Bugfix 2.0 Heatbed not coming on. ?

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)
(It then times out and "Heating failed- Printer halted"

I then uploaded my backup copy of Repetier to this Mega, and then all works well again with no problems. Proof that it is not an electrical wiring or a physical problem.

I have gone over the Congfig file many times now, and have tried "example" configs, (Those using Ramps 1.4 and Mega,) No luck.
Is it perhaps a PIN assignment problem?

Any pointers, please anyone.
My Config attached.

Thanks guys
Jan

Edited 1 time(s). Last edit at 10/13/2021 08:51AM by Jannie.

Sound to me like it has external mosfet on some other pin than D8 the default onboard bed mosfet

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 kept the ATX solely on the bed heater on the 11A input.

You guessed it.
I neglected to "activate" the PSU in the Marlin Config file, so it never actually came on.

Thanks for your thoughts on this.
Jannie