VIC 20 without audio tone

[fabry891]

Hi everyone,
I'have VIC20 that I bought in 80's.
During this days I tried to turn on it, after 20 years well-deserved rest.
If I try to lauch a game, I don't hear any sound, except like "pc speacker".
Reading the manual, I found POKE comand, so If I try to digit POKE 36878,15 (volume max) and than POKE 36874,222 or 36875,222 or 36876,222 I don't hear any sound (with the tv audio volume set to max), but with 36877,222 I hear a noise like "pc speacker"
Also, only once, the VIC20 connected to TV via RF modulator, the screen became grey color e it began to flicker, so I tried to connect the VIC via scart cable e TV said "no signal". Now this trouble never appereance again.
I would to replace the MOS 6561-101, but I want to learn your opinion first or any suggestion is much appreciated.

Thanks