Apple II+ repair details
jws at jwsss.com
Mon Nov 30 16:20:08 CST 2015
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:
> Terry (Tez)
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.
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