PAL SNES boots to a dim screen when turned on

When turned on with a game inserted, my PAL Super nintendo will show a black screen with 1 pixel on the bottom right of the TV.

The only exceptions are:

• Super Mario Kart, which shows the nintendo logo before fading to black and not loading the main game
• DOOM, which shows the red intro text, Ocean logo and then proceeds to an extremely dim and corrupted title screen. It allows me to start the game and the music and sound effects work, but it's impossible to see anything other than the severely dimmed gun shots.

Here's a list of games that I tested and got nothing but a black screen from:

• Super Mario World
• Doctor Mario and Tetris
• Arcades greatest hits
• Starwing

I don't know much about the SNES, but my research makes me think that the problem is something to do with either the PPU chip, VRAM chip or Ricoh 5a22 CPU.

Any ideas or suggestions as to how I can salvage this console, or is it done for?

Thanks in advanced.