On 19/06/2020 4:00 am, Francesco Messineo wrote: > On Thu, Jun 18, 2020 at 9:49 PM Gerrit Heitsch > <gerrit_at_laosinh.s.bawue.de> wrote: >> Yes, but often, if you have a dead C64 with a lot of 77xx chips on it >> it's one of those that is the problem (unless it's the PLA, but that is >> also known as '7700-xx' or '8700-xx'). > I've repaired dozens of C64 :) > Once I've found a C64 that had almost all 77xx logic chips and one by > one I had to change them all to revive it. > I don't desolder parts before making sure they are really bad usually. > I've never said they're reliable, but they aren't either all failed by > now for sure. > > What is really "disturbing" in the MOS reliability issue is that it > appears to depend on the part number too. > Either some parts were made in different farms, so the less reliable > farms were producing the less reliable part numbers > or it was heat-related maybe (like the hotter parts failing more often)... > I tend to believe it was a "farm" issue. > > Frank > Sorry Frank my previous replies were meant for the list not just you by email, apologies for that. C64 DRAM has a high failure rate but that's true of DRAM generally in all my vintage equipment these days it seems (yes some brands fare much worse like mT). I rarely had to replace was the 77xx logic on any Commodore computers though, I would say about the same rate as for other systems that used 74XX logic. 74LS245's are a relatively common failure item for instance. Not really surprising given the age of the equipment, it's to be expected. Last week I had to replace a SGS Z80 in a Microbee due to sudden total failure and I've seen a few Z80 failures from different manufacturers on different systems in recent years. The failures vary, anything from a stuck address line to complete and total failure. I've had to replace mask ROM's too in quite a few systems, they don't seem to have aged well. Environmental factors may well be a factor, IC's don't need to be use to be damaged - certain conditions may contribute to a higher than normal failure rate too. It is however my opinion that power supply ripple, not only over voltage is quite capable of damaging IC's in the long term including DRAM and C= supplies were notoriously rubbish back in the day and haven't gotten any better with age. That's not to say that it is the reason why you are seeing high 77xx failure rates of course but I wonder why my experience is different to yours. Perhaps I've just been lucky. Has anyone else here seen higher than normal failure rates of 77xx series logic vs their 74xx series counterparts?Received on 2020-06-19 12:00:02
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