Well the unanimous opinion is for a hollowed finger board so that is what I will do.
Many thanks all for your help! Allan

I totally agree with Ken.  I hollow all my fretboards to approx. 1/4" thick on the sides and top.  The hollow runs from the nut area and stops just short of the strum hollow. And, similar to John's suggestion, I also cut a wide slot in the soundboard under the entire fretboard.

updated by @kusani: 02/16/19 10:42:18AM

I would hollow.  Or even make the fretboard from three pieces of 1/4" wood in an open box shape.  The fretboard is a massive brace running lengthwise and anything you can do to lighten it up will improve not necessarily volume, but overall sound quality, because more things can vibrate.  

On a typical dulcimer, as opposed to a ukulele or guitar, the vibrations from the strings get transmitted to the soundboard through the fretboard.  A heavy fretboard will tend to dampen some of the vibrations.  That said, it you use a composite fretboard with light wood topped by a strong fingerboard, (think mahogany and ebony) you might not need to hollow out the fretboard.  If your instrument has good internal bracing, you might even try separating the fingerboard from the bridge/saddle portion.

Or maybe just take John's advice and go with a hollowed out fretboard.

Hollowing the fretboard saves weight and adds more cubic inches of soundbox, and usually yields a little more sound.

All things being equal, it's a good idea to hollow it out.  Another question is whether or not to cut holes in the top under the fretboard.  I like to cut them out, but doing that may not make it sound much different.  Best wishes on your repairs!

I have acquired an old dulcimer it is a bit of a wreck and will need restoration. Among things that need replacing is the fingerboard, I have just removed the old one and it was solid. Should I hollow the new fingerboard or not bother, I note that most string instruments have solid boards. The question is how much difference if any does hollowing the fingerboard make to the sound volume? 

Any opinions are appreciated particularly if based on experience. Thanks Allan