Hi, I tuned a piano about three weeks ago and it's holding tune fine, except for one note- the g# above middle- that falls out of unison every few days. The center string is holding fine but the right string in particular falls flat very quickly. Is this just a loose pin?

Loose tuning pin, bridge or hitch pin moving. Do you have any past experience with this piano? Has it held in the past and is just now giving some problems?

Check to see that the becket is not slipping out of the tuning pin. Sometimes the becket was not inserted far enough into the tuning pin and it creeps out slowly. It is easy to fix. Just take the tension off of the string, re-shape the becket and put it in the correct amount, lift your coil and tune. If it breaks you may need to replace the string or you'll have fewer coils on the pin.

It could also be that this string is touching either the tuning pin and/or string of the one next to it or both. Or touching near the tuning pin as it passes by it. If it is rubbing on either, it will be unstable. Sometimes, the same length of string (I.e., one long string making 2 unisons) is the same string that is touching it's sister string. If you move that string and the other changes and or visa versa, that is the problem.

JFK:

It could be a rendering problem. I am guessing this is an upright. The right string on an upright has the shorter non speaking length. So when the torque on the pin is released, the difference in tension between the speaking and non-speaking length can be more than the other strings. And if a string with a short non-speaking length also renders poorly, it may be a little while before the tensions equalize and the pitch drops.

There are many, many ways to set pins and render strings. What I am going to describe may help you understand the mechanics involved, more than a one-size-fits all method (I don’t believe there is one.)

Choose a note in the high tenor where the set of three pins are offset towards the pressure bar and the right string has a very short non-speaking length. Hopefully the center string is already stable. Mute the right string and tune the left string to the middle by starting below pitch and bringing it up with a smooth pull and as little flagpoling as possible. Stop exactly when the unison is beatless. When releasing the torque on the pin, the pitch may drop some. (This is a demonstration, remember.) Now apply a little torque to the pin in the direction to lower the pitch, but not enough to actually turn the “foot” of the pin. At the same time give the key a number of hard blows. Release the torque on the pin and listen to how fast the unison is beating. For this string, that is about how fast the unison should be beating when tuning with a smooth pull before rendering the string and setting the pin. Now do the same with the right string. On an upright, you can expect the overshoot required to be much more because of the shorter non-speaking length.

Since you are novice, assume stability problems to be an opportunity to improve your hammer technique before looking for problems with the piano, especially loose pins (DAMHIK). Except in the low bass, the tension of a string exerts only about 20 inch pounds of torque on the pin.

Loose pins are the most common cause of this symptom. The fact that you suggest it as a possibility probably means that it is likely to be the problem. Novice tuners often don't have a lot of experience with new pianos so they don't realize how tight tuning pins should be. If you are practicing on few old pianos you may not recognize when pins are too loose to hold properly because they are all a bit loose.

So. Thanks for all your responses. This piano is old- probably 75 years- and not well kept, original ivory keys partially missing and broken. (I'm refacing them this week). It's a Vose and Sons baby grand, style 51, serial 102344. It's not a piano I've tuned before this time. I did tbink it might be my poor pin setting, but given that I've set every other pin on the piano correctly it just seems unlikely?...
The string doesn't rub any others that I can discern and the becket is fine.

Check the bridge pins for any movement. If not, use a brass screwdriver shaped rod and tap above and below the pins so the string will re seat. Just a thought.

UnrightTooner, good thing I looked at this thread. Your tip about setting pins/strings may help my tunings be more stable. I hadn't thought of doing it that way before... But... why is it that once I've pulled up a string (usually in the 6th octave or higher), then give it several hard blows, it has dropped anywhere from 20-40¢?

88:

The shorter the strings are, the more the spring in the pin will change the pitch. Let’s say the head of the pin turns 5 degrees before the foot of the pin moves. For a low tenor string, this would change the pitch very little, but a treble string much more.

The percentage of speaking length to non speaking length of string is reversed from the rest of the piano in this section. When the section of string between the hitch pin and bridge and the section between the pressure bar and tuning pin levels out with the portion of the string that you are able to hear there can be quite a dramatic drop in pitch. If you have pulled above pitch then eased back, the drop in tension in this section effects the speaking length dramatically as this section is sometimes longer than the actual speaking length. This magnifies the effect on the speaking length as you pound the key to level the tension with your test blows. Down in the mid section and bass this portion of the string is a small fraction of the speaking length of the string so there is less drop when settling the strings with test blows.