What to check

Check the door unit microphone. A failed microphone tends to give a low hum on the incoming speech (from the door) which is a characteristic of the amplifier when there is no input.

If shorting the microphone connections stops the hum, it would suggest the microphone has failed.

Corrective Action

Replace the microphone.

Additional notes

Please note - there are two types of door unit amplifier;

3 terminal - marked E, D and C - 92ETXAMP (pre 2015 and 4 terminal - marked +, -, E &D 10ETXAMP (2015 on)

Links

https://www.entryphone.co.uk/shop/productdetail/ETXAMP-MIC

https://www.entryphone.co.uk/shop/productdetail/MIC-PTS