Anyone know what this is?

[gavo]

Hi,

I found the following interesting (to me anyway) little hack on the 128k motherboard of the US mac that I just blew up. My other 128k board doesnt have it - the board looks to have been added post-factory. I was just wondering what it was / does - looks like some kind of fix.

http://www.digitalvault.co.nz/mac/128-p1.jpg

http://www.digitalvault.co.nz/mac/128-p2.jpg

Cheers,

 

[gobabushka]

what markings does the chip have?

 

[Mac128]

That is a classic mini-mux board required for decoding 512K RAM. Note the RAM chips are 256 variety, so this 128K board has been upgraded by a third party to 512K RAM. This was more or less how the engineers left the back door open to make the Mac more powerful than Steve Jobs would let them.

The most surprising thing about this upgrade is that the new RAM was soldered in place of the old rather than adding sockets as most of these upgrades used. However, this one does have the resistor upgrade at location D15 without which the 128K board could sometimes hang.

I have never seen one mounted vertically like that. Most are top and bottom. Either way, it taps into the circuitry designed for RAM expansion. Thanks for the picture.

This one uses the same type of multiplexer mounted in the same location but expands the RAM to 1MB.

http://68kmla.org/forums/viewtopic.php?t=4182

And just for fun here is a path trace of how the minimux interacts with the RAM and CPU:

http://68kmla.org/forums/viewtopic.php?p=23300#23300

 

[gavo]

Thanks for the info Mac128. Actually, now that I look at it more closely I see my board also has the 128k ROMs and what I assume is a 800k internal drive (its certainly not the old 400k internal I'm used to seeing) so its obviously gone through a major upgrade at some stage.

Cheers!

 

[Mac128]

gavo, do you have a close-up picture of the back of that mux board? (i.e. p1.jpg) I'm guessing it's all traces? Put one up if you can ...