Suspect slightly tarnished chips changed out on A board; EDUC-8 now working properly

LOOKING FOR SLIGHTEST HINT OF COLD SOLDER JOINTS

I found a few chips that were much better than the ones I had replaced on other boards, but these were the very chips involved in the logic chain that clocks the accumulator (AC) shift registers, where I was seeing the erroneous behavior.

I pulled them, cleaned the leads of two using a method suggested by Gwyllym Suter, and put in a brand new chip in place of the third. The solder joints looked perfect so I restacked the card into the EDUC-8.

MACHINE WORKING PROPERLY

The bad behavior where the AC was being wiped out by instruction fetch activities was gone! I entered quite a few instructions into memory and ensured that they produced the proper results. I then toggled in the test program suggested in the article, which clears the accumulator and then counts up to 256 before halting. 

It worked just as it should. The machine is, as expected, painfully slow in operation but working just as it would have if I had built it in the 1970s from the Electronics Australia magazine articles. 


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