One of my Dragon 64s is set up so that I can test RAM easily. I’ve made some little PCBs up, which allow me to plug in a mounted ZIF socket into a standard DIL socket on the lower four RAM positions, as per the image below:
Whilst I was undertaking this testing, it became apparent to me, that before even getting to the memory test program, a RAM fault would generally appear on the boot screen. In fact, faulty RAM would fill the majority of the screen with a single character.
Through a bit of trial and error I discovered that the single character shown could be used to work out which chip(s) had failed.
Another way to reproduce this fault was simply to leave one or more sockets empty, with the populated sockets containing known good chips.
I’ve not replicated the testing on the upper row of four DRAM chips, but I have worked out what symbol corresponds to which of the lower sockets are faulty, as presented in the table below. RAM A-D are the bottom row of RAM, from left to right respectively. If anyone knows how faults would show in the upper row of RAM chips, please do let me know.