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Very thin coat of solder might work. Older RAM boards were 'tinned' this way. They do corrode more quickly than gold and need reseating or cleaning, but carts get swapped in and out more often than RAM.
The photos show it's stuck on luma value 9. I doubt it's the 4050. More likely it's saturated luma at the HDMI converter which is expecting a lower voltage range than the 800 provides.
A simple test to vindicate the 800 is to try the RF output on a TV which accepts the old analog standard.
A possible workaround is to wire a pulldown resistor to the 800's luma output. Maybe 75 Ohms to ground.
Any luck with this? Another trick we use with those tinned card edge connectors is to run an eraser over them to remove the dark corrosion.
Looking forward to seeing it on your excellent YT channel.