That is certainly a low quality cable. I don't think disconnecting the ground pins is a great idea.. but I also don't think it's causing your problem. It does seem like it could be some weirdness with your TV.
Generally when you have brightness issues it can be one of the following:
- Uniformly weak R/G/B signals (unlikely)
- Some issue with the sync (possible)
- TV configuration problem (most likely)
Personally I would spend the most time investigating the TV angle to start out. Does the TV have a Limited/Full range option? This may also look like "black level" or something along those lines. It's strange if
all settings are locked in RGB - perhaps this can be changed by activating the service menu in your TV. Although sometimes you can get a different set of options for RGB vs. Luma/Chroma based signals so make sure you investigate the menu fully.
Another option worth considering is if your TV has component inputs - get a cheap PS2 component cable instead. If your TV has component inputs this will look just as good as RGB and may sidestep whatever issues you are having in the RGB pipeline. Also if your TV supports 480p and above it will work via component whereas RGB likely won't.
If you do want to switch to luma sync, this may be possible if the luma pin is hooked up in your wire, you'll need a multimeter though and it will be fiddly. Here is a PS1/2 AV pinout I found (the C-sync stuff is weird but the pins look correct and that's all we care about):
You can see pin 6 is composite video and 5 is luma. So, first you need to figure out if the luma pin is available at the other end of the cable... look for any wires that have been cut off near the end of the cable, and check for continuity with pin 5 (it may also be that the cable is already sync on luma, though doubtful, so check your scart pin 20 wire also), check all the other wires for continuity with pin 5 also just in case it's wired in a weird way. If you can locate the luma wire, then you simply disconnect the composite video wire from pin 20 and solder the luma wire in its place. You will likely need to use some wire to extend it if it's been cut off. You could also just chop the end off the wire altogether and rewire the entire SCART head - though I wouldn't recommend this option, especially if you're uncomfortable with soldering.
