🚀 go-pugleaf

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gould!jhagen (Jarom) asks what to do with a 128K
mac. My upgrade experiences may be of help to him and others.
Although the Dr. Dobb's upgrade has been around for a while
I think my upgrade was an improvement over the kludge recommended
in the article (Jan 1985). They ask you to snip the old mem out,
DO IT! even with a desolder station you will not save those rams.
The worst part is the select logic, what a kludge, they wanted you
to solder directly to the first 74f253 (Dual 4-1 Mux), I say, don't
Buy 2 74f253's snip and desolder the one there and install a solder
tail to wire wrap pin size socket, similarily install an inline solder
tail to wire-wrap socket. Point is you can now wire-wrap the select 
logic to a proto board and plug it in, and always able to revert to 128K
of memory. (Testing code etc.) The proto board with chips will just 
fit back under the frame of the mac. You have to bend the mother board
slightly as you go. REMEMBER there is an error in the article, while
they correctly show in the picture pin 7 of the new mux connected via a
resistor to Dram pin 1 they wanted you not to connect pin 8 to pin 8 
of the original mux, connect them or you will not be joining GND's.
Certainly DO NOT join 7-7 or the Z outputs will be shorted to each other
and you will get a 05 FFFf (Address uniqueness error) on boot.
The wire-wrap solution is clean and easy. since I havn't destroyed
my board I am contemplating 1MBx1 on a small board. See below.


Cost of project: 
	JDR Microdevices:
		16 41256 150nsec( same cost as 200!) 2.95
		16 sockets	.17 each
		2 wire wrap sockets	2.00 each
		proto board etc....

		total ~$50.

Question: Can I add SCSI if I get the 128K ROMS and hack a bit more OR
are there major changes to board layout?

Now for the REALLY brave, pull the 68000 and drop a pin grid down to a
sub-board with an '020 on it, you will have to cut the case.
With appropriate PLA's you can time the sub-board to access on board ram
at say 15Mhz 0 waits and time longer for accesses to the screen ram above^.
Not to mention a 68851. We speak of ~$500 in parts and $aleph0 for your labor.

Anyone needs the article from Dr. Dobbs or help let me know. 

Hedley.

{decwrl|sun}!imagen!hedley

(408)-986-9400 x423