Door Entry buzzing

[binky]

Had a look at a door entry system this morning, Northern Bell, so a bit of a cheapo unit and roughly 15 years old to boot! Now I changed the power supply / control box about 10 months ago, which got it working, but the residents are moaning that it's not working properly again / possibly some of the flats have not worked longer than that. I am a little intrigued as to what is making it buzz constantly, it's so basic, that all you have is the doorbells, a small control box/ power supply and the handsets in each flat. Is it possiby a faulty handset that is causing the buzzing? 

 

[Geoff1946]

What's buzzing?  The door release unit, the power unit or the audio on the handsets?

Presumably it all works on low voltage DC.(?)    I would be looking at rectifiers and smoothing capacitors in the supply. 

 

[binky]

no door release, it appears to be the speaker. It's all 12v. 

 

[ProDave]

Assuming it's all analogue?

Poor or disconnected screen wire.

Failed PSU capacitors so 50Hz ripple on the DC

 

[Evans Electric]

Possibly a mains cable has been installed parallel to the  low volt stuff .    :C