Mac Board Swap

Over the course of building and maintaining an elementary school computer lab for the last six years, I've encountered just about every pre-PCI Mac, either through buying cheap ones at swap meets or having them donated. Inevitably some of these machines had bad power supplies or bad logic boards. Some were kept for their working floppies, and some had custom huge hard disks, while others were little more than a working logic board in a busted case.

Having all these parts machines lying about, I began to notice similarities in power supplies and logic board arrangement between otherwise completely different computers. The first time I noticed this was when I had a Quadra 700 and a Quadra 650 torn apart on the same table in the back of my room ("the shop"). The logic boards looked very similar. They had the securing holes in the same places, the power supplies connected at the same spot, even the ports (SCSI, ADB, serial, AAUI) on the back all lined up. Since I already had them apart, as an experiment I tried the 700 logic board in the 650 case. Lo and behold! Ding! Happy Mac.

This led me to compare other logic boards, which in turn gave me some insights into the evolution of the pre-PCI Macs. For instance: the IIcx sized logic board lasted seven-and-a-half years (3/89 - 9/96) through four different case changes. Obviously the Q700 logic boards were the same size as IIcx's, and they used the same power supply, but logic boards that fit the IIvi case (Q650, PM 7100) were the same size and shape, too. So were those in the Q800 (PM 8100) case.

So if you have a bad logic board in, say, a Q800, and you also have a working IIci, you could make a working computer with room for a built in CD-ROM drive out of the two. You could consider this a case upgrade for the IIci or a logic board downgrade for the Q800. You would have to do some dremmel work on the case's back panel to get to some of the ports, but it would work (at least till you could find a 8100 board to put in there).

Below is a table of pre-PCI Mac logic board "form factors." Please be careful if you try to do some of these swaps. Just because the logic board is the same shape, don't assume the power supplies are the same too. For example: even though the Performa 6360 uses an identical looking outer case, you can't put its logic board in a Quadra 630 case, because the 6360 board requires a heftier power supply. The 630 board might work in the 6360 case, though.

A similar case may not be a guarantee the logic boards will even fit. The LC 575 and LC 580 have an almost identical case design, but the LC 580 logic board is actually a member of the IDE friendly Q630 family, not the SCSI only Color Classic sized boards of the other 5xx's.

In the table, the main model names (Mac II, Centris, Quadra, LC, PowerMac) are used, but there may be Performa variations not mentioned.

Something struck me after I made the chart. The Macintosh IIsi design was never used again for anything else. Its power supply, case, and logic board are unique among Macs. Was there ever another desktop Mac design that was only used once, without even any upgrades?