I quickly scanned through the Sol-20 theory of operation manual and came across something that seemed particularly relevant:

"Seven-bit ASCII-coded data is written into RAM chips U14 through U20 from bits DIO0-6 of the Bidirectional Data Bus, and the cursor bit (DIO7) is written into RAM chip U21."

I had previously had problems with flakey 2102 RAM, so U21 could indeed be a possible culprit. I quickly pulled it out and replaced it with a known working RAM chip...Problem solved. Too easy!

After my recent Sol-20 fix I have been using the computer heavily. Firstly, I have been creating a library of .wav files by transferring the .ent files to the Sol via a serial connection from my PC, then [SA]ving them back out and recording them as a digital files. Secondly, I wanted to run the machine for extended periods to do a burn-in test. I think the last time the computer was turned for any length of time was over 30+ years ago. If something else was going to fail, I wanted to know about it sooner rather than later.

I powered up the Sol-20 today only to be greeted by two flashing cursors on the screen. The first cursor was where it should be, one character to the right of the ‘>’ prompt, but the second cursor was offset by another 8 characters. If I entered text via the keyboard both cursors would move. After 8 characters were entered the phantom cursor would then jump 8 characters behind the real cursor. With another 8 characters entered the phantom cursor would jump infront of the real cursor, and so on…

neoncluster.com

Screen showing two flashing cursors