[Premium-Rx] WJ8888 - Strange fault

[Danny Higgins]

Richard,

You haven't got an open circuit CMOS input due to a dry joint/PCB
crack/faulty plated through hole/open circuit resistor?  I've seen weird
faults where the circuit changes as your hand moves around, due to an open
circuit CMOS input.

Danny  

-----Original Message-----
From: premium-rx-bounces at mailman.qth.net
[mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of
RichardReich at aol.com
Sent: 08 February 2013 17:44
To: premium-rx at mailman.qth.net
Subject: [Premium-Rx] WJ8888 - Strange fault

Hello all.
 
I am in the process of restoring my WJ8888 to working function, and I have
the comms datastream up and running again.
 
However, there is a strange fault  - which has taken some finding  (weeks).
 
There is one particular CMOS logic device (2-input NOR gate) that will only
work with the scope probe attached to its output pin. If the probe is
removed, the device slowly stops working after about 30 seconds.
The data becomes spikey, and then just fades away - as if the chip is slowly
running out of gain.
 
Once the probe is replaced, the device works perfectly once  more -
instantly. The scope does not have to be attached to the probe  to have this
effect.
 
I have changed the chip (twice), and the one after it (twice) - it  makes no
difference - the problem persists. Its almost as if the chip now  requires
some capacitive loading to work at all. I am reluctant to add a  cap which
wasn't in the original design to fix this.
 
The circuit is not fast (a few kHz).
 
I wonder if anyone else has seen anything like this - and has an explanation
or solution?
 
Never seen anything like this behaviour before with logic devices. 
 
 
Thank you....   Richard
 
 
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