Louis Dionne
Montreal, Canada
February 1999

Introduction

The RCD/Hitec 535 micro receiver has been and still is a popular after market receiver. Low cost, reliable and small. I have 3 of them and found myself wishing that I could be using those on my airplanes that have a more sophisticated layout - like 2 servos in the wings for each ailerons enabling differential or flaperons. Having them on separate channels would give me easier adjustments from the computer transmitter and readily access to the basic aileron differential or flaperon program of my Futaba FP-8UAF computer transmitter. Hitec recently came out with a smaller version; the 555. Don't confuse the two models as I have no idea if the 555 uses the same chip (GD4015BD) nor if the channel connections are the same.

Ever since I read about the fact the microchip at the core of this receiver was already a 6 channel device I had been thinking about converting my 535 receivers. Well, after swapping one too many times my 8 channels receivers, I decided to act upon the conversion and dig out a few notes I had taken when Ralph Weaver and I put a scope on the microchip part a couple of years back.

Out came the tools, soldering iron and the definitive intent.

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Adding a Channel vs Swapping

Open up the case, by removing the tape. Then remove the case altogether. Under the receiver you will find a 16 pin flat chip soldered to the surface of the small printed circuit board. Pin one is at the left top side of the chip when looking at the chip with the small dot at the top. Pins are numbered from the top left, going down and then back up on the other side. Our channel 6 pin, Pin 12, will be the 4th pin from the bottom on the right side. Pin 13 is channel 5.

An alternate solution to get the 6th channel is to disconnect pin 13 and attach Pin 12 to the soldering pad where the Pin 13 was connected. You would disconnect pin 13 by applying heat to pin 13 pad and lifting the pin of the pad with a small - very small - tool; I got myself a dentist tool for this kind of work. This will of course prevent the use of channel 5 but will let you use the regular connector for the servos without any external changes to the plastic box or the receiver. Your receiver will look as before, simply tape over the label for ch5/bat and write instead ch6/bat. I did not try this channel swapping since I stilll wanted to use the 5th channel for my airplanes with retracts and twin wing servos. But others have done it.

Note also that you can expect the warranty to be void by making the proposed modification, on the other hand the modification can be easily reversed.

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Items Needed

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Step by Step

The receiver box has been removed. Turn the receiver on its back. The main chip is located under the neoprene rectangle. Remove it and put it aside for now. Cut the wire to the length you want, cutting the white wire shorter like shown on the picture below. Remove a short section of the wires and tin them, then cut the exposed wire to be about 1/16th of an inch long.

There are 3 rows of pins where the servo connectors will normally fit. The row at the outside is for the ground, the middle row is the positive and the row inside is for each individual channel. You will see that the pins for the ground row are all connected by a trace on the circuit board. There is another trace connecting the positive row. Solder the black wire of your pigtail to a connector pin on the ground row at the far right. Solder the red wire to the connector pin next to it but on the positive row.

Locate Pin 12 on the chip starting from the Pin 1 with a small dot beside it. Pin 1 to 8 are on the side of the dot. Pin 9 to 16 are on the other side with Pin 9 starting from the right side as viewed on the picture below. Solder the white wire of your pigtail to Pin 12 on the chip. Be careful here as the leg of the chip is quite small and make sure you dont more the white wire before the solder solidifies otherwise you will create a cold joint with poor conductivity and risk of failure. Make sure also that there no solder connecting to adjacent pins.

The hardest part is done. I simply use a piece of cardboard cut in a J shape to prevent the various other soldering points to puncture the pigtail wires. The small cardboard piece is place between the circuit board and the pigtail wire. Replace the neoprene protector on the chip.Now you are left with cutting with a modeling knife a small channel in the bottom plastic case. Reassemble everything and check it.

As you can see on the last photograph, I am using a straw to wind my antenna. A number of people don't recommend this, but I use this method to ease the moving of my receivers from one small plane to another and prevent an annoying cable floating in the back of the airplane and I can't step on it anymore. The antenna now fits inside the airframes of my SPEED 400 airplanes and I don't fly them very far as they get small very fast.

Good luck with your modification and enjoy your expanded receiver.

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