Inside doorbell won't ring and Ring on Battery

I installed my Ring doorbell earlier this week. I checked the voltage at the old doorbell when I unhooked the wiring. It was 20Vac, and remains at that level. The Ring doorbell won't ring the ding-dong chime inside the house. It is an old-style electro-mechanical doorbell. Two coils and strikers for tone bars. Not digital. Not musical. I checked the transformer in the attic and it still shows 20Vac with the Ring doorbell in place, but my Ring app says the Ring doorbell is running on battery. With the Ring removed from the mounting plate, the inside doorbell does go ding-dong when I short across the power screws with my screwdriver. So it seems like the power from the wires isn't getting to the unit and the button signal from the unit isn't getting to the wiring. So as best I can tell, I have a bad baseplate. Is there a way for me to test the baseplate? Should there be a consistent voltage (either AC or DC) at the interface pads?