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Help repairing a board with SN74LS123N

Discussion in 'Electronic Basics' started by Dave, I can't do that, Sep 3, 2008.

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  1. Hi people,

    OK, out of my depth here so be gentle.

    I have a 6" x 4" PCB with a display and other stuff on it and it is
    supposed also output a variable PWM signal of 5vdc at 5KHz. Tracing
    back from the output connector, the first chip is a SN74LS123N,
    Monostable Multivibrator.

    It is putting out a square wave but at only around 460mv, it
    fluctuates between 440mv and 480mv.

    Using a Tektronix TDS-210 scope, I am getting a wildly drifting
    frequency ranging from about 3.8KHz to a little over 8KHz.

    Would this indicate the chip is faulty and that by replacing it I can
    solve this problem?


    Could it be something before that chip?

    The chip has 5 caps in it's vicinity and 2 resistors, I am guessing
    that some or all set the 5KHz frequency. Not all of those may be
    connected to the chip but they are close by. It is difficult to follow
    the traces as it is a multi layer board. One of the small blue caps
    with 473M as the only marking looks like it has been replaced as there
    is resin marks around the soldering.

    I know it is probably a question like, "how long is a piece of string,
    " But I am totally dumb when it comes to digital stuff but quite
    capable of swapping out the chip. I live out in the boonies and it
    takes about a week for stuff to get here so if I have to order I'd
    like to get the right thing.

    Thanks for any crumbs you guys can sprinkle.

  2. bw

    bw Guest

    Standard troubleshooting. Is V+ steady at pin 16??
    Flex the board a little and look carefully at the solder joints. Put finger
    pressure on the chip and look for changes.
    Is is a DIP or flat pack, etc, etc
  3. neon


    Oct 21, 2006
    if it putting out some square wave probabbly the chip is ok the problem maybe output loading something is killing it. a 123 is a non resetable mono so shouldn't be any freq. changes. if it is part of a PWM probably its function is as a watchdog timer. i believe the problem is somewhere else. and by all means check VCC if it is stable.
  4. Hi John,
    A single line to the connector.
    As suggested by BW, I will check the 5v supply.

    Thanks a million guys,

  5. Hi BW,
    Yes, 4.92vdc
    It is a DIP-16

    OK, I took this to a friend of mine who is younger and has better eyes
    and steadier hand. {grin} I am 65 and trying to get this thing going
    as a hobby.

    It turns out the track scooted under and parallel to a resistor with
    several other tracks and I picked up the the wrong one coming out. It
    appears the lead from the connector goes to pin 19 of a GAL16V8AS

    I think this is getting too complicated as the tracks from the GAL16
    go to a large Intel P-80C32 so I think I may have to scrap the idea of
    getting this working as there are no schematics available for it. Now
    that I have discovered programmable chips, if any of that is fried
    then there is definitely no chance of getting a replacement as this
    was designed in 1992 and the company no longer has information on it.

    The board is a Universal Laser Systems P/N 12-900-D just on the off
    chance that it spikes a memory in someone who may have a schematic
    lying around. Ha haa.

    Thanks for trying and sorry for the misleading track following.

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