Thanks for your reply. Do you mean reversing polarity of the 240v. Sounds scary!! But if I have to I’ll do it

I am going by the assumption of the worst case situation in which your doorbell receiver plugs into a wall outlet. In that case, yes, you should reverse the plug to check to see whether the power supply is transformerless.

If your doorbell receiver is hard-wired into the household wiring then the only check you need to do is to measure the AC between the doorbell's output and earth. It is good if the voltage is not large enough to interfere with the operation of the interface being discussed here or to hurt you. If it is a large voltage, then need for isolation is indicated.

 

Hi

Hopefully those pictures are good enough. Thanks mate

The RF chip is the SYN510H this receives the door push code and feeds it to U2, there are two leds on the pcb. Does one light up when you press the door push?