Re: 1977 original apple II restoration (continued)...
- From: "Michael J. Mahon" <mjmahon@xxxxxxx>
- Date: Wed, 26 Apr 2006 13:45:19 -0700
Nama wrote:
So I just got the motherboard back from the board repair shop (they
removed some hacked wires and fixed a couple of damaged traces to bring
the machine back to original condition).
You probably removed a shift-key mod, etc... ;-)
Now it's up to me to actually get the machine working...
Firstly I swapped out all the RAM for some that I recently purchased,
in the hopes that the old RAM was bad (of course I do not know this to
be true). The problem now is that on 'power up' I get 10 vertical white
bars across the screen. Pushing reset does nothing.
Don't worry, the original RAM is almost certainly not bad.
You could have removed all but the first bank (the row nearest the
processor) and tested with 16KB. You could then substitute from the
pool of 16 chips you removed with a virtual certainty of not finding
*two* bad DRAM chips.
It sounds like your processor is not actually resetting.
I spent a good deal of time swapping out ALL the ic's (what a painful
job that is), one at a time (including CPU and ROM's) from a ][+
motherboard last night.
After swapping in and out all the chips I still saw absolutely no
change in the 10 vertical bars.
However, I do suspect that the ][+ motherboard I was using for spares
is also dead, but I figured that the chance of the same component being
faulty in the two systems was slim.
That's a good assumption.
I then swapped all the old ram back in, and only then did I see any
change in the screen. The pattern displayed was exactly the same as it
was befor the hacked wire removal and the trace fix. You can see the
pictures of this screen that I had previously taken here (reset still
does nothing):
http://homepage.mac.com/lord_philip/PhotoAlbum18.html
So basically, after the board fix with all the original ic's and RAM in
place there is no change to the screen as previously seen. Replcaing
the RAM, changes the screen to vertical bars.
So, now I'm stumped. Does anyone have any insights.
I have looked in the archives, and have seen reference to other users
experiencing 10 vertical lines but the solution to this problem is
unclear. Most suggestions recomend swapping the ROM's or CPU, which I
have already done.
What you are seeing (the bars) is a random pattern in video RAM, which
demonstrates that the RAM is at least "sort of" working, and the video
generator is working. The processor is not working, since it is not
resetting. (An early Apple ][ should not require holding CTRL while
pressing RESET, but you might try it.)
Failure to reset is a very fundamental problem. It can be caused by
the /RESET signal not reaching the processor or by the 1MHz clock
signal not reaching the processor.
If the processor is getting clock and /RESET, then it must be unable
to access memory/ROM properly, or the F8 ROM may be faulty. There
are tranceivers in the path to the ROM address lines and the path
from ROM data to the processor. They are three 8T97's and two 8T28's
in board locations H3, H4, and H5, and locations H10 and H11. These
chips can be damaged by inserting or removing a peripheral card while
the system is powered on, or by ESD when it is off.
The fact that you have replaced these chips is a strong probabilistic
argument agains them being the problem, but it might be worth another
try (just for them).
You should also take a *very* close look at the board traces to
verify that there are no tiny cracks or cold solder joints that
could disrupt signal propagation. A low-power (VOM-type) continuity
tester can be useful in this process--do *not* use a continuity
tester that uses an incandescent bulb or more than a single cell
battery!
If these techniques don't localize the problem, I'm afraid it may
be time to get out an oscilloscope and start signal tracing.
-michael
Music synthesis for 8-bit Apple II's!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it is seriously underused."
.
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