Is it normal if I connect a at powersupply to a slotexpander (sunrise) and to the MSX , and i switch the expander on, than the MSX starts too ?
I try this with 2 powersupplys on a MSX2 and a MSX1 , all the four cases are reacting on the same way.
The 3 jumpers they are used if the power comes from the msx are removed
The MSX1 gives a crippled screen.
After disconnecting the expander, the MSXes works normal again.

Since the slotexpander can write to the MSX al sort of inconsistent signals can appear on the databus when switching on. SO I can imagine the MSX going bananas when the slot expander is switched on/off. And I ahve seen that happen too.
It is like inserting cartridges in a powered MSX: it is going fine most of the time but not a good idea, many MSX machines died this way.
Switch MSX and external powersupply of the slotexpander on/off at the same time.

I think that I have the solution by disconnecting the -12 volt , I hope this is enough, I'll test it a few days on a MSX1.
Anyway, I shall switch them on at the same time.