The schematic helps you trace the LCD ribbon cable connections and the backlight LED driver circuit. Instead of replacing the whole front panel, you can replace a single Zener diode.
Official sources are scarce because TYT does not publicly host service documentation. Legitimate methods include: Tyt Th-9800 Service Manual Pdf
Warning: Avoid random “free PDF download” sites – many contain malware or are simply the user manual mislabeled. The schematic helps you trace the LCD ribbon
Because the TH-9800 is a near-clone of the Yaesu FT-8900, the circuit design is 95% identical. Search for the "Yaesu FT-8900 Service Manual." While the component labels differ, the alignment procedures and block diagrams are functionally the same. Experienced techs use the FT-8900 manual to repair TYT radios. Warning : Avoid random “free PDF download” sites