sundance-communications.com/forum Forums TDM Business Telephone Systems Comdial Telephones & Systems DSU II humming sound on some phone lines

What could cause a humming sound on some phone lines on a Comdial DSU II with Impact phones? The phone company checked the CO lines and they are all good. Listening to the lines with one of the Impact phones, line 1 is ok, line 2 hums when dial tone is broken, line 3 is ok, line 4 hums, line 5 hums. The lines from the DSU to the demark are ok, If I unplug them from the DSU and plug into a regular phone, no noise, but when plugged into the DSU the humming starts. There is also a similar humming sound comming from the DSU that you can hear when standing by it, maybe noise from the transformer? Why would it only affect some of the phone lines?

If the line humm is system wide then it could be the line ports or a few other things such as electrical interference but to be sure you may have to trouble shoot by putting the lines on different unused line ports to see if it's the line ports but from what you posted it sounds like your line ports.Good luck, it may be time to bring in a tech.

check the cabinet , is it Grounded? try taking the ground off, is it Not grounded? Try putting a Ground On.

Yes, the hum is system wide. If it was from electrical interference wouldn't it affect all the lines? I'm only using 4 line ports so there are unused ones I could try, is there a fix for the ports?

I know there is a ground wire hooked to the terminal block where the station ports attach, I'm not sure if it goes to the cabinet also, I'll take a look tomorrow. Thanks for the suggestion.

so? what happened?

Still trouble shooting, there wasn't a ground wire on the cabinet, but adding one didn't change anything. I'm not entirely certain anymore that the DSU is even causing the noise, it may be bad wires from the DEMARK to the DSU, because on some of the lines the noise is still there even if I unplug from the DSU, but much less noticeable, the DSU may be amplifying noise that is already present??

I think I've narrowed it down to line 2 & 4 which just happen to be the two ports on the DSU that have AUX lines. I don't have anything hooked to the AUX. Is there a way to programmatically disable the AUX line on port 1&2 and 3&4?

Is it possible that the DSU may be amplifying the humming noise on the line? I think there are two issues going on here, the CO lines on the two lines that have the humming sound have the same sound if I unplug the line from the DSU and plug the line into a regular analog phone, but the noise is hardly noticeable this way, but it's still there. When I plug the line back into the DSU the noise is very noticeable, but not so loud you can't talk on the phone. Do they make a filter that I could put in the line to filter out this noise? As I said in the original post I've already had the phone company out and they claim there is no noise so they aren't going to be any help here.

Do you think this would help with the noise?

https://www.hellodirect.com/hellodirect/Shop?DSP=30102&PCR=1:1:5:25:280:3050&IID=3716-RJ14&itemskuid=3716-RJ14