KOB Dial-up Circuit

by Greg Raven

A new dial-up Morse terminal circuit has been developed to allow the use of key-on-board telegraph sets. Actually this new circuit is a modification of the "Trump Solid-State Dial-up Morse Terminal" which has been in use for several years now.

The original circuit required a 3 wire hook-up to the telegraph instruments. The new circuit only requires 2 wires. This 2 wire hook-up has the advantage that KOB sets can be connected without tapping the connection between the key and sounder on the bottom of the board. Simply connect the terminal to the 2 binding posts of the KOB and you are ready to telegraph. You no longer have to remember which wire goes where, as the the polarity of the connections is irrelevant.

No circuit boards are available for the new circuit, however, the board for the original circuit can easily be adapted to the new circuit. Simply compare the old and new circuits, and you will see that only a few minor changes are required.

This new circuit was developed after several months of experimentation. Ed Trump provided some excellent tips on dial-up circuit design and the potential problems. Early versions of the KOB circuit suffered from "kick-back" caused by the inductive nature of the key and sounder circuit. Some of these problems can only be noticed when telegraphing over long-distance circuits. The problems were eventually explained and resolved with simple modifications to the circuit. The new circuit has been used for several months now with excellent results.

The terminal can be constructed with an external power supply as shown in the schematic. No modifications to the DCM-6 modem are required. It is also possible to tap the required voltages from the DCM-6. In fact it is possible to build the entire circuit into the modem which makes for a very compact and portable set-up.

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