Currently, the RPM gauge drops down to 0 when you turn off DC power. It should not. The RPM gauge (ИТЭ-2) does not need electric power other than it gets from its sensor. The instrument connects via two 3-pin connectors to a pair of pickups (ДТЭ-6) mounted on the engine, one for HPR and one for LPR.
Back of ИТЭ-2
ДТЭ-6
I remember when this bug was corrected on Su-25T years ago. :)