Posted By
TLC
on 2021-07-12
14:34:05
|  Re: 7501 & 8501 Again
@PaulK : possibly you have a pair of "partially working" (or, partially bad, whichever you'd prefer) chips. Sometimes an internal defect only manifests in just breaking some parts of the chip, rather than rendering the whole part defective. I also have, for example, one 8501 chip, which irregularly fails to drive one of the outputs of the onboard port. I've also had a dozen of TEDs with various nuisances; one that had an attribute bit stuck so everything was just always shown in colours; another one that failed to blank the upper and bottom 4 scanlines of the screen area on clearing bit 3 of $ff06 ( ---> vertical smooth scroll always showed garbage in the upper and bottom part of the screen), and so on. Sometimes this happens. Symptoms can be completely random  , according to how the partial defect has manifested. It may sometimes only affect some analog property, who knows. This is not a general difference of "the" 7501 and 8501 chips. To know more, you'd have to test the chips in some known-to-be-perfectly-working machine.
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