[asterisk-users] Help 60Hz Hum?

[John Novack]

Mark Bell wrote:
>
> Hey guys,
>
> I am trying to diagnose a hum in my FXO lines
>
> I am using an Adtran 750 with 8 FXO ports. I am getting a pretty bad 
> hum on the line during a call.
>
> I have checked the Telco side of the 66 block and there is no hum 
> there so it’s my problem to fix.
>
Is there hum on the line ONLY when the FXO port has closed the loop? At 
the demarc? When the buttset closes the loop?
Or is the hum introduced on the Asterisk end?
All of the FXS ports clean and clear?
Are you sure it is 60 Hz and not 120 Hz?
Longitudinal imbalance introduced by a defect in one port on the FXO 
card? All 8 ports on 2 cards?
Remember that standard analog Central Office lines ( loop start ) are 
very much like a balanced audio line with DC superimposed for 
supervision. The DC is introduced at the CO end with one side grounded 
at the CO and the other at ( open circuit ) -48VDC, and a ground on the 
subscriber end will introduce hum, or if low enough resistance will keep 
the line in an off hook condition.
Swap the Adtran cards if only on some circuits, and make sure the 750 is 
well grounded.

John Novack

-- 
Dog is my co-pilot