Hy Mu and all,
Older computer PSUs operate independently of the mother board but 250W total outpit comes down to perhaps an amp or two at 12V for powering the CPU cooler and cabinet cooling fan. Such is quite suitable for powering a scanner or other low power device, for more power rather extensive and complicated modifications are required. In any case they have great voltage regulation being designed to hold the output to fine tolerances with an input of 90-130VAC (US).
For more information without the speculation I just finished reading check out the various ham radio web sites. Remember, hams do it with high frequency. Oscillator aligator, VFO crocodile. For the Canadians a sked is NOT pulled by a team of dogs, your tewk is too tight. (DOH!)