Grauw, again thank you.
I won't come near the Ricoh chip. 
I'll just cut the connectors to the battery. I thought, maybe when the insulation is removed, the battery can easily be removed. But, on my nms8245's, I also had to remove those big blue buggers with a pair of plyers. Too much risk of damaging other parts by desoldering them. Maybe not for someone more skilled, but not me.
I've removed the battery, but still get a black screen. 
BTW, the polarity on the mb is clear, there's a large + on the left.
Today, checked the wires on the backside of the motherboard and did find that somehow during soldering I managed to create a short circuit. 
Miniscule, but still no bueno. Probably should have used a different tip (chisel).
Fixed the short circuit, checked the lines and they seemed ok.
Board back into its housing and connected power, the external drive connector and the small pcb to the mb. Did not connect the flat cable for the internal drive.
Scart connected, power cable, switched on and.... yay, blue screen, MSX logo, etc.
It gave a message 'Bad FAT in 0" and OK. I had no keyboard connected, so could not check if I could type anything.
Gave some good expectations, cause at least some sort of Basic screen appeared.
But, after a power off and on, only black screen again.
It's frustrating the heck out of me that it isn't working. And even though I have other MSX's, I want this one to work as well. 