On Jul 17, 2:15*pm, Franc Zabkar wrote:
On Wed, 16 Jul 2008 18:25:08 -0700 (PDT), Myauk
put finger to keyboard and composed:





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

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
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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
circuits?

Best Regards