Anritsu MS710a 23 GHz Spectrum Analyzer I/P repair query

[rickells]

hi,

Recently acquired  Anritsu MS710a powers on fine, but no legitimate I/P signal is displayed.
All other functions appear to work, including the internal calibrator signal when tested with a friends SA; including test RF signals(on friends unit).

Wondering if the I/P diodes or amp has been blown ?
Any hints about possible repairs, & schematics ?

thank you