Modem Damage From Phone Line?

[msandona]

OK, here's one for you .

About three weeks ago a user in my a group called me and told me that her U.S. robotics external 56K modem was no longer functioning. I checked out the modem and it was dead so I replaced it with an identical one that I had in reserve. When I replaced the modem I left the power supply and the modem to serial port cable in place in replaced only the modem. The user was then all set. I took the modem back to my office and tested it and it was completely dead so I tossed it in the garbage.

About a week later I got another call from the same user with the same problem. Again the modem was dead. This time when I replaced the modem I replaced all the cables and the power supply , assuming that perhaps there was a short in the power supply.

I called our telecom department and asked whether there was any discharge that could travel along the phone line that might be burning out these modems. I was told that this was not possible though this information was not coming from a telecom engineer but rather a receptionist who answered the phone.

This week I got another call from the same user - the third external modem was now dead . Thinking that perhaps there was some damaging discharge coming back from the computer along the serial port I decided to install an internal 56K modem.

Today the user called me and told me that she was working and all the sudden the computer went dead. I believe the phone line zapped the modem again. It appears that the discharge only upset the computer. The machine needed only to be unplugged and rebooted but appears to have suffered no permanent damage. And the modem is also fine.

my question is: does anyone know whether there is any sort of damaging charge thar can travel into the computer or modem through the phone line? Has anyone had a similar incident?

Thank you .
Matt

 

[franklin97355]

Yes you can get damaging voltages on phone lines. Get yourself a surge protector that has a port for the phone line. Don't get the cheapest one but you should not have to spend too much. There are some that provide insurance if something is damaged but you have to read the fine print and follow the instructions to the letter.

 

[carrr]

Yup. Lightning comes in phone lines and blows modems all the time. In this case, there might be a power source/line somewhere near that is bleeding over (who knows, when the wind blows right) and sends enough juice up the phone line to take out the modem. It could be as isolated as a wiring problem within the wall where the phone jack is located for this pc if nothing else in that office is symptomatic, or something bad on the wallboard for the phone inthe building. All phone lines carry a small charge, just not that much, normally.

 

[cdogg]

And a tip for customers is not to use their modem during a storm! Disconnect it completely from the phone jack.

You won't believe how many people out there don't realize this. ~cdogg

"The secret to creativity is knowing how to hide your sources."
- A. Einstein

 

[Jonmonster]

One thing that I did not hear anyone mention is that there might be the possibility that the user is trying to plug the modem into a digital PBX phone system instead of an analog port. Digital PBX phone systems use a higher current than analog phone lines and have been known to blow out modems when they are plugged into them. Some modems offer some additional internal surge protection from this which may explain why the last modem didn't actually fry. Could this possibly be the case?

-J

 

[msandona]

Hey:

Thanks to y'all for the info. I still don't know what was wrong with the line (our telecom people aren't talking) but the local phone co. was out to look at the line and it seems to be fixed. Just to be sure I'm running the phone line through a surge suppressor.

Matt

 

[mjinks61]

The problem was possibly the phone line cable connection. If there was an intermittent short in it, that could deffinitly cause a modem to fry. A standard land line phone carries +- 90VDC power. That is why a regular phone will still work during a pwer outage. Good luck, and Happy Computing

 

[ANFPS26]

In the USA a standard phone line is -48VDC. Jim

 

[jeffbouldin]

You are both correct 50 volts normal, 90 volts on ring.

I had one site that 3 times in a month the cell back-up for the alarm system blew. Found out that BellSouth didn't ground their telephone interface, every surge went to my equipment instead of the ground. After they installed a ground rod we had no more problems.

 

[excessive]

I have used USR external modems in remote locations with really shitty AC power, worse phone service, no surge suppression and plenty of lightening. The only thing that took out a USR was a direct lightening hit that exploded the ceiling light!!!

every time we had a good lightening storm I'd hear about internal modems, and even motherboards getting cooked...but never an external USR...except by direct hit.

I guess my point is you problem is not a small one.


a surge supprressor is an obvious first step
I'd be curious enough to stick a recording voltmeter on the line before the surge suppressor to see if there was a hint there. Induced currents can be a problem if high current a/c lines run close to and parralle to the phone lines for a distance.. Air conditioning, or powerful electric motors are suspects there.