UPDATED: March 9 2009 FT-736R sequencer

The inspiration for this came from interfacing a Yaesu FT736R with a 1296 amplifier from W6PQL.

There is a potential problem if the amplifiers 28 volt supply is not on. To avoid this purchased the "all purpose amplifier controller" with the amplifier and incorporated all the features provided in the universal amplifier controller board. This will probably work with the 4 event sequencer as well, but I did not have one to experiment with, so can only report results with the all purpose amplifier controller board and the 2 X 286 amplifier.

What makes this possible is an easy modification to the 736. This applies only to one "band module", in my case, 1296. The module keying is done by one wire, called TX on the schematic. This wire is routed from the control unit to the 1296 module and runs over the wall behind the front panel.  The voltage used to key the module is +5 volts in transmit, and 0 volts in receive. The band module control circuit acts as a current sink, so while the module is keyed, the voltage measured on this wire is about one junction above ground. (.7 volts) We need to use this +5 to send  a ground to the sequencer to initiate the transition from receive to transmit. .7 volts is not enough to keep the sequencer keyed, so the TX from the control unit is dedicated to keying the sequencer and the 5 volts to key the band module is from a different source, but still dependent on the sequencer. Due to the way the sequencer works, we need to do logic inversion twice. The transistor does it once to drive the sequencer, and the relay does it once to apply 5 volts to the band module.

While I was doing this I removed the 1296 module and turned the level down to 10 watts. It was putting out more than 16 watts prior to this. The covers do not need to be removed from the module to do this. Yaesu engineers thoughtfully provided access to the level and ALC pots through the cover. The module does need to be  removed to access the bottom cover.

Some discussion  of the 736 control is in order to see how this works. All of the the functions that initiate transmit are common to the PTT buss and are read by the microprocessor without distinction. This includes MOX, VOX, PTT  and some tone ID functions. Band modules are put into transmit according to what band is selected, and almost instantly, but only the selected band. The accessory keying jack labeled "standby" is only a few electrons behind the PTT, and not useful to prevent hot switching.

Why this works is the 736 band module cannot transmit until the sequencer enables the band module TX line. When PTT is enabled  TX for the selected band goes high and  is converted to a low by the NPN transistor. This is sent out to the sequencer mounted in the PA and the sequence is initiated. Fans run, antenna relay switches, bias is applied to the PA and a ground is sent back to a relay I added in the TX line in the 736, to send the 5 volts to the band module, and transmit is enabled for the band module. Since this is a sequencer, going back to receive is the reverse order. This is not  dependent on mode, so it works with SSB, CW, FM, VOX, MOX or anything that transmits.

I considered using the PTT to initiate the sequencer, but since it is common to all functions,  the 1296 sequencer would be keyed even if 432 or some other band is selected. If I had more accessories then they would all key from the PTT at the same time. Further, if you key from the PTT, it is already too late to start the T/R sequence because the 736 makes power in just microseconds. I needed something on the order of 10s of milliseconds. It took some tracing and testing to see how this works, but it is actually quite simple. Using the TX line on each individual module allows having separate amps or preamps that are sequenced, per band, individually, and TX initiates within a few microseconds of PTT.  The advantage is that I never generate power until the sequencer allows it. The down side is that a separate relay, transistor and jack are required on each band, as well as an external sequencer for each band. The parts are small and 4 bands worth of switching could fit on one single sided board.


I mounted the transistor and 5 volt regulator and relay on the screws that hold the 115 VAC switching supply. This is visible above as the brass squares above the board.  +12 and ground are handy on the regulator board about one inch away. I used an 1/8th inch stereo jack to get the keying in and out. There is a small area between the end of the heatsink and the side where the stereo jack can be mounted. It is useful to temporarily move the power supply to allow access to the inside of the back.


Peek inside between the supply and the side and you can identify the area where the jack can be mounted.  There is very little room, so be sure the jack will fit when the supply is in position. I do not use the internal supply, so I removed it, and have plenty of room . There is also the possibility of mounting a bracket to the back to hold  a jack to the outside. It's pretty easy to get a couple of wires through the back. I'm sure someone will come up with a creative method.

There are likely other methods that would serve to protect the preamp from the transmit power, but this is what I came up with. It works very well. The 5 volts was included with the thought I would install the preamp inside the 736, and this would also be the 5 volts for the preamp. As it turns out, I included a 5 volt regulator in the preamp case.

I am using the same 28 volts that supplies the amplifier, to power the sequencer and relays. This is an important point. If the sequencer and relays have separate supplies, then you are back to square one, and this will not protect from lapses in testing and operating.


The TX wire is available without removing the band module. In fact this whole procedure can be done just by removing the bottom cover. When looking from the front to the rear of the 736 you will see a 10 position plug/jack on  your left in the module. Pin 1 ( a red wire) is on the left next to the wall of the 1296 module. Count over to 6 and that is the TX line. I spliced the two ends and covered them with heat shrink tube and secured the splices with a tie wrap. The wires are routed to the new interface board.

Some additional views of the 736 and amplifier




Put 12 volts to the interface board and a meter between  D and ground. There should be no voltage here. The 736 control unit does not send a ground to the band module. The same is true here, we either send 5 volts or nothing. Ground  C, and you should see 5 volts on the meter between  D and ground.

Remove the ground on C.

Leaving 12 volts connected, make a connection between C and B. The meter  should show no voltage at  D. Connect +5 volts from the 78L05 to A. The meter should show 5 volts between D and ground., and when the +5 is removed from A, the meter should show nothing again.  That's it  !

Just keep in mind the sequencer wants to see a ground to initiate the sequence, and the sequencer sends back a ground from each event. The 736 sends a +5 to the band module on the TX line, and the band module wants to see +5 volts to transmit. The 736 doesn't want to see +12 or ground, so don't send 12 volts to the band module or ground the TX line. Either +5 or nothing. You've been warned !!!!

*NOTE Operating the 736  without the sequencer ( as original) requires a stereo jack to be installed with the tip and ring shorted to restore the TX line.


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