sundance-communications.com/forum Forums Wiring - Telecom Carriers - Trunks & Services Cabling Telephones Systems Wiring Need help figuring out digital line problem

Hi all, new here, first post - seems like a great community!

I do PC hardware & network support for a local government, and the burden of supporting the phone system also fell to me (350+ users on an NEC 2400 PBX). I have no formal training in voice networks (or anything else for that matter) but I make do.

Trying to figure out a weird problem that has me sorta stumped:

User reports phone not working (NEC Dterm Series E). Tones weak back to blocks at PBX, and I don't get a good light on my tone generator. I test the Dterm in another jack and it works great. I test another known good Dterm in the questionable jack, it doesn't work.

Then I test a Dterm with a AC-fed analog adapter in the jack - works great! Note, I'm not connecting to the analog port on the adapter, I'm connecting straight to the Dterm like normal. It is NOT an analog port. My theory is there is low voltage on the line and the AC power supply is juicing the phone sufficiently to connect.

If I take a standard Dterm (no AC adapter) and punch it directly to the tags on the block fed from the PBX, it works - but only if the house cross-connects are disconnected. If the cross-cuts are live, the phone never lights, even that close in the chain to the PBX. This reinforces my theory that there's a voltage sap somewhere in the building wiring.

Next... where is the problem? A short in the line? A second Dterm double-punched somewhere (don't laugh, there are some weird gremlins here)? A bad tag on a block? I've tried multiple switch ports as well (though the port works when patched directly to a Dterm so it's not that).

We have one main MDF in the lower level of the building (4 floors + LL). Our PBX is on 3F connected to the LL MDF via 5x 100-pair feed lines. Stations are fed from the MDF via house wiring - no other relevant IDFs.


Hopefully this all makes sense. Like I said I have zero training in phone stuff and this is just what I've picked up throughout my tenure here. Any tips or things to try in testing this? I'm embarrassed to say I'm not even supplied with a decent multimeter to test voltage with (I plan to bring one from home). What voltage readings should I expect on a digital line coming from an NEC 2400? How best can I troubleshoot this issue? I'm guessing it's cabling but I'm really not sure.

Thanks!!


Jesse

From your statement of
sounds like you have a short in the cable. Remove it from the PBX feed and test with a voltmeter. Looks to me like you're on the right track.

I don't know what a "tag" is but I assume you mean the pin outs that feed this station.

Thanks for the confirmation. I will follow up tomorrow after I've tested with a meter.

Oh, and I always thought I'd heard people called the metal 66 block pins "tags". Guess I was wrong :shrug:

I've never heard them called that, but I'm old.

I just call them clips...

I've always referred to them as clips myself.

I like the idea that was mentioned about another D-Term cross-connected to the same station port. That would certainly cause the problem you are describing. Since you mentioned that there aren't any other distribution points, that might not be possible. You did say that there are gremlins, so if you read any form of continuity across the pair with an ohm meter, it is either that or a short like Bill mentioned.

Don't forget that line cords and jacks are the culprit most of the time.

If you have a short in the wiring, you will get a solid green light when you flip the toggle switch on the tone generator/tracer to "continuity".

I am not familiar with that phone system, but "Tones weak back to blocks at PBX, and I don't get a good light on my tone generator." sounds more like an open or one wire is shorted to ground. A short across a pair kills the tone from a generator - you shouldn't be getting anything at the block with a short. The tracer unit when in "tone" mode gives off a faint orange light (77 HP model anyway) and will go out completely with a shorted pair.

Jack

Could be crossed, but I doubt open. He stated it wouldn't work until station jumpers were removed.

Not really. A high-resistance short can leave enough of a path for a tone to make it through, albeit weak. Typical causes for this would be corroded jacks, line cords crushed under chair mats or digital phones connected across the same cable pair.

Thanks for the corrections Ed and Bill.

BTW, welcome to the forum, Jesse.

Jack