Ahoeben, thanks very much indeed for the help. Now have a machine which appears to be alive again at least. Avedude (actually I used AvrDudess in “Wiring” interface mode) seemed to try to program the 2560 but kept timing out. What worked was to download the Arduino source sketch from your link (I actually removed the last “Extended” from that path and found what I’m presuming is the source for the “standard” (not extended) machine, I hope that was an OK version to use) into Arduino IDE v1.8.7, selected the 2560 board type and then compile/verify in the IDE followed by upload. After a while th