MMcGinnis
Heliman
Location: Kansas City, Mo - USA
| What is the most expensive part on the Blade CX? If you said the 4in1 controller, you are right. I just priced one at my local hobby shop at $55.00. Well, I have some good news for you guys. At least for those of you who are adventuresome and willing to try ambitious repairs. It would also be good to have decent soldering skills.
The good news? The most common failure in this 4in1 controller (stopping the rotor from turning while throttle is applied, as in a crash where the prop hits the ground). is SUPER CHEAP to repair. How about $2.00 - $4.00. That’s what it costs me to repair mine. Let me explain.
I took my CX outside for the first time last week while it was a perfectly calm day… or so I thought. I got it in the air and was having a blast when I decided to see how high I could get her to go. I got up around 80-100 feet when it began to drift away from me. Full forward and reverse couldn’t overpower the wind up there and soon it had passed over the neighbors tree line where I would not be able to see what I was doing as I was forced to bring it down. I thought the best thing to do would to be time the decent behind the trees in my mind’s eye and give a little extra throttle just as it should be landing to cushion the impact. Upon retrieval of my baby, I noticed a burning component smell coming from under the front canopy. Upon inspection, I found a component on the 4in1 controller completely burnt and split open. I was forced to purchase a new controller for $55.00 just to be able to see what the bad component is, as my bad one was unrecognizable. I vowed if I have my way, I would never pay this outrageous price again. I opened the case on the new controller and found the part number and manufacturer of the bad component.
It turned out to be a dual N-Channel International Rectifier IRF7313 HEXFET Power MOSFET device. A search of International Rectifier’s web site found this part for under $2.00. But they had a minimum order fee of $25.00. I then found this device on www.digikey.com for around $2.00 and a minimum order charge of $5.00 (if order is under $25.00). I purchased 3 of them at about $6.00 + $5.00 min order charge + $1.78 USPS shipping for a total of about $13.00. I received it in 2 days after ordering it. Way to go USPS.
Now the hard part… replacing a surface mount component with only a pencil soldering iron. This component has only 8 pins. I opened the controller case and popped the board out. It’s a little tough to get out as there is a double sided sticky piece of foam rubber in there, holding it together. Removing the old part was easy; just apply the hot iron to the pins while lifting the part off the board with the blade of a pocket knife. I cleaned up the pads as best I could when I noticed a couple of things. I discovered that the pc board solder pads were laid out in an interesting way. If you refer to the attached picture of this PC board, you’ll notice the culprit component located in the lower left corner of the board, right next to the rotor motor connector. The 4 pin pads located closest to the motor connector are actually just 2 extra wide pads. Each pad is soldered to the two left and the two right pins of the MOSFET device respectively. This fact makes soldering such a small component much easier, because if you create a normally unwanted solder bridge between these pairs of pins, it actually improves the solder joint and improves current flow capability. The pads on the opposite side of the MOSFET device are laid out from top left to top right as follows: 1 thin pad, 1 wide pad, 1 thin pad, 1 wide pad. The wide pads are required to handle the high current flow while the thin pads are for the low current control signal to the device. I discovered that the bad part had overheated so badly that one if the solder pads on the pc board had completely burned and broke off. It was the third pad from the left (thin) on my board that was burned off. I mention the dimensions of these pads because since we are working with a very small device, and most likely, your board will be burnt, it might be difficult to align your new device on the board to prepare for soldering. What you need to do is first put a tiny bit of solder on each pad, NOT the pins. Now orient the new device so that the dot on the device (upper right corner of device) is on the opposite side from the motor connector and closest to the edge of the board. Then put a drop of CA glue on the back of the device, and using a pair of tweezers, align the two thin pads with their respective device pins. If you try to align the wide pads/pins, you run the risk of the thin pads/pins not lining up. Once the glue dries, touch your soldering iron to the two dual pads and their respective pairs of pins (motor connector side of device.). Hopefully, you placed enough solder on the pads earlier to allow the solder to melt and create a good joint. I would recommend adding a little extra solder ONLY on these two dual pads/pins to create a solder ‘bridge’ between the pair of pins to improve the quality of the joint and improve current flow. Now the hard part: Heat each pin in the other side respectively to solder each pin to its pad. As long as you have a good hot iron, you should only need to touch each pin for a second to solder the joint. This little MOSFET device is VERY rugged and can handle a lot of heat, but don’t push it. If you find the need to add a little more solder to each pin, be VERY careful NOT to bridge any pins or pads together. If all goes well, you’re done. Connect a battery, the servos and motor wires up and test it out.
Now I mentioned that one of the pads on my board had burned off. If you study the attached picture closely, you notice something white obscuring the upper right corner of the MOSFET device. Since I had no trace to solder to, I took a tiny piece of wire that I soldered directly to pin 2 on the MOSFET and jumpered over to one side of a surface mount capacitor, basically creating my own board trace to replace the bad one.
While you’re operating so deep inside your beloved CX, might I recommend that you do what I did and cut slots in the case of your 4in1 to improve airflow/cooling. Also, you may have noticed that I have added a micro switch inline with one of my battery leads to create a main power switch. I then CA’ed the switch to the top of the 4in1 case where it sticks up out of the hole I cut in the front canopy to improve air flow into the motors and the 4in1. I am always annoyed with having to fiddle with the battery terminals, and I find this a nice touch for rebooting the gyro and for momentary power downs.
I have not done this yet, but I am looking for a couple of surface mount or otherwise miniature ~5amp fuses to solder inline with the motor leads to protect this MOSFET from being fried again. It would be MUCH easier to replace the fuse than to replace the MOSFET again. Besides, this board can only take so much heat before you have nothing left to solder to. When I find the appropriate fuses and complete the next surgery, I’ll fill everybody in on how to do it.
I hope someone finds this info useful.
Matthew McGinnis
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