System-80 working (sort of)

This poor System 80 has been subject to cruel and unusual punishment at the hands of previous owners.

I’ve got the Ready prompt (the equivalent of the TRS-80 Memory Size) but it’s been hard work. All of the 2102 video RAMs needed replacing, as well as several LS157 muxes. I managed to link A4 and A5 when replacing a mux – that cost me a few days.

Pressing Enter gives a screen of full graphics blocks – I think the video RAM read is not working fully. But to debug that, I need to understand what is left of a lower case support mod and (I think) software switch for 32 col mode