The lithium battery on the display board and the GPIB/RS232/Centronics board provides backup power for the NVRAM on those boards.
If the display board battery falls below 2.8 VDC (nominally 3.2VDC) it should be replaced. The best way to do this is to use a couple of fresh AA batteries and a holder so you can have a temporary 3 VDC supply source. Remove the board from the analyzer and carefully examine the polarity of the soldered-in battery. Keeping the same polarity as the board battery, solder the AA battery supply leads across the board battery on PCB traces that are away from the board battery so you can unsolder the board battery without disturbing the AA battery supply. Once the board battery is soldered in, remove the AA battery supply and re-install the board in the analyzer. Test the new board battery voltage before and after installing to insure the correct battery voltage.
The idea is to use the AA battery supply to temporarily maintain voltage on the NVRAM while the board battery is being replaced so that calibration settings in the NVRAM are not lost. This way you will not have to go through the service manual procedures to restore accuracy to the analyzer.
NOTE: Even if the lithium board battery is below 2.8 VDC, replace the board battery this way in case the NVRAM possibly still retains the calibration settings.