Note
QST, Jul 1977, p. 172.
QST, Mar 1980, p. 176.
QST, Mar 1982, p. 183.
The FR-4 worked as a digital PTO frequency readout with all Drake 4-Line transmitters and receivers without modification.It could be programmed for use with TR-4 and TR-3 transceivers on all bands but 80 meters. Its display had 100 Hz resolution. The FR-4 was also a 50 MHz frequency counter.