I did the same with my Japanese Playstation and X68000 XVI
I'm usually very careful, but must have had a couple of Brain Farts!
Hope you get it sorted
I've done this a lot more than I'd liked to admit, haha.
When your country's output is 220V and your collection is a mix of PAL/NTSC/NTSC-J mistakes are bound to happen.
The characteristics of AX660 transformer may be are: 2 x 110V as input and 9V as output. If this is the cas, AX660 doesn't provide -/+12V for MSX cartridge slot.
Does AX990 transformer also have two or more output wires? I think there are two because there is a also diode bridge just after the connector.
Have you ever tried the FM-PAC on these computers?
Did you mange to fix it in the end? I have a similar issue but I was not the one that blew it