Your Tek DM254 is likely some variation of an older Appa Tech 100 series. See
https://www.eevblog.com/forum/testgear/iso-tech-(appa)-idm105-dmm/I suggest the following tests.
1) If you haven't tried new batteries, put in new batteries and retest.
2) On the Appa 105, there are 3 spark gaps (white with pcb designation SG), 2 blue PTCs and 3 power resistors (wrapped in heat shrink and in between the V/ohm jack and the Caddock precision resistor network). These components are used for input protection and could have been subject to high/over voltage?
3) You can perform all these measurements in-circuit first.
4) The spark gap should measure out of range (0L) in resistance mode.
5) The PTCs should measure around 1k ohm (a guess).
6) The power resistors should measure around 1k ohm (a guess).
7) Report all your measurements for 4, 5 and 6 above.
8] Anything obvioulsy out of these ball park ranges is suspect.