The thing is working, but it would be nice to have the service manual and schematics for future use.
I've been in contact with Three-R corporation in Japan that took over the service and support for Advantest equipment in 2017.
They will not give me a copy of the 30+ years old documents due to company rules and guidelines.
Even though they dropped the support completely for this model in 2006.
Regardless of how old the information is, they never provide "sensitive technical information".

I guess they rather throw it in the garbage than letting people save old gear.
I find it strange, almost unbelievable, that not even a single piece of schematic would have leaked since 1988.