chris@endgame.gsfc.nasa.gov (Chris Shenton) (01/30/91)
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 makes me think that it is coming from the analog circuitry inside. Further inspection of the [AC] wiring in the vicinity revealed that the outlets in use were ungrounded. I moved all of the line plugs from the computer, modem, and monitor into the same outler strip, which helped a little. Plugging the strip into a grounded outlet helped a bit more, but did not eliminate the problem. Telebit tech support [one day turn-around by e-mail, BTW ;-)] had no further suggestions beyond grounding. Before I make the assumption that something is broken, I'd like to hear the experiences of other T2500 users. Does/did yours have this problem, and if so, how did you solve it? Of course, any suggestions from telecom-guru-land are always welcome. Thanks in advance. Steve -- chris@asylum.gsfc.nasa.gov, ...!uunet!asylum.gsfc.nasa.gov!chris, PITCH::CHRIS
bruce@camb.com (Barton F. Bruce) (01/31/91)
In article <CHRIS.91Jan29153400@endgame.gsfc.nasa.gov>, 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.
mark@runx.oz.au (Mark Webster) (02/07/91)
In article <CHRIS.91Jan29153400@endgame.gsfc.nasa.gov> uunet!media!thanatos!steve= 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. There are two jumpers near the phone line at the rear. Move them across to the other position. They are grounding one side of the phone line. (See T2500 appendix P F-3) -- ------------- Mark Webster, PO Box 222, Wahroonga NSW 2076, AUSTRALIA Phone: +61 2 487 1299 -------------