Yamaha ELectone FC-20 volume "control sliding switch" that control the volume of the upper &lower keys there are no sound only if I put some force on this "switch" Will this be very costly to repair?

It sounds like there may be an issue with the power supply to your Yamaha Electone HS5. Here are a few things you can try to troubleshoot the problem:

1. Check that the power cord is securely plugged into the back of the Electone and into a working outlet.
2. Make sure that the outlet you are using is functioning properly by plugging in another device to see if it powers on.
3. If the power cord and outlet seem to be working properly, try resetting the Electone by unplugging it from the wall and waiting a few minutes before plugging it back in.
4. If none of these steps help, there may be an issue with the power supply unit inside the Electone. In this case, it is recommended to have the instrument serviced by a qualified technician.

If you have speakers connected to it, disconnect them to see if the static stops. If there are no speakers clean the on / off switch and any volume controls with electronic switch cleaner.

Other ideas here

https://www.google.com/search?q=electone+organ+static+noise

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https://www.manualslib.com/manual/779362/Yamaha-Electone-Fx-20.html#product-Electone%20FX-10

first, how old is this unit? according to this page https://en.wikipedia.org/wiki/List_of_Yamaha_products#Electone_(electronic_organs) they were built between 1983-1988. In other words, it's over 30 yrs old and it may have had some internal electronics finally die.

https://www.manualslib.com/manual/779640/Yamaha-Electone-Fs-30.html

https://www.manualslib.com/manual/779640/Yamaha-Electone-Fs-30.html?page=29#manual

Yamaha electone organ have no sound el 25 need help thnks...

Likely one of the cables to that manual has either oxidized contacts or come loose. This could be at either end of the cable. With it in "failed mode", try wiggling the cable(s) going to the upper manual and the problem MAY show itself.

Not likely iit is a fuse. The keys are likely scanned as a matrix with two contacts per key. You need to get a maintenance manual for thee organ. Contact Yamaha America parts for music products. They may have the service manual on CD... they used to get $15 for the CD of the manual. Unless you can find something simple like a cable knocked off this could be hard to troubleshoot.

This is likely in the power amp or the drivers for it.

The key to troubleshooting is a schematic. Try to procure one.

You MIGHT try using circuit cooler in the area of the power amps to isolate a bad component or cracked circuit board.

the problem is the main output amp. It is an IC so it is better left to a tech to put it in. Just had the same problem with mine. Seems to happen when unit is left on overnight.

Check if you may have knocked off the speaker leads OR ones to an expression pedal.