I have an early 80's (Mexican action?) Hamilton Baldwin 243 which I purchased a few years ago and had tuned. It plays and sounds well, (tuner's opinion), but the keys often do not respond to a 2nd key press, i.e. the action is not resetting. The tuner said it is "weak jack springs", which may be typical for this era 243, so I am wondering how to proceed. Can I stretch the existing springs, or should I replace them? If replacement, can anyone help me with the correct spring dimensions / specifications? I repair Hammonds and vintage synths, so I am confident I can do this without buggering it up. Any help is greatly appreciated.

It is not likely to be the springs. It is more likely to be that the jack center pin bushings are too tight. Get another opinion.

Could also be a slow whippen center that is not letting the whippen fall enough for the jack to reset.

If that is the case the action would have to come out.

The problem is probably as BDB and Dan S have described. It is a simple repair for a piano technician. But special tools, materials, and experience are required, all of which you probably don't have, even if you have lots of experience fixing Hammonds and synths. The only thing those have in common with a piano is the "digital" interface (you press a key with your finger to get the sound).