Re: 11/40 misbehaviour

From: alphadoc (all.the.fun_at_the.fair)
Date: 08/24/04

  • Next message: SCC: "Re: VAXstation 3100 strange-ness!" 
    Date: Tue, 24 Aug 2004 19:31:26 +1000

    I am very sure I have some 8881 chips etc, that I scrounged years ago. They
    are new 'very old' stock. Please post back on this newsgroup if you have to
    replace chips and are looking for a source.
    Thanks.
    RHB wrote in message <221a1b70.0408120649.19421c51@posting.google.com>...
    >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: SCC: "Re: VAXstation 3100 strange-ness!"

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