On Henri, my '74 Sov, there is a fuse box available on the right side of the rear closet. You need to get on your knees with a flashlight to see it. Henri is a rear-bath, so depending on your configuration, the fuse box may be in another spot. It is a light blue steel enclosure with a flip-down face that hides the glass "barrel" fuses.
I found that over time, the terminal that hold the fuses in on several lines had corroded badly, and we wound up having to replace the fuse box and the transformer is it fastened to.
No more hum, and lots of power available for lighting etc.
If it's to be, it's up to me.