Hitachi U 2800 Spectrophotometer User Manual

My Hitachi U-2800 spectrophotometer is passing all initial self tests, beam is coming in sample compartment (alignment OK). But on blocking sample or reference windows only slight change in Abs reading. Remains mainly negative reading. Suspect either Si detector electronics (pre-amp, amp) or A/D gain problem. Since only straight scan line is observed on scanning full wavelength (no peaks). Can someone help/guide as to where could be problem and how to resolve it? If some can give me Service Code for this instrument, it will definitely help.

Hitachi U 2800 Spectrophotometer User Manual