Add a 7th Channel to Futaba 6-channel Transmitters

The Futaba Skysport series of radios is popular. It's a basic, 6-channel, dual-stick, aircraft-style transmitter that can be ordered on ground frequencies. This model also includes 6-channel radios available under the Tower Hobbies brand. With no computer, no multi-model capabilities, no mixing, and missing a lot of the bells & whistles of high-end radio sets, it's a pretty good value for warship combat at usually around $120 or less. Something that can strike one as odd, though, is that almost all analog Futaba radios of 4 channels or more come with a FP-R127DF 7-channel receiver. Why is this? It seems like a little overkill, when the transmitter only outputs 4 or 6 channels - or does it?

Put a scope on the output of a FP-R127DF receiver's radio, & you'll see something interesting: 7 discrete channels of R/C servo pulses! I won't bother you with the details of where to see this, but trust me, they're there. What this means is that, inside almost every 4-6 channel Futaba transmitter is a 7-channel transmitter, just waiting to get out! Now, let's see a show of hands: Who would like to have another servo channel (or even more), especially if you own them already? Or, to put it another way, who wouldn't like to pay for a 4-channel radio and get a 7-channel radio?

Basically, what Futaba does is this: They build 7-channel radios. However, they sell 4- and 6-channel radios. Futaba's economy of scale improves by only having to build one basic type of radio. When you buy a 4-channel Futaba radio, you're actually buying a 7-channel radio with 3 channels disabled. What if you could "add back" those missing channels? Well, the good news is that you can, and this article shows you how to do it.

First, understand the basic modular concept of how a R/C "transmitter" works. What we generally refer to as the "transmitter" is that particular box that contains the user inputs - sticks, knobs, switches, etc. - batteries, and a "Radio Frequency" or "RF" section. Fundamental to understanding what's going on is to think of this box in terms of two basic modules: The Inputs module and the RF module. The Inputs module collects all of positions of the various user input devices and produces a servo pulse for each, with the width of the pulse determined by each control's position. These pulses are then strung together in a repeating sequence, and sent to the RF module. The RF module sends them to the receiver - we're not concerned with that here. What we are concerned with is that stream of servo pulses - particularly the fact that the standard Futaba encoder chip always builds a string of 7 of them.

The encoder chip is the heart of the Inputs module. It's basically a microprocessor that takes variable-voltage inputs from the various proportional controls, as well as switched inputs from the channel reversing switches, and outputs a continuously-repeating string of servo pulses to the RF module. The inputs for all 7 channels are there, on the encoder. All we have to do is identify them, connect an appropriate input device, and voila - we've got our 7 channels!

If any quotation of mine ever makes it into a book of famous quotations, I hope that it's this one: "Beware of any plan that includes the words, 'All we have to do is ________.'" I'm always skeptical of anything that sounds too easy to be true - it usually is! However, in this case, it really is that simple. All one needs is these instructions, some basic electronic tools and supplies, and the guts to attempt a modification that is guaranteed to void the factory warranty on your Futaba radio set!

Modifying The Transmitter

First, I'd like to acknowledge my basic source for these instructions, Philip Lawson's web page at http://el-image-private.lboro.ac.uk/play/radio_control/futaba.htm. Philip's instructions are basically pretty good, except that he arbitrarily re-numbered the pins on the encoder chip. I find this to be confusing, but if you ignore this oddity they're easy enough to follow. I've attempted to rework Philip's instructions into a step-by-step procedure, that anybody with minimal electronic assembly/repair skills should be able to follow with no trouble. Note that these instructions include installing a surface-mount capacitor, which can be a delicate operation.

Tools and Materials Required

Step	Actions
1	Remove the back cover of the transmitter and disconnect the battery from the main circuit board.
2	Using the Exacto knife, cut the circuit board trace between the pads for pins 20 & 21 of the encoder. See Figure 1. for the location of these pads. Use your multi-meter to verify that there is no electrical continuity between pins 20 & 21.
3	Solder the 22 nF capacitor to the pads between pin 21 of the encoder and the ground trace of the circuit board. See Figure 2. for the location of the existing pads.
4	Solder the 100-ohm resistor onto the end of a piece of hookup wire other than black or red. Apply heat-shrink tubing over the resistor and end of the wire, leaving about 1/16" bare resistor lead at the end.
5	Solder the resistor lead from Step #4 to Pin 21 of the encoder.
6	Strip about 1/16" from pieces of red and black hookup wire. Solder the black wire to the 0V pin, and the red wire to the +5V pin, as shown in Figure 3. The Channel 6 input connector is a convenient place to find 0V and +5V, but any other suitable points may be used.
7	Connect the opposite ends of the red, black, and other-colored wires to an appropriate input device circuit, cutting to a reasonable length. See below for some suitable examples. Should there be any need to drill holes in the transmitter case, drill or cut an external project box, etc. now would be the time to do so. Such details are beyond the scope of these instructions.
8	Replace the battery and rear case.
9	Test the modification by powering up the transmitter and receiver. Use any servo to test each channel, including channel 7, of the receiver to make sure that the actions made by the input are acted out by the servo.

Example input circuits

Figure 4.
Sample Proportional Input.

Figure 5.
Sample Switched Single Input.

Figure 6.
Sample Switched Dual Input.

Figure 7 shows the complete modification made to a Futaba model T6YG transmitter:

Figure 7.
Modified 7-Channel Futaba Skysport Transmitter.

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