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Way back when my 28C was new there was a hardware hack floating around on how to add memory (32K, I think) to a 28c. It involved cutting some internal plastic supports and the usual soldering in of a new chip but that's all I remember.

Does anyone else remember that hack or how to do it?

This is a tough one.

I made the hack with the help of a fellow on this page Wlodeck.

I removed the adhesive overlay on the right half kbd and
drilled the heatstake pins. THATS THE EASY PART.

then I found a CMT 32Kb memory module and with the help of Wlodeck found the relevant pads for nibble, daisy in and out and power.

Then assemble heatstake reflow and done.

Iøm sure that comp.sys.hp28 has a description from
about 1986-87.

best regards