the HP 8720B Network Analyzer has power down after power lost inhouse. Sometimes the display rotate 180° (with assignment from side buttons).
first open device and check for odor and defects. nothing.
disconnect cable
Clean and Gage all Connectors
step by sep one card out, see wich card is the problem.
in me case nothing, after clean and reconnect all is running.
for display error (rotate 180° with assignment of front buttons, overhead install?)
change some small caps on cpu board and digital board.
https://doc.xdevs.com/doc/HP_Agilent_Keysight/HP%208719D,%2020D,%2022D%20Service%20guide.pdf