The last time I used my SOL-20 it was fine, other than the keyboard. I set up my SOL-20 this weekend to see how it would work with the OAE OP-80A papertape reader. I never got that far as when I fired up the machine I was greeted with a screen full of 909090 rather than a SOLOS prompt.
I took a look at the troubleshooting section of the Processor Technologies Sol Systems Manual, page 250. It suggested which TTL chips to test, but I had already checked and ruled out those, so I next tested the RAM installed on U3, U4, U5, etc. on the motherboard in the section circuled above. This is the operation scratch RAM used by SOLOS in C900. I found all of the bad ones and replaced with 2102's on hand. (or equiv chip is the 91L02)
I have one more SOL-20 to fix.