Execute the self tests as described in the manual, and see the results.
When the battery inside the "non-volatile" RAM is exhausted, the calibration constants are lost. The remedy is to replace the RAM and recalibrate. Note that the "non-volatile" RAM stopped being manufactured a long time ago, so any "new" devices on fleabay will probably have a nearly depleted battery.
Workarounds are discussed elsewhere in this forum, and on the tekscopes groups.io forum.