The attached schematic if for a wireless doorbell model WL-100A-A. It doesn't have any markings to indicate the manufacture.
It has been installed for a number of years and was supposedly working? I recently noticed people have been knocking more than ringing the door bell. I discovered pushing the remote button at the door does nothing. A quick battery change and still nothing. I removed the cover of the unit plugged directly into the wall and discovered that sweat smell of burnt Bakelite.
My initial question was "where is the AC step down transformer?" An internet search of transformerless power supply (PS) reveal a number of hits. One immediately caught my attention because of the warnings about mains voltage. This is my first experience with a PS that doesn't use a transformer. After doing some reading it seems to be a common design. I thought it wouldn't pass muster from a safety perspective. The warning of AC mains being potentially exposed on the lower voltage side of the circuit made me stop and take notice.
After some napkin drawing and initial troubleshooting I thought maybe someone in the E14 Community has experience and can offer advice.
There is no DC voltage. With 120VAC mains applied there is a reading through fuse F1 and nothing any point further. From observation the printed circuit board areas around A, B, C & D are darkened. A resistance measurement of R1 indicates at open. The surface of the component is scorched such that I am assuming its value is the same as R2. R3 & R4 read slightly less than their colour code, no shorts or opens. Diodes D5-D8 appear to test OK. The schematic component type is not correct. I was unable to read the markings and the diode was a fritzing listing. No shorts or opens are measured on the diodes. The same goes for the zener diodes.
I don't measure a short across the 12VDC rails. My first thought was to connect a supply at that point to see if the device still works. I'm not sure what took out the front end but based on what they say about currents for this type of PS design it being caused by the load. Then I recalled reading about having no load on this type of supply. Not being familiar with the design and all the warnings I figured I would post to this life line for some advice.
