Above are some pictures of the sequencer that I use with my FT-897. Schematics of the actual sequencer can be seen as a .jpg file. There is also a text page as a .jpg file, this was the short article I wrote in 1989 for the 2m EME Newsletter (on a typewriter, remember those..??!)
The first pictures are of a sequencer I recently built up on veroboard, this one will be used together with my FT-817 for som MW projects.
The other one, in the box, is the one I am using now with the FT-897.
I am feeding that box with 12 volts, and it has a sequencer board, some external relays to drive the coaxial relays, preamp and the amplifier. As you can see, this sequencer board has some DIL relays, they are only intended to drive external relays that actually control all of the above. There are indeed more relays on that board than needed, but the boards are "generic" and I use similar boards in other places of my EME station. The one I use for my 432/1296 station switches both 12V and 24V, and then I make use of all relays on the board.
As you see in the circuit diagram, one 741 is for activating the antenna relay, with a delay on break to make sure there is no RF when switching back to receive. The other 741 is for delaying the keying of an external amplifier, and it has instant break to stop the transmitter/amplifier.
The sequencer is connected to the CAT/Tuner connector on the back of the FT-897. I use the TX Gnd to activate the sequencer via a DIL relay on the sequencer board. Remember, the sequencer 741's are triggered by a +12V impulse.
I also route the +13.8V available on the same connector, via a 1.5k resistor, to a set of contacts on the relay that keyes the external amplifier and then back to the pin TX Inhibit. When this pin is held high, no RF comes out of the radio on any band.
This way, no RF can be transmitted from the FT-897 before the amplifier is keyed, and this also makes sure that the antenna relays are engaged properly. Actually, no RF is coming out of the FT-897 unless the sequencer is actually keyed properly, which has proven to be fool proof, so far..
It might look and sound complicated, but it is not.. describing it in words is complicated. No preamp has ever been blown because of sequencer failure.. so far..