Re: 11/40 misbehaviour

From: RHB (dcs8506_at_bellsouth.net)
Date: 08/12/04

  • Next message: Rich Jordan: "Re: Help with SBB in BA353" 
    Date: 12 Aug 2004 07:49:34 -0700

    Rich Alderson <news@alderson.users.panix.com> wrote in message news:<mddvffp2oe4.fsf@panix5.panix.com>...
    > dcs8506@bellsouth.net (RHB) writes:
    >
    > > Do you have a scope to look at levels on the unibus ?
    >
    > There is a scope available, yes. Please teach me to fish: What am I looking
    > for?
    >
    > > Does the system run but die intermittently or is it flat out dead ?
    >
    > Runs intermittently. Was running for hours at a time, now dies in under 2
    > hours.

    The old DEC field service procedure on tough problems usually
    entailed:
    1. Check all the fans above each P/S, H7444, H745,the small one above
    the 54-9728 regulator plus above and below the boards. Make sure
    they are running.

    2. Check the power supply voltages at the CPU backplane, memory,
    device backplanes not at each regulator. Believe pins A2 was +5VDC and
    C1 was -15VDC. They mite look good at each power supply but be lower
    at the backplane pins due to the cable loss effect.

    3. Un-interleave the memory and swap the starting address among stacks
    and see if symptoms change.

    4. At the CPU backplane unibus slots A & B hang a scope probe on the
    Unibus address pins and look at the "1" and "0" levels and the
    quiescent levels. The old 8881 and 380 driver receivers chips ( I
    think that was the part numbers) would deteriorate over time and hold
    down the "1" level to -2vdc or less and cause traps to 4, etc. Or the
    "0" level would not be 0 VDC but maybe -.8, etc.You can compare know
    good signals to suspect ones. I can get back to you on exact pin
    numbers to check as I don't have it right in front of me. If you find
    a suspect signal you then pop driver/receiver boards out till the
    suspect level goes back to normal.

    5. Obviously shorten the bus as previous posts suggest.

    6. Check and clean dirty contacts as previous post suggest.

    7. This mite be a stretch but here goes....back in the days of tubes
    and discreet components one the procedure was to run the intermittent
    failing program and lightly tap a few times on the board handles with
    a screwdriver and see if the error occurs. Many a bad board was
    detected this way without diags. Step 2 of this procedure was to use a
    heat gun or hand held hair dryer and blow hot air on suspect boards.
    Step 3 was equally or more bizarre and will detail that later if need
    be.

    Granted this might seem like a fishing expedition but after 30+ years
    of fixing PDP's these worked and still do. Good luck.

  • Next message: Rich Jordan: "Re: Help with SBB in BA353"

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