When i turn on my yamaha f20 guitar amp, it produces a hissing/ static radio sound. Is it the power capacitor or something else?

Try putting guitar trough the FX Loop, if there is still no sound, you may have a problem with the Power Supply.

That means that if too much power is going to the speaker or extraneous signals are being picked up by the wiring, it can come through the speakers as static. Stopping static requires finding and eliminating that extra electricity from transmitting through your speakers.
If you're still hearing distortion, turn down the volume on your amplifier until the distortion goes away. If the volume is too low to be easily heard, the problem may be that your amplifier produces too much power for your speakers to handle or that your amp and speakers have different impedance ratings.

The way I've found this amp works is it always needs a certain amount of "Gain" from the gain nob in order to produce any sound.Unfortunately the more gain the more hiss and crunch at least thats been my experience, it could also be you have one of the filters "guitar sounds" left on.The guitar sounds nob has to be left on clean to attain pure clean.There could also be a connection problem.I have this at the moment.I have to bend my wire in the right place to keep the thing going.Another issue I have with his amp is its been replaced my first one just conked out completely I couldn't get the cash back when I asked so I just got another one.This new ones been fine but not without hiss.I think its a hissy amp I hate to say it.Not the best amp by a long shot.

Chances are there is nothing wrong with the mixer. IF you accidentally turned on Phantom power, that would cause the problem. Phantom is ONLY used with condenser mics and a few DI boxes. Turning it on can create noise unless balanced lines are used everywhere.

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.

do you have a low hum from the speaker when u turn it on? if no, then take it to the repair tech. probably power on indicator is okay but power supply does not reach the pre-amp or the power amp section. case maybe bad filter capacitor or bad series power line resistor. get it checked. i sincerely doubt internal speaker loose connections.

The hissing is caused by a bad connection, most likely at the cable jack. Try replacing it and while you're doing that check the rest of the components - especially the potentiometers, used ones generate a lot of static.

If you can hear music with the volume up, but like it was turned down low (from the power amp), then yes there is something up with it. If it's the same on both left & right channels, then it's something common to both. For instance the power supply, or an IC that has both channels going in it.
If however the power amp does produce sound normally with another device, then either the turntable has a fault, or the pre-amp has, perhaps on the pre-amp inside it for the cartridge.

It is because the AC-90 is not grounded. If you look at the power cable, there is just + and -, no ground/earth. This is because ground is not usually needed when using acoustic guitars/ mikes. If you use an electric guitar, or a slide, there will be problem. I have a AC60, that does go to ground. No hum. AC90=hum. Can probably be fixed by attaching new power cable and soldering earth to the chassis, like on the ac60.

the buzzing is indicative of an open ground connection, check the cord by substitution or with an ohm meter, if the cord is good look at the connections to the jack inside the amp ( im guessing you built the amp from a kit). Is this a solid state amplifier? Tube amps have lethal votages and should not be experimented on without knowledge of electronic safety. Also if there is more than one input to the amp try plugging the guitar into another one and see if its still the same.