Thanks Dust. Glad to say the problem is solved, and it was all my own doing. I'll keep it short. The solution hit me like a ton of bricks. My printer runs on two PSU units, with the bed running off a separate ATX supply. This ATX PSU used to be the "main" and only PSU, and was originally, and still is, GREEN WIRE controlled. Some years ago I put the Ramps 5Amps on a new separate PSU, and keptby Jannie - Firmware - Marlin
Homebuilt printer running 5 yrs now Repetier firmware with no problems. Ramps 1.4 on Mega 2560 I would now like to try Marlin, so I uploaded Marlin Bugfix 2.xx ON A SPARE Mega 2560, upload with no problems using Visual Studio Code. Both hotend and heatbed report exactly the same ambient temperature. 18C (Thermister OK) After setting temperatures, the bed does not heat up. (Extruder no problem)by Jannie - Firmware - Marlin