String Pair Out of Tune

Background:
1) Bought Sam Bush Gibson F5 2001 model brand new.
2) Changed tarnished tailpiece out with a James tailpiece 2+ years ago. The 3 screw holes lined up hole for hole perfectly and it's on solidly. Had to slightly bend the tailpiece so the strings would approach the bridge at correct angle.

Problem:
About 1 year ago I noticed the D string pair frequently became difficult to tune together (same pitch). Several electronic tuners would show that they were tuned alike, but their actual pitch I hear would be very slightly, but noticeably, out of sync.

Attempted solution:
5 months ago I concluded that the tuning machines must logically be the culprit. Unable to justify $700 for a set of Waverly's I bought a set of gold 18:1 Grover planks and installed (had to drill out the holes slightly (scary)) them - they are solidly in place, even if the top and bottom screw holes did not line up perfectly.

Result:
The new tuning machines seemed to fix the pitch difference problem at first, but now the issue has returned.

Additional Info:
The problem is there whether using new or old strings or using different brands of "medium" bronze strings. The bridge and nut are original and do not seem to be in any sort of bind when tuning. No other string pair have this issue; only the "D" string pair. I'm baffled.

Asking for any advice for what areas I should troubleshoot next. Thanks.

Bob L.

Changing the tuners was a waste of money and perhaps devalued the instrument. I suppose it benefits tuner manufacturers that so many people blame their tuning machines for set up-caused intonation problems, but it's hard to say how many perfectly good tuner sets have been replaced when the problem is in the string nut.
The problem might be the bridge slots, but is more likely the nut slots.
In order to play in tune with one another when fretted, both strings of the unison must be the same length (and of course same gauge and construction). If both strings do not rest on the edge of the nut slots, right at the end of the fingerboard, they will not play in tune. The same goes for the bridge slots. It is also possible that both nut slots are not the same height. That is, one slot might be deeper than the other.
A good set up will fix the problem regardless of which of those problems (or combination) is the cause.

Sounds stupid - but how do you wind your strings on the pegs? I've seen this happen when the strings are wound too high on the pegs. This makes it so rather than having the strings bend over the nut (god that sounds dirty) which puts pressure on the nut and helps keep the strings in place, the string is more loose which can cause it to go out of tune faster. Especially if it's just one string and all the others are good, might be worth a look.

Also, same vein, if the strings are really old or just really worn out, they could be the problem.

Anytime I have an tuning issue - changing the strings is one of the first things I do. Sort of the "turn it off and on again" of music instruments - it's a good first step to rule out a cheap, easy, and fast fix.

Didn't think of either of those areas. Thanks very much. Oh, and I'm not worried about the market value because it sounds too good to ever be on the market in my lifetime. And I always keep the original parts. 🙂

Thanks. I've always wound strings on the inside from top to bottom of the post.

Bob-L, I’m just curious here, why are you so certain that neither the nut or bridge slots could be the problem? I’m genuinely curious about your thinking, because an excellent luthier has given you good advice, and I would venture to guess that any good luthier here would agree that the nut would be the most likely culprit and the first place to start; also, that it is very highly unlikely that a tuning machine could cause the issue you describe.

It may be that sunburst was too quick to pontificate about tuning gear companies and devaluation of instruments, but his advice about the actual problem is sound. I’m really curious as to how you can dismiss that advice without investigating it?

I did read your post fully and understand that old strings are not the problem.

Mark, he said he had not “thought of those” areas of investigation. Not that the advice was wrong.

Amazing how often the idea "it's a tuning problem, so it must be the tuners" misleads us...