Getting to the root cause of the strange adding on the PC/Adder card - tarnished chip leads

USED SOME NEW-OLD STOCK CHIPS WITH SIGNIFICANT PIN TARNISH 

One of my objectives building the EDUC-8 was to leverage the large supply of chips I had, resulting in a low cost for the project. Quite a few of the chips I have had developed major tarnish on the pins. They tested fine on the Retro Chip Tester Pro so I decided to put them into the PCBs.

I had some incorrect behavior and once I began to scope lots of signals, I found some that were not correct as generated by tarnished chips. The chip tested good beforehand, so it has to be the solder joint itself. 

DISCOVERED THAT THEY WOULD NOT SOLDER RELIABLY

I had attempted to overcome the problem by flooding them with flux and heating them longer than usual. Some of the joints looked a bit suspect but I stuck to my guns because I still have the low-cost objective in mind.

What occurred in the solder joint was great flow and surface tension to the pad of the board but a very thin layer of oxide insulating the pin itself from the solder. There was a channel that the pin sat in, which wasn't conducting properly. 

PULLED ALL THE SUSPECT CHIPS, WAITING FOR NEW STOCK WITH GOOD PINS

I decided to unsolder any of the chips whose joints had the characteristic appearance that might signal an oxide protected channel inside. The more tarnished the chip was, the easier it fell out of the board after I removed the solder. 

New chips are on their way and once installed, I can get back to the project. 

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