I have been out of touch for a few days while traveling and helping the teams at TechWorks! in Binghamton with their 1401, 1440 and other IBM gear. Two way exchange of advice and ideas plus a chance to see some historic IBM and other gear. One non-computer example is part of the lunar module simulator used by NASA to train the Apollo astronauts for their landings on the moon.
We are currently chasing a problem in the 1401 system here which causes Add instructions to inject a 1 bit into any result. If the result of the added characters already had the 1 bit set (was odd) then no harm, no foul. However, if the result was even, the added bit causes a parity error in the work going into memory.
The signal only passes through a few gates from the point where the adder converts the qui-binary result into BCD, with a signal called Arith 1, to the point where it drives the inhibit line on the 1 bit core planes. The way core works, inhibit lines must be active to keep a core from flipping on, leaving it at the zero state created during the readout of its previous contents. If no inhibit, then the core is set to 1 at the end of the memory cycle.
Our tools were inadequate to watch the signals and find the spot where it is failing, but we will have access to a more modern storage scope tomorrow when we hope to find and eliminate the problem.
We will also help inspect and clean some 1311 disk drives, as well as one of the 729 tape drives for the system. We have already done some inspection of their 1402 reader/punch and are finding the part number for one missing rubber belt.