Intercom and door lock no longer working

I have one of these outside the main external door and once upon a time, the three flats could use the receiver in their flat and talk through this and hear the visitor. We could then press a button which unlocked the door). Now neither of these functions work but I notice that there is some power in this unit as it is lit up.



Does anyone have experience of this and suggest how I may go about troubleshooting this please.

I do own a 2 pole voltage tester and this provides a voltage reading too.

make and model please - we need to find the manuals - and ideally circuit diagrams

Not always easy unfortunately.
For a novice a good start is go to this website
http://www.intercomsrus.com/intercom_handset_finder_tool.htm
Find your handset and Doorbox and you will see the connection functions.

Do a little drawing of your wiring colours transposed onto that and using your tester you may be able to break down whats going wrong.
Forget about the speech bit to start and concentrate on the CALL buzzer and door LOCK bits, this should be a case of measuring voltages across the relevant connections from your made up drawing.

These voltages would be measured between CALL ,LOCK terminals AND COMMON with relevant buttons both ways both OUT and then PUSHED IN
The light on it may be a help, but the voltage could still be lower than needed, so a good start is measure that voltage first
Normally 12volt , same voltage would be found in your Call/lock to common tests above.

If no 12v present in the main door box then your problem may be the power supply located elsewhere.
In the Doorbox you will find the input cable from that power supply so test voltage there as one of your first tests

It’s an old bell systems intercom ... And they normally use 12v Ac

And I think the O wire is the common, so probably where the O wire connects at the speech unit at the door, would account for total failure of all functions on all handsets.