I was wondering if it is normal that my piano down-tunes while I am playing.

Jacob Brothers from 1921 according to the link shown:


https://www.mypianofriends.com/i-have-a-jacob-brothers-piano-model-number-57866-but-i-cant-find-any-infor...-107545.html

Quoting:

I have a Jacob Brothers piano model number 57866 but I can't find any infor... by Guest 5 days ago
9
I have a Jacob Brothers piano model number 57866 but I can't find any infor... I have a Jacob Brothers piano model number 57866 but I can't find any information on it? Anyone know anything regarding this model?
Posted on Questions & Answers
Answer
4 days ago C. Anthony
Guru
Blogs: 48
Forum: 16,526
Votes: 420 What type of information do you want? you have the piano in front of you!
specifically what do you want to know?
the SN shows 1921 date.
Have you tuned it this year? If the piano will not hold tune to A=440hz, the value would be 0. However, I would think it would hold 440...but I may not.
Call your local piano technician to check it fully.
CAPReply

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Electronic organs typically do not require tuning
A pipe organ produces sound via hundreds or thousands of organ pipes, each of which produces a single pitch and timbre. The goal of tuning a pipe organ is to adjust the pitch of each pipe so that they all sound in tune with each other.

http://www.total-piano-care.com/ try this site to get the information for your piano

Take a look at this page from the user manual.

Well friend,
According to page 24 of the manual here http://support.casio.com/pdf/008/ap_45_e.pdf
It says
Demo tunes
While holding down the MUSIC LIBRARY button,
press the PLAY/STOP button
.....and try to play a specific demo tune ........
After playback starts, press the tone button that
corresponds to the tune you want to play.from the list.

ALSO in the Troubleshooting section page 35 For DEMO tunes UNPLUG HEADPHONES

If it doesn't work, then you need to get a Technician to look at it.

Well my friend, not knowing exactly what model this is, makes this a difficult question to answer as the market for upright pianos is really shrinking, but that's not to say it has no value. I can tell you that all pianos need tuning after being moved even down a flight of stairs so don't worry about that detracting from the value. More details would be helpful in answering this question.

Have a professional tune it for you. It may be that some strings have loosened/tightened.

Realize that it MAY be stretch tuned like a real piano. A real piano is tuned increasingly sharp above middle C and increasingly flat below middle C. At the ends, the the off tuning is about 20 to 22 cents sharp and flat. The Clavinova should emulate this. In a real piano this makes it sound better and prevents some cross coupling of harmonics. Read up about stretch tuning on the internet. The Clavinova gets its reference from a quartz crytal oscillator and they just dont' get out of tune.

An autoharp makes chords by muting all the strings except those that make the chord. There would be 2 ways to change the chord structure. 1) retune the strings knowing which ones will be muted by each key so that the result is another but valid chord or 2) make your own "keys" so that they mute different strings.
The tuning of an autoharp is selected to allow you to play songs as long as they are in the right key or two. If you set it "like a piano" as you suggest, you could play those chords but not the chord sequences that make up most songs. Most songs revolve around sequences or intervals such as the popular 1 - 4 - 5 blues progressions. Most music has some predictable progressions and that's why the autoharp is tuned the way it is.
You can always experiment with different and odd tunings but if your goal is to play already written songs, I'd stick with the normal tunings - there are alternates.

If you are marginally interested in the process of tuning the piano, and your main motivation is to save the cost of a professional tuning, bite the bullet and engage the services of a qualified tuner. Trying to do it yourself will become overwhelming very quickly. If, however, you are genuinely interested in the skill of tuning, and willing to invest the time and effort to become good at it, then get yourself a star tipped tuning hammer and some mutes (eBay will do) and start to see what you can do to make your piano sound better. Begin by getting the whine out of individual notes by tuning the strings to each other. Search out the "Every which way you can" tuning article by Kent Swafford on your favorite search engine for an introduction to aural tuning. If you're inclined to start out with an electronic tuning aid, check out Tunelab Pro - a free, fully functional demo is available for download to your laptop. Remember, however, that an electronic aid only shows you whether you've hit your target for any given string. Learning how to finess the physical motions of tuning so that the string stays on target for more than a few minutes is another process altogether. If your first couple of forays into trying to tune don't drive you around the bend, then consider finding a qualified piano technician who would be willing to provide lessons for a fee. If your interest holds up, investigate the resources for learning that are available through the Piano Technicians Guild. Their source books and monthly journals are second to none, but unless you are committed enough to the process to become an associate member, the non-member prices for materials are pretty high. "Piano Servicing" by Reblitz is another excellent source of knowledge. It might be in your local library.