Looks like the top saucer is a direct signal from the MPU and does not go through the decoder on the SDB. Check continuity from MPU J4 pin 5 to SDB J4 pin 11. You need to test from the back of the MPU to the back of the SDB to verify the pin inside the connectors are good. The easiest way is to take a row of header pins and plug one into each connector, then check end to end via the header pins.

Actually, if you haven't repinned both of those connectors I would go ahead and do that. MPU J4 is especially susceptible to corrosion.

Here are a couple of pages from the schematics. On the first page you can see the top saucer connects to SDB J5 pin 7 (output) and routes back to J4 pin 11 (input).

The second page shows SDB (board A3) J4 pin 11 connects to MPU J4 pin 5.

Xenon_saucer_1 (resized).jpg
Xenon_saucer_2 (resized).jpg