Doorbell killing power adaptors

1st gen doorbell.hardwired with bell transformer for a couple of years, no issues. Then battery drained and would not charge. I checked AC voltage from transformer, nothing!purchased identical transformer, connected it, no charging. Checked voltage output, zero!purchased plug-in adaptor, checked voltage output before connecting doorbell. Good voltage. Connected to doorbell, no charging. Checked output voltage, zero!does anyone know what's going on!I can manually charge the doorbell but I'd rather gave it hardwired.