COM120B's can't measure fast enough to properly test a DMR MS radio transmitting in TDMA mode, they don't support any test patterns (O.153, 1031, etc.), and won't be able to display BER in digital mode. Not even the later 2975 supported it. It took the switch to a Linux based platform when the other protocols were finally implemented in the Aeroflex/IFR/Viavi portfolio.
You would instead configure an analog mode on the radio to perform the tuning, which you're still measuring and adjusting the same major basic parameters of the transceiver. Even modulation balance is still an alignment done in the analog realm with the deviation meter.
The COM120s were my favourite service monitor model and I've lugged one around for years, very fast boot up compared to the later models and still acceptable runtime from a lead acid battery. But the reality is the internal real time clock chips and the displays are reaching end of life and they need special care taken to not lose options (read out and make backups!) and try to avoid leaving static graphics on the display for periods of time. If you find one that's been unplugged for years, chances are the RTC module may have discharged. Then my last one finally lost its power termination temperature measurement. Plan for the common failures and you end up finding out something else goes...