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Old 10th Dec 2020, 7:31 pm   #157
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 8,625
Default Re: I found it! A very sorry looking MK14.

...Not good news on Buzby's new-OS PROMs unfortunately.

It turns out that the 'darkened' IC ('2o') is actually the good one, that reads / verifies perfectly against the code file I have here. But when I read '1o', the first 256 nibbles read correctly but every address from 100-1FF reads as 'F', so that is looking like a failure of the internal address decoding logic to me. I've done basic checks like resistances of all pins to the VCC and 0V pins but A8 on the dud IC reads no differently to pins A0-A7.

The only noticeable difference is that where '2o' has a resistance of >6M between its supply pins, '1o' has a resistance of 171K.

If it had been a case of finding a single bit stuck low somewhere I would have tried overprogramming but I don't think there will be any benefit to doing it in this case, where clearly the whole upper half of the IC has gone Missing In Action.
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