Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!lll-winken!uunet!zaphod.mps.ohio-state.edu!rpi!uupsi!camb.com!bruce From: bruce@camb.com (Barton F. Bruce) Newsgroups: comp.dcom.modems Subject: Re: T2500 has nasty hum Message-ID: <1991Jan31.011648.39523@camb.com> Date: 31 Jan 91 06:16:48 GMT References: Organization: Cambridge Computer Associates, Inc. Lines: 19 In article , chris@endgame.gsfc.nasa.gov (Chris Shenton) writes: > My recently purchased a Telebit T2500 appears to put a nasty > hum on the phone line. Briefly, when there are no > connections to the modem except the phone line, all is fine. > If the power xformer and/or the RS-232 cable is hooked up, > the hum appears. Turning off the power switch on the back > of the modem does not change anything. If I place the modem > on the line and "listen in" the hum is unbearable, which May be that the phone line xfmr in the modem has an inter winding short. Providing anything that acts like even a half baked ground to the modem is them hooking that to the phone line. Doesn't need to be the xfmr, and could even be some surge/static suppression device on the line in the modem is shorted (maybe while doing its job). If you have a VERY GOOD 110v to 110v isolation xfmr that allows NOT grounding the secondary, try running the modem AND a connected terminal both from the xfmr. Many TV repair shops use such an xfmr just to keep themselves from getting killed working on 'hot-chassis' sets.