Long time reader but first time poster. I'm troubleshooting my Rev.3 (7850). After isolating Ram problems and swapping 1 or 2 LS, it now does boot and start disk II.
The main default comes from HGR command, which doesn't clear the screen (Hires1), so I suspected the $2000-$3FFF RAM area, but corresponding chips are tested good. After issuing HGR command, the screen doesn't blank and shows partial white blocks. Otherwise, HGR2 works: Hires2 is cleared. Text (1&2) and GR work well.
Any hint appreciated ! Thanks.