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Hum on phone.

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Nathan Sharpe25/08/2018 23:10:02
149 forum posts

Hello all. While I accept that this is not Model Engineering , could anyone advise please. I have one NTE5 BT master socket with both line and ADSL on the faceplate , a MK2 Openreach unit. Infinity broadband works perfectly but the BT badged phones have so much hum on them that on speaker they are unusable. The hum fluctuates with the incoming voice , even using 1471 as a test this still occurs. I have removed the faceplate and sub plate on the socket to use the actual master socket and still get the same problem. The line has been checked by BT who report no fault on the line . The phones are all BT branded/badged and are compatible with each other. They consist of a base station with two sub,s and are all on the same ringmain for power. I can find no appliance on that ring that is causing the hum . My opinion is that the wall wart power supplies are on their way out and that the electronics are ageing , these phones are now five years old. I am intending to replace both the master socket and the phones . Would there be any advantage in also fitting ferrites to the wall wart cables to try to reduce any future appliance based hum ? Should I instead try to reduce any mains/earth induced hum on the appliances ? While these phones have fallen in price and replacement every few years is not a major outlay it does annoy me that they are failing. All advise will be useful. Nathan.

Alan Vos25/08/2018 23:37:27
153 forum posts
7 photos

When did the hum first appear?

Don Cox25/08/2018 23:49:33
50 forum posts

If I read this right the fault is present with everything else disconnected, ie front face of the NTE unplugged complete with your internal wiring and equipment. I would suggest you get a known working ordinary (not cordless) phone and try that in the bare NTE socket, if the fault is still there then it points the finger at the B.T. line or the exchange. I would further suggest that anyone who has cordless 'phones should also have a non cordless one always plugged in somewhere. If one night your house catches fire one of the first things to go may be the mains and with it your cordless 'phone, how are you going to call the fire brigade then?

Mike Poole26/08/2018 01:11:02
2604 forum posts
63 photos
Posted by Don Cox on 25/08/2018 23:49:33:

If one night your house catches fire one of the first things to go may be the mains and with it your cordless 'phone, how are you going to call the fire brigade then?

On your mobile?


modeng200026/08/2018 07:19:33
220 forum posts
1 photos
Posted by Mike Poole on 26/08/2018 01:11:02:
Posted by Don Cox on 25/08/2018 23:49:33:

If one night your house catches fire one of the first things to go may be the mains and with it your cordless 'phone, how are you going to call the fire brigade then?

On your mobile?


Assuming it is charged up!

not done it yet26/08/2018 07:39:13
4728 forum posts
16 photos

It should not be difficult to isolate the problem.

Try in another ring circuit? Even in another house? Try a line connected phone instead. Try a different power supply unit? Even a suitable battery arrangement?

There are so many options to narrow down the fault source, that it should soon become clear.

Most likely your cordless set at fault. Might even be the speaker or lines picking up stray interference from elsewhere.

john carruthers26/08/2018 08:12:57
604 forum posts
176 photos

Try a different wifi channel?

Mark Dickinson26/08/2018 08:25:35
48 forum posts
4 photos

Try unplugging all devices from the phone socket, unscrew the face plate so you access the master socket. Get a cheap plug in corded phone, plug into the master socket and the dial 17070 , option 2. If you still have hum it's BTs problem. If the hum's gone it's one of your devices.


Mark Dickinson26/08/2018 08:25:36
48 forum posts
4 photos

Double post

Edited By Mark Dickinson on 26/08/2018 08:26:17

Clive Brown 126/08/2018 09:14:11
443 forum posts
14 photos

Could it be that a broadband filter is missing / duff? We once had bad hum. It turned out that an extension bell fitted to the system needed a filter, as well as the 'phones.

Brian G26/08/2018 09:24:04
705 forum posts
28 photos

Have you tried disconnecting the phone from the line and calling between the phones in intercom mode? That would pretty quickly identify if the phone line has anything to do with the hum.


SillyOldDuffer26/08/2018 09:27:51
5895 forum posts
1280 photos

+1 for checking the line first with a dumb handset. BT aren't always right.

As hum can be caused in several ways, the approach (as others have indicated is to logically eliminate potential causes). Possibilities:

  • Hum incoming on the BT line due to a BT fault. (Handset check as above)
  • Failure of smoothing capacitors inside a wall-wart - try a different wall-wart (same volts, amps & polarity)
  • A signal problem between the base unit and sub-stations - unplug both sub-stations. Change channel if you can, otherwise replace unit.
  • Failure of decoupling or power capacitors inside the base station - replace unit.

Ferrites won't help. Once it's confirmed the BT line is OK, I wouldn't mess about trying to fix an aged phone. If changing the wall-wart doesn't fix it, replace the phone.

For interest, if you have to replace it, have a look inside the old phone before dumping it. I expect you'll find its made in a way that discourages repair by either amateurs or professionals - SMD components, multi-layer boards etc.  Not impossible, but irksome.


Edited By SillyOldDuffer on 26/08/2018 09:29:27

Nathan Sharpe26/08/2018 17:27:18
149 forum posts

Thanks to all for your advice. I've ordered new phones and a new NTE5. Nathan.

Ian S C28/08/2018 14:05:28
7468 forum posts
230 photos

T find less noise with an old transformer type wall wart compared to the modern electronic ones. Mind you the old ones could be a problem too if they weren't filtered properly.

Ian S C

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