Hi, just wondering if you've made any progress with your A500, and in particular if you've been able to check the serial output from DiagROM. I looked at your video and I saw a similar colour pattern from DiagROM at one stage while I was troubleshooting my A500, and DiagROM was often reporting CPU exceptions such as AddressError and IllegalInstruction. The cause was one bad RAM chip, though the machine also had a bad CIA B and keyboard issues.
I think it is plausible that memory issues could be causing the Kickstart yellow screen as well, if it results in a CPU exception at boot time.
It's not very well documented, but DiagROM has some useful operating modes controlled by different mouse button combinations at power-on (note that you need to release the buttons before DiagROM checks for stuck buttons). RMB tells it to use the beginning of memory rather than the end, and LMB tells it to use Fast RAM if available (note that no video output is possible in this mode, so you will need the serial connection). You can also combine the two. I found I could run DiagROM without crashes if I installed a trapdoor RAM expansion and ran it from Fast RAM.
