John wrote:
Alrighty. I did some googling, found the "Understanding the Apple ][' PDF, downloaded it. I haven't read it much yet, but the board layout has proved infinately usefull. I spent a bit of time with a speadsheet and a Mouser window open, and it looks like I can replace every single 74LS chip in the ][+ (there are 41 of them) for a mere $28.47, plus about $6 shipping. Not bad, and it probably would help the machine's longevity if I replaced them all anyways.
Not necessarily. The probability of failure of an integrated circuit is highest when it is new--"infant mortality"--and is the reason that high reliability types are "burned in" for quite a while. The historical failure data on ICs of this vintage indicates that if they have survived this long, they are very likely to continue to survive. These relatively coarse-scale ICs have no "wearout" mechanisms, and as long as their packages remained sealed, they should be subject only to very infrequent failures. Originally, it was thought that ICs would have a "bathtub shaped" failure rate curve, but data indicates that the "end of life" upswing isn't there--at least for values several times their predicted lifetimes. -michael NadaNet file server for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."