I played around with the voltages a bit.
Mi-8: nothing is on, battery knob I and II, indicate 24V. I start AI-9W, voltages checked, momentary jump on the ammeter to 300A. AI-9W is working, I turn on STG-3, voltage increases to 27(28)V, STG-3 -100A, batteries - 0A. STG-3 off. procedure for starting engines, switching on generators and rectifiers. After switching on rectifiers, the STG-3 ammeter reading drops to 0A and from that moment on there are no indications. AI-9W off, STG-3 generator does not work, does not supply voltage. Checked.
Mi-24: battery knob, no indications. I turn on L. battery, R. battery, battery knob - 26V. This is the first difference between Mi-8 and Mi-24. Then everything looks similar, until the rectifiers are switched on, where the STG-3 readings show 80A and only after switching off the generator do they drop to 0A. The rest is similar to MI-8. Checked.
To my knowledge, this generally looks like correct operation.