Reposted from Neil Thomas (retro man cave) if you’re having issues as your TV doesn’t send out the required EDID info it needs to set the 50/60hz automatically for you. Neil posted the following -
Posting this as it might help some owners. TheC64 seems to default to 60hz mode if it doesn't get EDID info from your display and there is no option to manually set it to 50hz (holding power for 3secs or factory reset does not present the option). I had to use another display to get the 60/50hz option and then plug my original display back in to run at 50hz. The first 10mins of this stream exhibit the problem, troubleshooting and the fix. I hope it's useful for some.