A dead CPU board is very often due to a chip located on the front border of the CPU board (the second one starting from the top.
Its a 74HCT244, labeled (if I remember well) : IC240. Its anyway easily recognizable ; it has a few wires modification on the solder side.
Change it to an 74LS244, taking great care to take note of the wire modifications and installing the circuit on TWO HIGH QUALITY tulip single on line (cutable) connector.
The change from HTC to LS is okay for this chip. Don't change them all !!!!!
Thanks to Dr.C (
dr.c@keyboardsrecording.fr) for providing this info.