Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!lll-lcc!styx!ames!ptsfa!ihnp4!kitty!larry From: larry@kitty.UUCP Newsgroups: sci.electronics Subject: Re: Wireless Audio Link Message-ID: <1667@kitty.UUCP> Date: Tue, 24-Mar-87 14:39:57 EST Article-I.D.: kitty.1667 Posted: Tue Mar 24 14:39:57 1987 Date-Received: Thu, 26-Mar-87 01:45:48 EST References: Organization: Recognition Research Corp., Clarence, NY Lines: 83 Summary: Replace cable, if possible. In article , jg2u#@andrew.cmu.edu (Joseph Gerard Gottlieb) writes: > <1. You have a defective cable, possibly with leakage from conductor-to- > < conductor and conductor-to-ground. Such leakage could be rather > < high resistance and caused by water permeating a damaged section of > < cable. Since this may be high-impedance leakage, it may not have > < been recognized as such; i.e., any attenuation and noise may have > < been attributed to equipment and not the line. Many installations > < of conduit which run underground will fill with water. This is an > < accepted fact-of-life and generally not a problem, unless the sheath > < of the cable is damaged. How did the cable get damaged in the first > < place? Most likely by the electricians who pulled it! > > DING! DING! DING! The bells and whistles are going off. As for your other > points - I have already looked into those problems. But this one escaped me. > So I have just gotten off the phone with the electrian that ran the wire. He > remembered pulling the wire well (from three years ago) - why ? Because they > used a 4 ton come-along and a cable griper to take the slack out of the run > (since they couldn't access the pull boxes along the way). Consequently the > Chief Engineer at the time tried (and failed) to make them replace the entire > line (it was all one piece). And to top it off the conduit is always filled > with water! AAAGGGHHHH!!! > Thanks for the response. But now what? Since there are 18 wires and 9 > shields could I find enough good wires an possibly go digital (as many people > have suggested) On the assumption that the cable is the cause of your transmission problem, the first thing to do thoroughly test the cable to ascertain the extent of the damage. The simplest test is to disconnect the pairs from the apparatus at both ends, and using an ohmmeter test the open-circuit resistance of each pair itself (i.e., conductor-to-conductor of given pair), and of each pair conductor to ground. I would venture that each of these measurements should be well above 1 megohm for any of the above measurements, since the inherent insulation leakage resistance of any polyethylene or PVC insulation on your cable for such a short distance is pretty high (probably several megohms for 2,000 feet). A few words of caution on the making these resistance measurements: 1. Allow sufficient time for the ohmmeter excitation voltage to "charge" the capacitance of the line under test (you'll understand what I mean the first time you make a measurement). 2. If you have a conductor-to-ground fault close to the opposite end of the cable && there is a ground potential difference between the two buildings (a common situation), you may have AC and/or DC foreign potential across your measurement leads. This foreign potential can introduce a serious amount of error in your resistance measurements. Always take a measurement using both forward and reverse polarity on the ohmmeter leads; both measurements should agree - if not, you have a foreign potential problem. You can also measure the voltage from ground to each conductor to verify the presence of foreign potential. If you do have a foreign potential problem, then you need a resistance bridge set up to do cable-fault testing in order to get useful measurements. The ideal cable-testing device for your application is a time-domain reflectometer (TDR) designed for cable-pair testing (most TDR's are used for coaxial cable testing; that type of TDR won't work properly on pairs). A TDR display will instantly show the precise location of any resistive, capacitive, or shield-related cable faults. Unfortunately, your institution probably does not have such a TDR. I would suggest that you try contacting your local telephone company or electric utility company to see if they would loan you a TDR and a person to run it as a "public relations gesture"; all it will take is 10 minutes or so. While electric utility companies don't deal much with twisted pairs, many of the TDR's that they use for fault-locating (like those made by Biddle) happen to also handle pairs. Once you locate your fault, you should replace the section of cable between pull boxes. Be sure to use an encapsulated splice kit (like those made by 3M that use Scotchcast (tm) resin) to maintain the watertight integrity of the cable splice. The ideal situation of to replace the ENTIRE cable; who knows what stress may have been applied in excess of the safe mechanical strength rating that will suddenly become a problem as the cable insulation ages? If you find some good and some bad pairs, you could use only the good pairs - but I view this as only a stop-gap measure, prior to replacement of the entire cable. I view cable faults (especially where moisture may be involved) as a type of "cancer"; while the fault may go into remission, it will invariably come back - worse. This also goes for presumably "good" pairs in a faulted cable eventually going "bad". <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"