If you have the "mechanical" friction pegs with the tension screws, and the screws won't tighten enough to hold the strings in tune, it means the plastic peg buttons are compressed. The screws can't go any farther.

You can take the peg apart and either add a small washer between the screw head and the top of the button, or make a washer out of leather or soft plastic and put it between the bottom of the button and the metal housing it sits on. This will allow the screw to be tightened a lttle more.

These screws shouldn't be any tighter than necessary to hold the string in tune, or the buttons will become compressed prematurely. 

If you have wooden violin-style pegs, the various remedies already given work. Sometimes I use "peg dope" (jeweler's rouge) or blackboard chalk for the peg to have more grip in the holes.

What kind of pegs do you have?. Are they wood violin or metal type with screw in the end. The metal type can be replaced with new ones for under $30. Robert

It depends on the instrument. At least by the late 70's, when I got my first McSpadden, they had friction tuners that are different than wooden pegs, although they are still 1:1 straight-through tuners. Peg dope won't help with these, but if this dulcimer is even older, then maybe they did use pegs and it would. Basically, if it has a screw head on the end, it's not a peg and you can tighten the screw to keep it from slipping.

I've always found slipping wooden pegs to be nicely cured with a minor treatment of Peg Drops liquid. You can't lose anything by trying it out before opting to completely change to geared pegs.

Wood pegs tend to slip more in the Winter dry indoor weather. The Peg Drops liquid is made with rosin and provides a nice even very thin film of 'grip'- makes tuning/moving them smoother, and pegs then will hold.  You need to unwind the string and remove each peg in turn, put 2 or 3 (no more than that!) drops and evenly turn the peg in place to spread where the peg touches against the wood. Then restring the string, tune up with a little less tension than usual, and let the drops 'cure' overnight before tuning up to normal.

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Just bear in mind that if you alter McSpadden's design and you want to sell the dulcimer later, the modification may affect the value of the dulcimer.

Follow Steve's advice. A problem in the late Fall and winter is a lower humidity which causes the scroll walls (sides) to shrink a little. This causes the peg to slip. Tightening the screw in the peg button should take care of it. Should you choose to replace the friction pegs with geared tuners, that can be done. Depending upon the type of tuners you choose you may need to enlarge the holes for the tuners.

Ken

"The dulcimer sings a sweet song."

If really want to replace the pegs not just tighten them, Gotoh, Five Star and Schaller are well known brands of tuning machine makers.  Stewart-MacDonald (stewmac) has quite a variety for sale at a range of prices.  Don't look just for Dulcimer tuning machines, individual guitar tuners (either open geared or closed) work just fine.

Look at the end of the pegs, first. Theirs usually had a screw in the end which could tighten the grip. It worked well on those I had of theirs.

i own a McSpadden with friction pegs that continue to slip. The dulcimer has a scroll head. Any recommendations on replacement tuning pegs?