Keys C and F# don't work in all octaves. - Yamaha MM6 61Key Synthesizer Keyboard

I believe you have broken the connection between that section of keys and the electronics. If you are unfamiliar with electronics, you are better off to take it to a repair shop. You can c ause more damage by poking around inside it.

Whether it happens to all of the key?
If yes, transpose setting may have changed. Change the setting to normal (0).
If the normal transpose, but tuning is still not standardized, check Pitch Setting.
To Restore Standard Pitch setting:
Hold down the A-1, B-1, C0 and C # 0 keys simultaneously and press any key between C3 and B3.
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If the above procedure is still no effect, try resetting the factory settings / Initialize overall: :
This is the steps:
Initializing the Settings
When you initialize the settings, all backup data stored to this instrument will be initialized and reset to the factory default settings.
Turn off the power to the instrument. Then, simultaneously hold down the C7 key (right-most key on the keyboard) and turn the power on again.

NOTICE
Do not turn off the power to this instrument while it is initializing the data in its internal memory (that is, while the button indicators are flashing).

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Notes:
If this instrument somehow becomes disabled or malfunctions, turn off the power to the instrument, then perform the initialization procedure.

Those keys come in an octave set. You have to remove one octave to either side also to free them.

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

Please have a look at the instruction manual again. The 8ve keys are assignable/editable for up or down. You can change the range of the scale this way. I own a WX7 and a WX5. They both work this way.

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.

Take it in for repair. There may be a broken line for the velocity sensing of that octave of keys that is causing the velocity to be sensed low. The repair is NOT DIY if this is the case.

The next step gets very complicated without test equipment.

It is important to realize how these keyboards work. ( I assumed you cleaned the black rubber dot that forms the contact.)

The keys are "scanned" by the electronics. Usually 12 bits are read at a time and the computer scans through the banks of keys that are usually 1 octave at a time and two per bank to enabling reading the velocity as the contacts for a key close one slightly behiund the other.

If there is any deformation of the contact bearing dome, that will cause a velocity sensing problem.

The keys are scanned as a matrix. To avoid current going backward through keys in other octaves, there are disconnect diodes that prevent coupling of one octave with another and between the two sets of contacts (for velocity sensing) within a single octave.

Do verrify that you cleaned the board and the black rubber dots with 99% isoprophyl alcohol and NOT rubbing alcohol as the latter contains an oil that worsens the problem.

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