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Discussion in 'Electronic Repair' started by Myauk, Jul 17, 2008.

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  1. Myauk

    Myauk Guest

    Hello All,

    I am trying to repair a pressure alarm unit, the pressure transmitter
    will send 4-20 mA to the unit. The unit receives the current and let
    it pass through a resistor (178 ohms) to convert to voltage, then it
    uses an op amp-amplifier stage to amplify the signal across the
    resistor. The output of the amp is coupled to comparator circuits.
    The output of comparator circuits is connected to SR NAND LATCH.

    The Reset input of low active SR NAND LATCH circuit is pulled high by
    a resistor and a capacitor. (Power-on reset)

    The problem i found in the LATCH circuit of a typical alarm card is
    that when the Set input is low (as the fault "low" signal comes from
    the comparator unit), the circuit has high output which drives the LED
    light, but when the Reset input is low (i connected the reset intput
    to ground to give it a low signal and I also made sure that there is
    high signal for "Set" input), the output does not go low.

    But, if I add an additional resistor (1k ohms) between the output and
    ground, the latch circuit works normally.

    On the other hand, i fear that adding 1k ohms resistor at the output
    might affect other operations of the alarm cards when it is installed
    in engine control room of the ship.

    Any suggestions?

    Best Regards
    Aung Ko Ko Thet
  2. Franc Zabkar

    Franc Zabkar Guest

    I've had to add buffer amps to compensate for faulty digital gates in
    unobtainable ASICs, so I think I understand what you are trying to do.
    What is it about your SR NAND latch that prevents you from replacing
    the faulty gate? Is it part of a custom chip?

    - Franc Zabkar
  3. Myauk

    Myauk Guest

    Thank you so much for your sharing

    Actually, it is just a 4093 IC using all four gates inside it, with
    resistors and capacitors mounted around it. It is easy to replace the
    gates if they are faulty.

    I have some experience in power supplies and PWM circuits but trying
    to repair the alram cards is new experience.
    This is my first time trying to repair the alarm cards.

    Can I assume that it happened because of the faulty gates?
    I will try replacing the gates today.

    Can anybody suggest me some idea for testing such SR NAND latch

    Best Regards
  4. Franc Zabkar

    Franc Zabkar Guest

    I have repaired one alarm with a faulty CMOS gate. That's the limit of
    my experience. :)

    Coincidentally, my faulty part was also a 4093:
    I would think that CMOS inputs would be more prone to damage because
    they are connected to the outside world.
    Your testing method seems fine to me, ie apply all combinations of 1s
    and 0s to the inputs and test for the correct outputs.

    - Franc Zabkar
  5. Myauk

    Myauk Guest

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