The 3Com / US Robotics Sportster models 0839-08, 0839-09, 5686-01, 56886-02, 5686-03 and 5686-04 appear to have nearly identical component layouts.

Although the modem can accept commands through the serial port and dial out, it cannot standby in "hangup" or "hook on" mode. The hook on mode provides the physical equivalent of a handset returned to the telephone cradle. This indicates that the direct access arrangement (daa) has some componenet damage.

In older modem designs, the daa requires replacement of the component that provides a voltage decrease such as closing a relay or a pull up resistor. What component provides this electrical equivalent in these particular modem circuits? Is this component obtainable by the average hobbiest?

Tom Glenchur, December 2004

The circuitry for decreasing voltage for the DAA in modern modems is called a voltage regulator. They've been around for years, and USR uses TO-220 packages, not those damn tiny things. I think the average hobbyist could replace it if he could solder. You'd have to remove the old one, decode the part number to figure out which rating it is (Can't be too hard, every USR I've ever seen used Motorola regulators), buy a new one from Radio Shack for $1.39-1.99 depending on where you live, and solder it on.

Lincoln, January 2005