I have had my Steinberg upright piano for over 40 years during which time it has been regularly tuned every 6 months. Recently, however, the strings in the upper register go considerably flat two weeks after tuning. What could be causing this?
If the tunings over the last 40 years have been stable and have only recently become less so then you will need to look closely at what may have changed. Assuming the piano has not been moved then the most obvious thing to look for is a change in the heating system of the house. Anything which affects the temperature and most importantly, the humidity of the room in which the piano is kept is likely to have a detrimental effect on the piano. Has the central heating changed in some way? A more efficient boiler or new radiators, even the installation of double glazing may dry the air to the degree that the wrest plank (the block of laminated wood which holds the wrest, or tuning, pins tightly in place) becomes over seasoned or too dry which in turn causes the pins to become loose and unable to hold the high tension required to keep the strings at pitch. The fact that in your case the top treble seems to be affected most might indicate the presence of an over efficient radiator near that end of the piano. If you are very lucky, removing the source of heat (e.g. turning that radiator off) may restore the wrest plank to its former condition. Otherwise there are several options which you must discuss with your tuner. The easiest and simplest possible solution is to hammer the pins, knocking them further into the plank to gain a little more purchase, but this may only be a delaying tactic – a better answer would be to replace the wrest pins with a larger size, maybe only the treble section if the others really are still tight. This may be an ideal opportunity to consider restringing the piano; however, the cost of this may outweigh the value of the piano, again something your tuner will be able to advise on.