The Backlight only need '12v' power applied to come on.
Most of the set runs on 5v via a regulator - but not the final few Audio OP amps which will generate some hiss..
Perhaps you ruined the first CPU (the instructions are too brief !) but the current one should have been fine.? ? I would proceed as the troubleshooting guide
in?
and?
Do all the DC checks in the build manual but start with is the 5V power there and at 5V !
?
Bruce Akhurst