QST, Mar 1973, p. 168.
QST, Aug 1974, p. 152. (420A)
QST, Dec 1976, p. 184.
QST, Aug 1977, p. 152.
The EK-420A incorporated measures to eliminate faulty keying due to contact bounce. Its susceptibility to RFI was also reduced. All transistors, ICs, and the keying relay used sockets.
