Bad chip that I had already replaced - 74LS08PC

TRACING A DIFFERENT PROBLEM

I found my 'ready to move' latch on the control board being forced to reset but as I traced the signal back I discovered that it was due to a NOR gate U4G producing a low output with two low inputs. Rather than immediately pulling that chip, I looked for any evidence that the traces were being shorted to ground.

This NOR gate output hooks to two other chips, one of them is U2J that I had previously double replaced (first incorrectly and then to put in the right NOR chip). I looked very closely at the traces to see if my desoldering and resoldering had somehow shorted the trace to another one, but didn't spot anything.

Based on a hunch, I snipped the lead for pin 12 of U2J chip from the board and bent the leg up. It was still reporting a short to ground! That means I have a bad 74LS08PC chip and would need to replace this chip site for a third time. I fired up the board with that leg lifted to verify that the U4G NOR gate is giving proper outputs.

This time, I installed a DIP socket so that I never have to resolder again as the traces are quite funky after all the desoldering. That allowed me to beep out the connectivity and install rework wires before I had a chip in place.

I found that pins 1 and 6 should be connected but the trace soldered incorrectly onto pin 2. This required a small bit of rework under the board beneath the socket. I then had to address some misrouting of traces for pins 11 and 12, but once it was repaired, the board was ready to have the hopefully working 74LS08PC chip inserted into the socket.