Comfort II PCB and power supply hum problem

[ghylwood]

Andrew supplied and I fitted the replacement lower power board to my Comfort II as suggested by Chiu. The hum is still present on the telephones and keypads; so the problem does not lie with this circuit board.

I have checked for parallel earth paths and have located and subsequently isolated 2 of them – one on the 2 DMO1 Fermax interface boards and the other on the UCM Comfort to PC interface, this was finding an earth via the 232 serial interface from the PC.



The comfort II board is mounted in a Comfort I enclosure on insulated mounting posts. I did not have a problem with the old Comfort I board.

I have measured the resistance from 12volt to earth and 0volt to earth and the reading is now infinity, so no parallel earth paths exist and the power rail is floating.

If I remove the AC supply plug to Comfort board the hum goes – it doe sound like a mains hum.

I applied a 200ma 12volt load to comfort and with nothing else connected to the board, except phone ac and battery - the hum returned.

I then removed every connector from Comfort II except the out going phone line - the hum goes away, but as soon as any form of 12 volt load is applied, the hum returns.



After reconnecting the power supply, when it had been running on battery, the hum returned as it was now drawing current to recharge the battery. I have changed the battery and this makes no difference either. Does anyone else have this problem or is the top part of my Comfort II board faulty?



The problem lies in the power supply side – Any idea\'s appreciated? Graham Kemp :?

[slychiu]

Can you connect an earth to the Comfort PCB like in the old board and see if it makes any difference

Also is the UCM connected to a PC which has a connectionb to earth?

regards

[ghylwood]

Yes I have done that - I Disconnected the UCM's RS 232 cable to the PC (which does earth the comfort board!)I tested to make sure the power rails and AC input were floating from earth.

 Then tried again and earthed the negative and then earthed the positive power rails on the comfort board. - It makes no difference at all

Then for curiosity I earthed each of the 12volt AC supply wires connected to the Comfort board - That made no difference at all.

The background hum is of such a level that it sends the duplex audio switch into 'talk to door station mode'. As soon as I remove the AC supply to the Comfort board, it switches back to listen from Door station to comfort mode - Also the hum disappears.

Thanks   Graham  

 

[slychiu]

This is a mystery
Have you tried disconnecting the keypad?
Do you have a RGR04 whicis plugged in? Can you try disconnecting that?

You say that with just the Comfort board and telephone connected, you can hear the hum with no keypads and any other modules?

[ghylwood]

Yes I have removed the RG04 and also the kepads and the siren; in fact every thing unpluged, except the phone line, battery and AC supply. Then I have tried it with the phone line removed and the intercom connected and the hum is still present.

If you would like to log in yourself and listen, I can send you a password. It actually distorts my recorded message. Do you think it could be anything on te top board that would cause the hum? I dont have an osiloscope, or I could have looked at the DC power rail and audio out from the board. It is on Keypads, door stations and telephone.

Graham

[slychiu]

Can you unplug the phone line and sign in with the local phone, see if the hum is present

If so, give me a password but send to chiu@cytech.biz

[slychiu]

Graham
I managed to sign in on a mobile phone but could not on a landline. I could hear the hum on the landline, it was not obvious on my mobile. I dont know if it could be due to corrupted parameters. Can you download a blank template FS34 (Comfort Ultra) into your system. There are tuning parameters which may be affecting the volume or audio settings, perhaps they were inadvertently changed

You say that disconnecting the telephone line and signing in on a local telephone still has the hum?

If that does not change matters the other section of the board may need to be replaced,

[ghylwood]

[font=\"Times New Roman\"]Thanks for looking at my problem Chiu, I'm back home now.[/font]

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[font=\"Times New Roman\"]I have loaded a new and blank FS 34 and the hum is just the same. I had actually tried that before, when I was testing, just so that I could unplug everything without the bells ringing.[/font]

[font=\"Times New Roman\"] [/font]

[font=\"Times New Roman\"]Yes, disconnecting the phone line and signing in on the local phone makes no difference. The only thing that removed the hum is to unplug the AC supply to the board.[/font]

[font=\"Times New Roman\"] [/font]

[font=\"Times New Roman\"]So do you think I should ask Andrew to send me the upper part of the board to replace now?[/font]

[font=\"Times New Roman\"] [/font]

[font=\"Times New Roman\"]Thanks   Graham[/font]

[slychiu]

Yes, lets try that

[slychiu]

If the fault is on the digital side, can you ask Andrew to send back the replaced board so we can investigate what caused it.