Terada mandolin

Hi - I have a 60s (?) Terada Japanese mandolin. Looks beautiful, but the neck is like a rollercoaster & the tone is .. well, "interesting" (dull). I rebuilt it from a bare-bones reject, fitting bridge, hardware etc. At the moment it's still unplayable. I've taken the bridge down so far I'm almost at the adjusting pins but the action is still way too high.

I plan to remove the fretboard, then look at what I can do to the neck to straighten it out.

Could I lay another piece of hardwood upon the old neck then re-glue the fretboard to that?

If I could raise the whole fretboard, I might be able to have the bridge high enough to be adjustable and also maybe fit an old Telecaster neck-pick-up at the neck (without having to cut into the body) and try it out as an electric mandolin.

Sounds like sacrilege I know, but if I'm going to have an instrument it HAS to be playable!

I'm pretty sure that nobody is going to consider anything you do to that mandolin sacrilege. It's your instrument, you live in a free country, and alligator dentistry has saved many an unplayable instrument. Besides that if you find it fun and entertaining then my feeling is you should go for it.

If the neck’s warped, the fretboard may not lay flat, even with clamping, but otherwise the concept is ok. Even our ancestral family violin has a narrow wedge under the fingerboard, put there sometime before 1925 or so. One question is what’s the glue. A modernish mando might have a glue that won’t easily release with heat, but a clothes iron is one way to find out. If your shim is not a wedge, you’ll have to raise the nut too, so a wedge might look better. And then there’s the neck, which is going to get fatter in the important area unless you do make it a wedge.
Have fun!

I'd say go for it. I learned a lot about mandolins by rehabbing a totally shot Osborn Sammo mando, followed by some more focused repairs on a sadly neglected/abused Stahl. Good luck!