cy384 | The Ultimate Centris 650

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2021-07-31 23:30:05

I've been getting into software development for the classic Mac OS, and wanted a pre-PowerPC machine to play with. The Centris 650 (or the slightly later Quadra 650) is one of the last machines of the era, but also cheap, expandable, and hackable. As a bonus, it's capable of running the final version of A/UX. This page will document its current state, link reference material, and track plans for further work.

The Centris 650 runs a 68040 at a wimpy 25 MHz (or, if you're unlucky, a 68LC040 with no FPU). Mine came with a XC68040HRC25M, mask revision E31F, a 1993 chip manufactured on a 0.8um process, rated for 25 MHz. If we want to overclock, ideally we want a fastest-rated, latest-revision chip. Since the 68040 was sold from 1989 to 2005-ish, it underwent many updates and die shrinks; the final one, which runs the coolest with the most headroom, is the L88M mask revision. Unfortunately, these are pretty rare (I blame the Amiga fans), and any you see on ebay are probably fakes (usually older, relabeled chips). I ended up with a 40 MHz MC68040RC40A, mask revision E42K from 1999. Swapping in the new chip is fiddly but straight forward.

The clock speed is determined by the oscillator between the CPU and the closest Nubus slot, which is then multiplied by two. The stock part is a fairly large 4 pin plastic SMD at 12.5 MHz on 5V. It happens to have an enable pin, so it's trivial to disable by grounding. While I was buying parts for this project, an obscure item came up on ebay: a NewerTech MacClip Junior. It's an aftermarket clock generator that clips on top of the stock oscillator, with speed configuration via DIP switch. My particular model is labeled for the 800 and 804av, but it works fine on other Quadras. It also includes a cute heatsink with a tiny fan, though the stock heatsink is probably sufficient. For reference, here's the list of settings and the clock speeds I got: