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The heatsink cools the LM7805 power regulator, not the CPU. In the original 48k Spectrum, the heatsink was internal.

And if you use a modern switch-mode replacement, you don’t need a heat sink at all.

I know some folks like to put small heat sinks on the ULA though.

Soviet clones had ULAs built from individual 74xx ics. No need for heatsinks!

Excellent! I need to mess around with them at some point.