Core memory issue resolved by reseating a card

USING AN OSCILLOSCOPE, I TRACKED THE ERROR TO A MALFUNCTIONING INVERTER

The inhibit line is asserted when we want to write a zero in a bit, since a write cycle will otherwise flip all the bits in the word to a one value. The line for bit 13 was not inhibiting properly, thus when we asked to write a 0 in the bit, we got a 1. The machine intended to write a 0, thus it calculated the parity bits for that value; the incorrect bit resulted in a parity check on the next read of the word.

Specifically, this occurred with bit 13 of words in the first 4K of the 8K core memory, pointing at only a couple of cards that were associated with that symptom. When I checked the inverter I found my error source. I pulled the card and reseated it, after which memory passed with flying colors in all 8K with any bit pattern.