The time has come. We wish you a safe journey, AMD 386SX CPU. You have served us well. Armed with a heat gun and flux, work begins on my attempt to create a 50 MHz Amstrad Mega PC.

Now, that 50 MHz target needs a little explaining. The original Am386SX runs its core at half the CLK2 input frequency, so a 50 MHz CLK2 signal gives it a 25 MHz core speed. The replacement TI486SXLC2-G50-PQ, shown in the repair photographs, is a clock-doubled processor designed for a 50 MHz internal core with a 25 MHz external bus. I’m not trying to make the entire Mega PC motherboard run at a 50 MHz external bus frequency!

Of course, there is more to a CPU swap than getting the old chip off and soldering another one in its place. The SXLC2-G needs a 3.3 V core supply, even though its I/O can tolerate 5 V, and the chipset, BIOS, cache-control signals and surrounding circuitry all need to cooperate. Similar-looking processors with similar clocks aren’t necessarily interchangeable. Plenty of opportunity for things to go horribly wrong, then!

I’m using a general-purpose heat gun for the removal, which is hardly the ideal tool for a soldered QFP processor on a vintage multilayer motherboard. Proper temperature-controlled hot-air rework equipment would be a much better choice. Still, this is very much an experiment, and the AMD 386SX’s journey is about to begin.

Edit: Everything is on fire! What have I done! Flames everywhere!

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