iz8dwf has another new video troubleshooting a RAM problem on a C64. I haven’t seen this method before an interesting process but not perfect process. His notes are below:
This a very short video of repairing the C64 S/N UKB 462942, back in early 2016. The only problem was two partially failed 64Kx1 DRAM chips that were identified by “piggybacking” good chips over them.
Always make sure that the original chips have clean pins and the piggybacks make good contact on all pins and don’t cause any short on adjacent pins.
This method doesn’t work all the times (for my experience, it works about 50% of the times). See for example this one:
and this one:
where my attempts at identifying bad chips with the piggyback method failed.