now D0AB runs as it should. I wonder what actually happens.

    Earlier this year I was attempting to load paper tapes from my REMEX to the PDP8E. I want to do a little more MAINDEC testing, then resume work on the REMEX. Mike

  2. #2
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    Welcome back Mike.

    Putting the 'T' away for the winter break?

    Dave

  3. #3
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    Well...... The Model T is not put away quite yet, but since the weather is cooling off, I thought that I'd try the PDP8E. I'm sure there will be a few more fall Model T tours before the snow flies and maybe a couple even after that. What I want to do is, see if I can save and load programs to/from paper tape. I have a copy of Digital-8-5-u-sym and maybe this program is what I need. Yet I'm not sure. Can I save a text file to paper tape and then read it in using EDIT? I have some reading and trying to do. So, while the weather is a little under the weather, I'm flip switches and watching lamps blink. Thanks for the welcome. Mike

  4. #4
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    If we could have weather like today all summer long I'd not have dreams of moving to Resolute. And finally the mosquitos have receded a little.

  5. #5
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    Sure has been wet here. All through August and September seemed to rain every other day or so. And you are right about the mosquito's. Made life in the garage tough, but they are gone now. Resolute? I spent some time in Churchill and they have these flies, that will make you want mosquito's. Anyway, bring on winter, I'm ready. Mike

  6. #6
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    It's going to rain for days. I'm almost ready for winter. Need tyres for my daily driver, a cover for the hot tub, and to finish plumbing my oil tank. Everything else (I think) is finally ready.

  7. #7
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    Mike,

    Did you save E17 at all?

    Even if you snip it out of the board - if you take care you should be able to still test the device outside of the M8300. It is just a D-type flip flop. You can set yourself up a little test rig with some switches, pushbuttons, LEDS, resistors and a bit of breadboard.

    Actually understanding what the failures are may give you some clue about how they may fail in the future - or what to look for when things do fail for unknown reasons.

    Dave

  8. #8
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    Dave, The output of the flip flop was stuck high and nothing would change it. The data pin could be up or down, the clear didn't do anything. I suspect that the bottom transistor of the output circuit opened up. So, how do TTL circuit age? Could there be some thing that caused both output transistors to come on for an instant? Mike

  9. #9

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    To fix the boards in my PDP8/e and /m I had to change out several TTL chips.
    It seems hard to tell what happens inside the chips. I've read here on
    the forum that several model chips are prone to fail like the SP380 and 7474...
    But I had to replace the 8881 as well. That is a quite simple chip.

    I've seen boards which have been in a non heated storage for years.
    Those had a lot of failing chips. Probably a coincidence, but the most
    plastics are hygroscopic so I wonder if the freezing cold killed the chips.
    Tiny bits of water expand in freezing environment and could crack the plastics a bit...

    At my previous work we have done some tests with plastics and moisture.
    Some plastics even grow a little in size due moisture, this even tore the glued
    ceramic parts from it! Not even freezing here, just moisture at one side of the plastic.

    The hygroscopic sensitivity strongly depends of the used material...

    Regards, Roland
    WTB: RX8E / M8357 and Case for Altair 8800

  10. #10

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    In getting and keeping an 8/I and a PDP-12 running we have replaced LOTS of 7474 ICs, and quite a few 7400, 7440, 7438. We have several tubes of new 7474 ICs because we replace so many. The failed 7474s are from several different manufacturers. If I repair an M216 flipchip with just one bad 7474, I replace them all.

    There was some speculation that the 60s vintage ICs didn't have a passivation layer, the plastic that the chip was made from isn't as moisture resistant as with modern chips, and the seal between the leads and the case isn't as good as on modern chips. All of this makes the chips moisture sensitive. I have a quite a few defective ICs. It would be interesting to get them delidded and inspected so we know what goes wrong.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

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