The upgrade card plugs into a socket that replaces the MC74F253NDS
chip in position F3 on the logic board. The upgrade card also has a single
in line plug of 7 pins that fits in matching socket at position E3. This
E3 socket is adjacent and in-line to a 10 pin resistor array. The upgrade
card has two unidentifiable TTL chips: one at E3-4 (adjacent to the 68K),
the other at F4 (adjacent to the F bank of RAM). The upgrade card also has
two resistors between rows E and F.
The ROM sockets appear to have been original to the logic board,
because solder work matches the rest of the original components. The ROM
part numbers are: 342-0220-A (ROM LOW) and 342-0221-A (ROM HI). Both are
dated 1984 and copyrighted 1983, Apple.
For this fat Mac, I beg to differ with your opinion of the analog
board as being the source of the problem. As in a previous post, the
solder job was shoddy. I'll wait until I have a second Mac for spare parts
before I work on the analog board. My source of RAM chips is from 8088-
and 20286-class computers. These motherboards have from 256KB to 1MB
socketed.
Could you send me the 'digibarn' picture privately? Email me off list
if you'd like a photo of this board.
Regards,
Edwin
At 01:02 PM 3/20/05 -0800, you wrote:
>O.K., here's the thing ... I have a 128k that sounds like it had exactly
>the same upgrade yours did. Though, it sounds like yours went a step further
>and added the Plus ROMs to make it a 512ke. -- not sure where your extra
>circuit board went though -- it's also possible that it was upgrade past
>512k of RAM and the add-on board was designed for that purpose. I have a
>"tiny" circuit board (raised on pins so it sits just above the 68000 chip)
>with ONE chip in a socket marked: 74F253 PC 11445, and a 2.2k ohm resistor
>that sits at right end of the 68000 chip (see pic below -- where is yours).
>But it doesn't replace anything, it was added to the 128k board in the only
>open leads on the board (which was originally designed for 512k in the first
>place). I find the following picture extremely helpful in talking about the
>logicboard mods as there were so many different 3rd party hacks that done:
>
>digibarn
>
>ANYWAY, I don't think the logicboard mods are your problem. I have 2 128ks
>and a 512k all but one of the 128ks has been modified and all of the
>modified boards work in my "good" 128k chassis. The other 128k and 512k
>exhibit different problems, one is the exact checkerboard pattern you
>describe without a startup "chime". The other does nothing and starts to
>smell like burning electronics and "chime" a lot if I don't turn it off. As
>I said, the logicboards have nothing to do with the problem. I would say
>your problem is with the Analog boards which were notoriously plagued with
>problems. If something goes in a compact Mac, it's not likely to be the
>logicboard, even if it has an "unofficial" mod (Apple did not condone these
>3rd party solutions -- they would just as soon swap your logicboard and
>charge you twice as much or more).
>
>Anyway, I don't have all the links at my fingertips, so I'll just tell you
>that you can do a search online for analog board repairs (there are two or
>three excellent sites), which eventually I'm going to visit myself so I can
>get these old boxes back in service. BTW -- my "good" 128k exhibits symptoms
>of the checkerboard screen if I turn it of and on too fast. Fortunately a
>slow power-off returns everything to normal. So I imagine this is a
>harbinger of things to come, so I'll be looking up the sites myself and post
>them when I do.
>
>Oh, also, the 1256-15 chips: where do you get them? I've been toying with
>experimenting with an upgrade to see how far I can take the RAM in those
>sockets -- obviously when they de-soldered the original 128k RAM they put
>those sockets in for an upgrade or they would have just re-soldered the 512k
>RAM directly to the board. Also, what is the difference between the 1256-15
>and -12, -10, 64-12 chips? If it is actual RAM capacity, then they may not
>work with the resistor board that's been added to provide the additional
>power for the added RAM tailored for the -15 chips.
>
>Good luck.
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