Apple II+ repair details
terry at webweavers.co.nz
Mon Nov 30 17:52:41 CST 2015
>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.
On Tue, Dec 1, 2015 at 12:22 PM, Terry Stewart <terry at webweavers.co.nz>
> 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 <jws at 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:
>>> 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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