Wednesday, July 29, 2015

This and that on 1401 and Pertec drive

I spent time at the Computer History Museum helping to work on a problem with one of the two 1401 systems - spurious validity checks when the card reader encounters a group mark character. This is quite a devious problem, We haven't gotten to the bottom of it yet, but did solve another issue where a shorted transistor pulled an output line that should swing -6 to +6 instead drop to -12V. That transistor was replaced on its SMS card but the wider validity check issue remains.


It appears the 7493 counter chip for the +5V Timer Board might be the actual bad one, since the counter is now ignoring its reset lines and is counting up to shut down the drive after 5-6 minutes. Since the reset line is hard wired to ground, it should stay in reset.

I did only a couple of short tests tonight, but will get back to testing tomorrow when I have more time.

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