I'd start with taking photos of all stuff and posting
. We missing F5500A teardown thread in metrology section
Outguard section troubleshooting perhaps good start. If power is all good (there are +12, -12 and +5V), check resets and check that there is life between outguard CPU and inguard CPU on optical interface.
For schematics perhaps you can reference to 5700-5500A zombie,
57LFC manual, which have schematic diagrams and resemble 5500A for most of the parts. Also check if there is life between front panel and outguard CPU. I have 5500A A2 board and front panel LCDs, which I do not need.
Be careful not to get zapped, there are high voltages inside the inguard box.