Gord,
the action on the SD1 is weighted with metal weights under the keys. The Yamaha on the other hand does not have metal weights but Yamaha calls this keyboard "synth weighted". The technics I can't comment on because I don't carry this keyboard.
The other main difference is the way aftertouch feels on these two keyboards. On the Solton, when you push down on the key for aftertouch, the key actually moves a little bit which is how the aftertouch sensor works. On the Yamaha, when you apply aftertouch, the key doesn't move at all. You are only putting more pressure on the key.
If you would like or need a real piano hammer action, try the Roland KF90 which is like the roland VA5 in a digital piano body (FP9)
George Kaye
_________________________
George Kaye
Kaye's Music Scene (Closed after 51 years)
West Hills, California
(Retired 2021)