Ttl Selcal Parts List

Integrated Circuits

11,12 - SN7404 hex inverter

4G1, 4G2 - SIM7420 dual four input nand gate

8G1, 8G2, SG3, 8G4, 8G5 - eight input nand gate, SN7430

SR1.SR2 - five bit shift register, SN7496

FF1, FF2, FF3, FF4, FF5 - SN7476 dual JK flip-flop

MS1, SN74121 monostable multivibrator Transistors

G1, Q4 — 2N4124, NPIM silicon G2, Q3 - 2N4126, PIMP silicon Q5 — relay driver transistor, 2N697 or equivalent

Capacitors

O.t ¡IF Sprague type 225P Orange Drop, 100V dc (4)

Resistors, %W composition 560f211} 680£2 (1) 10 KS2 (2) 1 Ki2(2) 15 Kfi(1) 470 £2(1)

Diodes

1N270 germanium (3) 1N4004 silicon (1)

Relay

Allied Radio Type KIM 105-2C-1 2D, DPDT, 12V dc coil, 80 mA

modulators mentioned already contain a motor control relay, and it might well be put. to use in some way or other to be used in conjunction with the SelcaJ output signal. The method of control seems to be an individual thing which is best suited to the station at hand. Each one seems to be slightly different as are most RTTY installations overall.

Conclusion

In building this sort of circuit, be sure to include adequate bypassing on the 5 volt Vec line around the circuit board. In addition a shielded enclosure for the circuits is a must to eliminate stray rf from getting in and upsetting circuit operation. Adequate filtering and bypassing of the signal lines going in and out helps here too. However, in most cases, a nearby transmitter will not be operating when the Selcai is required to do its job, unless of course, the station is transmitting on one band while monitoring another band at the same time. In such, a case, rf elimination can best be achieved on a cut and add basis for the individual station layout.

A couple of Selcals using this circuit have been built by other amateurs using hand wiring techniques. These operated without difficulty. The author wiil be glad to try to answer individual queries regarding the circuit, construction techniques, etc. The considerable simplicity of this design makes for much easier wiring, even on a point to point basis, eliminating much of the chance for error in the final product.

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