The output on my newest Commodore 64 (1989) is pretty marginal. The colors are okay(ish) but the text lines are clearly bleeding and the picture is very muddy. Most vintage computers don't do red text on blue very well, but this one is particularly painful.
My non-expert electronics assessment of the signal is that there are probably a couple capacitors on the board that need to be replaced, particularly in the RF Modulator.
I performed Adrian Black's RF Modulator hack on my beloved Bread-bin Commodore 64 (board model 250407). I've had that computer since I was 8 and the results were pretty dramatic, even if I was less than thrilled about attacking it with a soldering iron and wire cutters.
As far as I know, there isn't an equivalent hack like this for "short board" Commodore 64Cs. This one is a PAL board. My Retro Tink upscales it just fine and my monitor has no trouble switching to PAL video modes.
I'd prefer to keep my vintage computers in their "stock" configuration. Replacing capacitors and doing a simple bypass of a couple components is one thing, but a rip and replace was something I've resisted.
Unfortunately, the picture on this 64C has been degrading so quickly that I've become concerned about it losing it's effectiveness in my desk rotation of vintage computers. I don't want it to be relegated to a non working museum piece, so I set out to find a way to improve the picture without being TOO dramatic about it.
I settled on something that is more rip and replace that I'd previously wanted, but after much hand wringing about it, I took the leap and broke out the desoldering gun. The C64/C128 S-Video Bypass/RF Replacement offered by Video Game Perfection arrived in the mail earlier this week.
The procedure is straightforward if you have any soldering experience at all. The old RF Modulator can be a little tricky to get off, but patience is your friend. Just keep at it and don't get frustrated and break something.
There are plenty of techniques videos for removing these from boards. Jan Beta is a good place to start.
Some examples of the picture quality before the upgrade:
The same screenshots after the RF Modulator replacement:
My results are good. It's definitely better, but I guess I had expected a near perfect picture. I know, unrealistic.
The colors are less muddy and it's sharper. I didn't hook up the oscilloscope for before and after, I will when I do this same upgrade on my flat Commodore 128 next week. Having snapshots of the actual signal would have made me feel better about it because I think that might show a percentage improvement in the signal that might be more dramatic than my eyes can see.
Overall I'm pretty happy with it. It's a simple upgrade that most people can handle on their own and it was only $19 US. It's certainly part of a "future proofing" attitude I have towards these beloved computers.