4 equidistant strings/McCarty tulip dulcimer
Instruments- discuss specific features, luthiers, instrument problems & questions
Jim Hedman: Hopefully, you are enough of a geek to ferret out that for this particular dulcimer the required nut and bridge adjustment for "correction" is impractical and the residual random fret placement errors (now sharp and flat) would still be troublesome even if attempted.
As Bridge just hinted at with his 'gut feeling' comment, the problem here could be that you are expecting equal temperament sounds on an older dulcimer that may have been fretted in just temperament. This would result in an instrument that not only sounds badly intonated when in DAd tuning, but even in DAA tuning when chording it across all strings.
Our member Robin Clark has written extensively over on the ED forums about equal vs. just tempered scales on older dulcimers. I don't know how much you guys already know or don't know about this subject, but here are a few of Robin's posts on it over on ED (which will sadly shut down at the end of this month):
http://everythingdulcimer.com/discuss/search.php?st=0&sk=t&sd=d&sr=posts&author_id=6698
There are also a lot of great discussions here on FOTMD concerning the equal vs. just temperament factor with older dulcimers:
https://fotmd.com/search/results/all/1/25?search_string=just+temperament
One thing I would say is that many folks wind up thinking their old dulcimer was improperly fretted and they proceed to pull out all the original frets and refret it in equal temperament. This is a disservice to irreplaceable older dulcimers and will immediately and severely devalue them forever (that's assuming they were not actually badly fretted by an inexperienced maker). If one needs an equal tempered scale due to how one wants to play (intended repertoire, playing style, tunings), one would do far better to simply use a more modern equal-intonated dulcimer. The old just-intonated dulcimer fret spacings sound sweeeeet when you play noter style in ionian tuning, as they were designed to be played.
Again, I do NOT know if a just temperament is the cause for the frets being 'out of tune' in this instance, and I don't know anyone's expertise or knowledge level here. But any time an old dulcimer is involved and thought to be badly fretted, the intended fret scale should be high up on the list of factors.
I'm hoping Robin Clark will jump in here, since he has done a lot of research and experimentation on the subject.