The output pin of 74HC138D was bad often,what should I do?

[ketian]

Like the following picture,    the cs-1,cs-2,cs-3,cs-4 has one load for each ,We add a  2000 ohm pull-up resistor near to the output pin. But factory report that there are many output pin of  74HC138D keep high level all the time, we must change another one to repair this question.

I want to kown ,whether the pull-up resistor make it? or other wrong at there?

[Bulat]

Everything is ok in your scheme. To answer the question "what is wrong" you need to investigate the faulty boards. Recommended points to check::

1. All pins of the 74HC138D are really soldered. In other words there is a good contact between each pin and PCB pad.

2. Pin #1 of the 74HC138D  is at correct position.

3. No accidental shorts between neighbouring nets on the PCB.

4. Function of input signals of the 74HC138D. To get an output 'low', you need a proper combination of A0, A1 and E1/E2 inputs.

Regards,

Bulat

[ketian]

We received the report that feedback 74HC138D was wrong after them work a long time.so ,

1) it  should be soldered good;

2) all pin should at correct position;

3) when we change another one can repair the question ,so,the accidental shorts should not be exist;

4)  The pcb suit for the scheme,so,the function is ok!

When we analyze the bad sample ,we find one of output pin can not give a low signal ,but not all pin.if we change a new one,there is no wrong,all the pins can give the low signal.