Hugo Holden
Veteran Member
Well that is good.Hi, i repaired 74ls00 s' leg and i inserted on socket but i have always same bad signal on UD2 pin 9!
Time to wait for new 74ls00! I update this thread when it arrives!
Thanks so much!
It confirms that the fault is inside the original A4 74LS00, and was not something like a short external to it.
But what Daver2 and I are both interested in is how that IC failed in the way it has, with interacting inputs.
As you know, one "theory" is that the die inside the chip has lost its +5V power. If that was true A4's outputs would be very defective or absent too.
But it is possible that it is just the gate connected to pins 1,2 and 3 that is defective and somehow the inputs have developed leakage between each other, but this is not common either.
If you can, please leave the scope Ch1 connected to D2 pin 9 showing the bad signal, and scope around on the other pins of this defective 74LS00, for example put the scope CH2 on output pin 3 of A4 so we can see what the output of gate A4 is doing.