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This FAQ is only for off-warranty 4100 modems with broken power switches that would otherwise be useless paperweights. If it is dead, then nothing you do to it will damage it any more, right? This is desperation modem surgery.
1. With the modem totally unplugged, remove the Phillips screw on the bottom of the modem. Then insert a thin screwdriver or knife blade between the top and bottom halves of the case and gently pry them apart. There are four corner tabs clipping the modem halves together and a plastic hinging latch and tabs on the front that must be separated. It is best to work from the back corners to the front unhooking the four tabs. Once the four tabs have been unclipped, the modem halves will hinge open and the front latch and tabs will unhook easily.
2. Then remove the modem printed circuit board (PCB) from the bottom half of the case by lifting the front corners of the PCB with the blade.
3. Flip the PCB over and locate three pins in a row soldered to the opposite side of the PCB from the power switch. The three pins are circled in red on the photo.
4. These pins must be bridged with solder to short-circuit the broken switch as shown below.
For the first test of this revival method, the power switch was gently pulled from and twisted off the PCB with needle-nosed pliers as shown below, but this step is believed to be unnecessary for reviving the modem.
5. Then replace the PCB back in the case, snap the halves together, and insert the screw. Your modem should work properly once it is powered up.
Since you have shorted the power switch, the DC-power supply, barrel connector can now be used to power cycle the modem.
A special thanks to guhuna for beta testing this modem revival method and providing the photos of the soldered switch pins and the pulled switch.
Thanks for the tip. Managed to revitalize a 4100b that kicked it after two years of service.
Thank you very much; looking forward to fixing this. However, for a short-term fix until I get to my brother with a soldering tool, I've inserted a paper clip in beside the power switch to keep the switch leaning to the left. This seems also to make the connection for me and powers on the modem.