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Modem Repair Questions


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1/2/2004 6:36 PM
bob p Modem Repair Questions
here's a question for the DIY crowd: has anyone ever repaired a dead modem?  
 
i've got a handful of dead 56k PCI modems. they all have similar problems -- they either won't get a dial tone, or they won't hang-up after making a call. my guess is that a defective relay is the culprit in all of them.  
 
so i'm wondering... does anyone have experience in repairing modems? is relay failure a likely cause of most modem failures, and if so, is it worthwhile to try to repair them?  
 
tia!  
 
bob
 
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1/3/2004 6:05 PM
MBSetzer

I haven't fixed it yet but I have one with the same problem, I really believe it is just the relay and I remember the nearby lightning strike after which this one's performance was compromised.  
 
It did not completely fail to connect but the speed is now 14.4 max on a good connection, and it was only running at about 9600 for the first few months after the damage, so it's getting a little better. Always made better than 46000 on the same line for years. Still it never hangs up afterward so I have to unplug it from the phone line each time I want to hang up. From the failure to hang up there is definitely damage to the modem, but I have still not tested the line fully to rule out any damage to the wiring in that office.  
 
I just consider it a little more security enhancement, that machine was never used for any incoming data just outgoing faxes anyway, now it takes me 30seconds to fax final papers to a client instead of 10 seconds.  
 
To operate the damaged modem, I keep the phone line cord unplugged while the modem initializes, then right after the modem says it is *dialing* the number, I plug in the cord. There is a short timed period where the modem is waiting for the dial tone and as soon as it appears it will then dial.  
 
Also, recently I was using one phone line on one PC to call the other line on a different PC on the same desk, and checking out the connection speeds. This was misleading because my office has the full Western Electric wiring harness from the '70's still in the walls. the two pairs I am using for these lines are bundled in parallel from the desk back to the service entrance, as well as being parallel in the branches leading to the other rooms. So there's a lot of stray capacitance which is better at coupling these lines together the higher the frequency of the signals. Plus I think there was some permanent damage left over from the flood, where moisture inside the cable sheath has increased the dielectric factor allowing crosstalk at lower frequencies than before. The problem is the modems check for echo and if there is too much echo they bump the speed down automatically to maintain intelligibility. There is no significant crosstalk from the voice to the data line as long as there is only voice on the main line. But during sign-on and initial modem-to-modem negotiation, when the signal strength is highest, there is a lot of bleed-through of the data tones into the voice line.  
 
Anyway, I can still communicate to the outside with good 56K performnce using a fully functional modem, but rarely can connect to another good modem in my same office as fast as 33.6K.  
 
Mike
 
1/8/2004 5:56 AM
bob p
Mike,  
 
i Googled the string "modem repair -cable" while looking for info, which was very difficult to find.  
 
i don't remember where i saw it, but i found a post on a board somewhere where somebody talked about repairing modems following lightning strikes.  
 
supposedly, a lightining strike can be as far as a mile away and still take out your modem. the parts that usually take the hit are low wattage SMD resistors that are positioned at the connection of the line to the circuit. supposedly the voltage spike in the line can take out your SMD resistors. the post that i was reading mentioned taking them off and replacing them with 1/8 watt resistors stood up on end. IIRC the values were low, about 100 to 270 ohms, and the values weren't critical. the post didn't make any mention of replacing relays.  
 
i have the impression that my modem's problem is relay based, but i have no good reason to believe that a resistor could not also be the culprit. i'm still looking for a good source for replacement relays -- specifically, i'd like a better option than taking an older/slower modem that works in order to rip the relay out of it in an attempt to fix a faster, but dead modem. with my luck, i'd end up with two dead modems instead of one. :D  
 
a couple of my modems are dead and won't dial out, but one of them has the failure to disconnect problem you've mentioned. it works perfectly, except that it won't hang up. i guess that i could continue to use that modem as long as i remember to always unplug it manually and/or reboot the system, but i've been hoping to find a better solution.
 
1/8/2004 8:22 AM
Stefaan Van Slycken

Hi Bob,  
main problem for you will be the double-sided (or even multi-layered) PCBs, they're hell to work on. I don't know but I thought dial-up modems were dirt cheap nowadays?  
 
kind regards,  
Stefaan.
 
1/8/2004 3:52 PM
bob p
as far as multilayered PCBs go, all of the modems that I've looked at have a relay that goes all of the way through the board for soldering/mounting, so multiple layers shouldn't be a problem. i honestly don't know about the SMD resistors, but i would guess that you're dealing with only the top layer there.  
 
you're right, 56k PCI dialup modems are pretty cheap, but 56k ISA modems remain rediculously expensive. i went to a PC show a couple of weeks ago and bought 3 take-out modems from computers that were being parted-out at the end of their lease life. they were pretty cheap...  
 
unfortunately I appear to have been taken -- all three of them are dead and the merchant isn't being cooperative about a refund. fwiw, he appears to be a russian immigrant with less a than honorable business model -- i get the impression that he's selling some parts that he knows to be bad, as i've had a 4 out of 4 failure rate. :( in the spirit of DIY, i'd like to try to salvage at least one of the modems if its possible...
 
1/10/2004 7:38 AM
Steve A.

bob:  
 
    Can you pop the cover off the relays and get to the innards? Sometimes just cleaning the contacts with a piece of paper and rebending the metal a bit will do the trick...  
 
    Back in 1989(?) I got a new TV for my mother since it would not turn on properly... you'd hear it click but it would not switch on. I took it apart to look at the power relay and cleaned it up a bit after writing down all of the numbers on the relay case- I figured it might work better for awhile until I could get a new relay. I never did order a relay for it and the TV still works great! ;)  
 
    Of course the relays in your modems might be a lot smaller with no access for cleaning (in some cases I have held a board sideways and upside down, trying to get contact cleaner to soak inside a relay).  
 
--Good luck!  
 
Steve Ahola
 
1/10/2004 5:58 PM
bob p
thanks for the tip Steve! I had thought that my only option would be to replace the relay. for some reason i hadn't thought about taking it apart. if getting good exposure is a problem while its mounted on the board, it wouldn't be too hard to unsolder the relay from its 4 mounting points... (spoken in a fearless tone with soldering iron in hand...) :D
 

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