20 Sep
2013
20 Sep
'13
7:16 p.m.
On Fri, Sep 13, 2013 at 12:17 PM, Ethan Dicks <ethan.dicks at gmail.com> wrote:
Hi, All,
A list member recently sent me a broken Mac128 logic board...
So for anyone following this saga at home, I fixed the "jail bars" by
locating and reparing a hidden busted trace from the DRAM at G8.
The next result was either a 04114E or 03114E Sad Mac code
depending on which ROM set was installed (original 64Kbyte
ROMs or "enhanced" 128Kbyte ROMs that came with the Plus
and were installed in upgraded 512Ke Macs). I grabbed FDisadm
and vMac and figured out what the ROM checksum and RAM tests
were doing - something I learned was that the tests are *different*
in the two ROM versions, something not mentioned in the
"Sad Mac Code" page that has been handed around for decades.
In particular, the reason that the original ROMs return 04xxxx and
the enhanced ROMs return 03xxxx is that the newer ROMs
left out one test (and do it slightly differently, vis-a-vis table-
driven vs calculating alternating and grouped-bit test patterns),
though I suspect that the actual values written to RAM end up
being the same, they are just sourced differently.
So... at first, I found a lot of tales on the Web, mostly in mailing
lists and forums (fora), of 04xxxx codes that resisted DRAM
replacement. Single-bit 04xxxx errors were corrected by replacing
the indicated chip, but when 4 to 6 to 8 bits were claimed to be
defective, replacing RAMs (and up to 50% of the TTL on the
boards) was claimed not to work. I went along with that, because
after all, it's reasonable to have 2 or 3 of 16 RAM chips go when
these particular (Apple-badged) chips have documentably failed
in countless machines. What's less reasonable is to have 1/3
to 1/2 of the DRAMs fail. Well... not finding any other obvious
causes, I translated 04114E to a table of DRAM locations
and tried piggybacking known good chips. Of the 6 bits reported
as defective, I got 3 or 4 to work by sticking a good chip on
top and firing the Mac up. So I plan to replace those with
NOS replacements (I have tubes of 4164s from the COMBOARD
days) and continue to debug things. The parts won't match,
"devaluing" the board, but at least it will work.
That's the present status - original fault fixed, and secondary
fault halfway diagnosed.
Thanks for everyone's input. I'm confident I can get this
original Mac returned to its 1984 glory in time for its 30th
birthday in 4 months.
Cheers,
-ethan
I know that Apple-branded DRAMs of this era are dodgy.
I could just
clip them all out and replace them with fresh, *tested* chips (I have
a DIP DRAM tester), but I am concerned that the fault is elsewhere
since a known-good DRAM always produces a stuck-on bit (the "jail
bars"). That smells to me like a bad gate upstream of the RAM field.
Looking at the available schematics, unbuffered DOUT D0-D15 go several
places - a pair of '244s to buffer the data bus, a pair of '166 shift
registers for video data out, a pair of '161s for sound data out, and
a 16R8 PAL named ASG that appears to be related to sound and diskette
PWM. The DIN side of the data bus connects to the '244 bus buffers,
the CPU, the ROMs, the IWM, the 6522 VIA, and the Z8530 SCC. I would
think that if that side of the bus had a stuck bit, the CPU would be
fetching bad data from the ROMs, so there wouldn't even be a Sad Mac.
I can do some catastrophic repair by blindly pulling all the RAM and
replacing it, and I can even pull the CPU and socket it so I can stick
my Fluke 9010A on the board and do some serious memory tests and
exercise the bus and poke at it with an oscilloscope. What I'm asking
for at this point is any information that can help me target my
efforts. It seems like this sort of fault would have been common in
the day, but mostly when I google "Sad Mac" or "jail bars", I get
stuff about replacing individual RAM chips or changing SIMMs in an
SE/30.
So anyone have any advice that could help me not remove and replace
320 solder joints on a multi-layer board? Yes, I can do it, but it's
a lot of time to do it well. I'd just like to get this old Mac
working with an original board (I have working 512K boards, but the
point is to have an Original Mac in time for display next January).
Thanks for any suggestions,
-ethan