As Dave from Artek says, it is helpful to know the serial number of the unit. What you most likely want is the "Operation and Service Manual" for this unit. The part number for the manual is 08558-90102.
Keysight probably still has this posted on their website as that is where I got mine, but it is not perfect and there are a few schematic pages that are not scanned completely/properly. If Dave at Artek has this manual, I would consider ordering it from him - Dave's manuals are, well, pretty much perfect.
Is there a specific issue you are having with your 8558B? I have several of them in service and might be able to assist you with debug and replacement parts (other than wafer-switch rotors).
Ken
