>Peter Coghlan dropped me a note privately, saying the signal of the F8 ROM could also be caused by the pin not connecting properly. The replacement ROM could have had been sufficiently >different in that the legs were at a slightly different angle etc. This would also make sense, as I couldn't understand why their wasn't anything on the earth rail (and why the PSU didn't shut >itself off).
Incidentally, I did check when the machine was off to see if that F8 earth pin did connect to ground. It appeared to, but then I was putting some pressure on the pin when I was taking the measurement. It might have been enough to force a connection in the socket. Terry (Tez) On Tue, Dec 1, 2015 at 12:22 PM, Terry Stewart <te...@webweavers.co.nz> wrote: > Thanks for those comments Jim, Yes, something to think about. > > Peter Coghlan dropped me a note privately, saying the signal of the F8 ROM > could also be caused by the pin not connecting properly. The replacement > ROM could have had been sufficiently different in that the legs were at a > slightly different angle etc. This would also make sense, as I couldn't > understand why their wasn't anything on the earth rail (and why the PSU > didn't shut itself off). > > So, there could be a socket problem still lurking which may come back to > haunt me later so I'll check it out at some stage (In fact, I might just > replace the socket). I'll have to fish that F8 ROM out of the rubbish bin > and try it in another working Apple board. I did put it back in the first > board at the time just as a double check and got the same result as before > so I concluded it was toast. > > Terry (Tez) > > > On Tue, Dec 1, 2015 at 11:20 AM, jwsmobile <j...@jwsss.com> wrote: > >> >> On 11/30/2015 12:18 PM, Terry Stewart wrote: >> >>> Speaking of Schrodinger's feline, here are details of my recent Apple II+ >>> repair for those who might be interested: >>> >>> http://www.classic-computers.org.nz/blog/2015-11-29-more-repairs-to-my-appleII+.htm >>> >>> Terry (Tez) >>> >> >> Terry, >> nice dialog on your repair job. >> >> On the last comment about the ground pin of the defective rom having a >> signal, if the apple board is a 4 layer board the ripple from the short to >> the internal signals from address current, or other signal current being >> propagated to the ground pin, I suspect the resistance in the pin itself >> may have provided the needed high resistance to show the signal. Unless >> you scrap the ground solder protect off and look at the voltage out in the >> actual ground conductor, I suspect the voltage went down to a very low >> level very close to the pin. >> >> Also where were the decoupling capacitors located with respect to the >> pin. I suspect that might have gotten rid of more of the voltage, but they >> were probably nearer the Vcc end of the chip. >> >> If you can track down the schematic, it might be that your missing pin >> doesn't do much unless you perform some special operation, such as some >> controller addressing or memory operation or such that you don't normally >> do. It may have also had a fit to the other part of the pin if it was >> present in the socket to actually work. I didn't hear if you found that, >> or maybe it fell off when you pulled the chip out? >> >> I suspect the short developed from your theory about stress, or perhaps >> the chip was programmed by a bad programmer. We had a programmer that we >> found developed a tendency to program eproms and like programmable chips >> and it destroyed the chips capability to actually reach ground again. >> >> The programmer made chips that verified, but when you ran them in a >> circuit and probed the lines with some sync to the system clock, rather >> than seeing the signals on the data lines going to zero on the datalines, >> you could see a hodgpodge of crap at 1.5 to 3 volts which is TTL la-la >> land. The chips programmed in such a programmer as a properly working Data >> I/O had clean lines as did reference chips from years earlier. >> >> Due to the fact we didn't program many chips, and I found a cheap >> programmer to hook to a PC, we never found out what broke in our programmer >> (which was a home design admittedly). But it was build to standard, but >> had something happen to start killing eproms. So that sort of fault may >> have been induced in your chip and got bad enough to kill off your Apple >> some years later. >> >> thanks >> Jim >> >> >
