Icom IC-740

Electronic Keyer (IC-EX243) Modification

The original keyer has a small flaw. When it is powered on it sets the key down momentarily. By design this happens whenever you switch to CW mode. When you switch any other mode the power (8V) is disconnected from the board. This is actually a behaviour of the chip Curtis 8044 as described in 8044print.pdf.

To prevent this unexpected keying I made the following modification, see mainly the schematics diagram.

Mind the extra BS170 tranzistor. It just overlaps the ICOM label on the board. You won't find it on an unmodified board. Three holes had to be drilled for it.

The bottom view of the board. Except the tranzistor all parts of the modification are soldered on this side.

Schematics diagram

<<< Main Page