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?