You can try laptop-schematics.com to see if there's a schematic available, but these are so old now that you may be better off just replacing the system board (or sourcing a working, used system).  The entire system can almost certainly be purchased for not much more than what the schematics would cost -- and very often for less than what a replacement system board (which is almost certainly needed) would cost.  
My old Dell Latitude C640 recently cannot boot.  When the power button is pressed, there is no power suppy to bios flash memory as well as other parts of the system. Can schematics diagram of this old laptop model be available for me to troubleshoot the problem?  I need to keep using this laptop because an very old application must use this laptop's PCMCIA interface (due to compatibility constraint).  Any help will also be appreciated.[IMAGE]
Looking for Dell Latitude C640 Schematics diagram