Are the JV3 SP and SPII main boards backwards compatible with the S models?

[spencer2004]

Could I fit a main board from a JV3 SP or SPII model in a S model provided that the correct firmware and parameters are installed?

They look almost identical other than the JV3 S mobo has 6 head data cables instead of 8.

 

[Solventinkjet]

No they are not. The S series main board is missing a row of transistors. Even if you were able to boot up somehow, it would only fire 3 heads out of the 4.

Edit: Just re-read the question and realized you are going from the 4 head to a 3 head. It may work but I still think the cable configuration is different and it probably won't work. Worst case scenario it blows a board and/or damages heads.