Finally got around ordering new powersupply – “Universal Powersupply” for the Apple /// wich i had problems with booting up. Hope this will make things better.. and no need to start investigating the motherboard itself. Will comment more when i receive this and how the installation goes.
“Universal Powersupply” from Reactivemicro.com (Apple /// version)
And new powercable. It’s surely good idea to replace the cable for some 35+ year old computer while you are at it.
Wish me luck!