I have a yamaha p-155 88 key digital piano the upper octave has no sound

What Kind of yamaha??? What model?

What is your piano brand and type?
Damage to the contact rubber can cause note plays loudly or,
note does not plays at all.
It is caused by:
1. Carbon rubber contacts dirty / dusty (yellow arrow). Clean with a pencil eraser (or cloth). Gently rub it.



2. Contact rubber is torn (your case). If this happen,it can not be repaired or glue. So, you have to replace one octave.
The yellow arrow indicates the side that is often torn.

Rubber side that is torn, usually not seen, unless we move / pressing gently on top (In the rubber contact position is not installed on the board.)
Examples of this picture was yamaha psr.

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try to clean under neath of those particular keys very gently with cotton buds .2)is it the case with in all the octaves 3) has the K.B been in humid places

First try looking in your transpose settings. Refer to your manual to see if any arpeggiation/unnatural chord settings were inadvertently created by you or made by another user. If you believe it is a software problem you can not solve, back up your memory bank, remove your memory card, and hard reset your keyboard. Insert your memory card and check to see if your problem still exists.
If it is a technical problem, it sounds like you may need to get it checked out by an engineer/repairman. Sometimes when a digital keboard's keys collect too much dust/debris they can trigger multiple keys that aren't being pressed. Think about it like a key getting caught on an upright piano and pulling back the hammer next to the one that was played. A digital piano works off of one or several printed circuit boards or PCBs. The printed circuits usually can not short circuit because the wires and circuits sit in fixed positions. If an alien conductive fiber or series of conductive fibers connects a conducter to another, or if there is any humidity inside your piano it may cause circuits to misfire when a circuit is connected in the area. This could explain why notes you are not playing are being activated in the area in which you are playing.

If you are in a humid area and you suspect that this may be the cause you can attempt to draw out the humidity before getting a an expensive diagnostic done. Find a box large enough to house your keyboard. [If it is on a stand take it off and] place it inside the box. Cover the keyboard with plastic wrap (so as to prevent any foreign objects from getting into the instrument) and very carefully surround the piano with white rice. Remove the plastic wrap, close the box and leave it alone for a few days. The rice draws out any moisture from the electronic components. Very carefully (again so as to avoid getting any rice inside it) remove the keyboard from the box; remove any dust from the body, face, keys, and other components. Try your keyboard now. If humidity/moisture on the PCBs was the problem it should be solved. If not it is time to take your piano for a tune-up.
Hope this helps!

It sounds like there may be an issue with the piano sounds on your Kurzweil sp4-7 keyboard. One possibility is that there may be a problem with the software or firmware of the keyboard, in which case you may need to update the software or contact the manufacturer for assistance. Another possibility is that there may be a problem with the internal hardware of the keyboard, in which case you may need to have it repaired by a professional technician. In either case.

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

If they are the same note- different octave- it may be a bad solder joint near the main oscillator IC.

If they are the same cluster (adjacent white keys or adjacent black keys) in the octave, it may be a faulty wire connection.

In playing fast you have to make sure to release the note fully and let the key come up. These keys have TWO contacts that are used in the scheme to measure note velocity. If BOTH contacts don't open, the note will NOT re-fire. The second symptom you have written would be a characteristic if one of the disconnect diodes has shorted. These diodes direct the strobe signals that scan the matrix of key contacts to avoid other octaves responding when multiple keys are depressed including those in other octaves. Servicing of the unit is required. It is unlikely you have the tools to DIY this problem. ONE THING to try is to tip the unit and gently tap it to see if some debris MIGHT have fallen in that may be shorting some of the matrix connections.

Either the first or second closure matrix line for the "A" keys is bad.

This may be a cracked or shorted circuit board.

A bank of diodes is used to prevent sneak electrical path if more than one octave has notes pressed.

First thing is to clean ALL the key contact areas and the conductive rubber pills with 99% isoprophyl alcohol.

This involves dis-assembly of the key area. If you are not competent in electronics, best left to a pro shop.

The keys are scanned in a matrix, two contacts per key and all the individual notes are in common (all the "A;s", "B's"...) And each octave is strobed for the first set of contacts and then for the second set of contacts. an the whole octave is read in parallel.

The reason for two contacts is one closes first and then the other as a key is lowered. The time between these is measured and is the note VELOCITY which for a piano controls the loadness of the note. IF ONE of the two fail, the loudness will vary as yours does.

Hi ...
There are flexible cables that connect the keys to the main electronics. The connectors for those cables sometimes age poorly..
It is fixable. But dont "get taken" on a repair.
If it's just resoldering a few points,it shouldnt be more than $100.00
to fix