Some early upgrades (Brainstom?) would use a Kelly Clip & socket to replace the CPU. I kow a couple of guys who pulled the 68000 chip and installed a socket for a 16mhz 68000 upgrade. You will need a new xtal.

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The Brainstorm avoided the need for a new crystal by connecting to the 16MHz oscillator that already exists on the Plus/SE. It's a little ugly, but it works. The Brainstorm snakes a wire (through a 47 ohm resistor, or some similar value) to a pre-existing 16MHz signal. The Brainstorm people were very clever. Their upgrade board is a model of economy of design.