ClausB

Could they make a decent case for a 400, which has all flat sides?

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.

I don't recall. Maybe we used whatever FJC's video recommend.

A little 3D wireframe animation:

Star Raiders! Oh, wait...

They might have lowered prices on the 400 and 800 sooner to slow the success of the VIC20.

Yes. It doesn't matter unless you're replacing faulty DRAM chips.

Could they be Freddie chips? Or maybe PIA chips for banking?

Gotta look at the data sheet.

No, the 7-bit ANTIC would only refresh half of a 256 cycle DRAM, so the other half would fail.

128 cycle 64K DRAM chips are more compatible because some 800XLs have the older ANTIC with 7 bits refresh addressing.

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.

Have you tried adjusting contrast and brightness on the monitor? It appears that the luma signal is saturated.

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.