Well I see on the X10 web site there is now the one you have [I think] MT12A and a newer MT13A [has a dim capabilities].
Anyway short test today.
XPCP Coupler. One input on power line other on an oscilloscope with a 10 Ohm Load. ACT CR134 Repeater coupler off.
MT12A New MiniTimer 4.25 VPP
Radio Shack 49-1000 Security Console [OEM from X10 and has wall wart interface] 4.5 VPP
SC503A Maxi-Controller 5.5 VPP
TR16A Telephone Responder 5.0 VPP
PSC01 Powerflash 5.5 VPP
TM751 Old Style Tranceiver 5.5 VPP
TM751 New Style Tranceiver 5.75 VPP
CM15A Older revision 5.75 VPP
RR501 Tranceiver Tried two to verify. They may have a power supply that is limited. 6.5 VPP With +4 V and clamped -2.5 V.
You could be correct. If the TM12A is in a system where the signals are marginal. The older MiniTimer may work and the new one not. I don't have the older MT10A to compare the new one to.
Added module: I just got my hands on the new MT13A Mini Timer. It has the exact same wall wart power supply XM18A; as the MT12A. So my guess is it also will have a lower output.