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