Flight controller repair or buy a new one?

A few months ago I received the Eachine QX110 V-Tail mini racer for review. First flights went smoothly, even after minor crashes it was flying like a new one. Courageously, I tried to do some flips in “Acro” mode and I landed upside down. Lots of smoke and fun was gone.

Arrived home, I powered up the drone – one motor started to spin permanently. When I saw this it was clear that there is a problem with the flight controller (FC).

While brushed motors (like the QX110 has) are simply driven by MOSFET transistors integrated into the flight-controller, brush-less motors are driven by ESCs (Electronic Speed Controller). As construction quadcopter, ESCs are 3 types, individual for each channel, 4:1, and integrated into the flight controller.

Using a magnifying glass I carefully inspected the drone’s FC and I found out that a tiny SMD-Transistor was exploded – the label can’t be recognized. Checking the corresponding IC on another channel I realized that I need an “A08K” Fet. Five minutes later, my order was placed for 10 pieces.

To ensure that I’m ready to fly asap, I ordered from Banggood a brand new flight controller too.

Eachine QX110 Flight controller repair

In just 8 days, the package arrived. Surprise! I didn’t get exactly what I ordered. Instead of an A08K, I received A09T (AO3400). I contacted the seller, he said that “these two N-Channel MOSFETs are 100% compatible“.

In order to repair the flight controller, I had to remove it from the frame.

Using a $15 soldering iron, I desoldered the defective MOSFET and soldered back the new one. You need some dexterity because the tiny transistor measures only about 1.5 x 3 mm.

After I put everything back, checked the wires, and soldering I powered up the quad-copter. No more instant motor spin.

Using Betaflight – Configurator I checked if every motor spines freely and correctly. Yep, the operation was successful :)

Flight controller replacing with newly purchased one

About 3 weeks later, finally arrived the replacement flight controller.

Luckily all the motors and the VTX camera are linked to the flight controller through connectors, so no soldering required. I managed to replace the flight controller in just 10 minutes.

After I put everything back, I realized that I received an upgraded FC board with different FETs (7700 BV2U1C), instead of 3 pin ones it has 8 pin ones!

Note: the receiver board is integrated into the flight controller and you need to rebind it with the transmitter.

Which procedure was more cost and time effective?

• 10 pieces of transistors cost me $2, which allows repairing the FC 9 more times (in case the same damage will happen). The new flight controller costs about $30. Repair vs Replace 1:0;
• In order to replace the transistor, I needed soldering iron, magnifier, screwdriver, and about 30 minutes. Switching the new flight controller took 10 minutes and only a screwdriver is required. Repair vs Replace 1:1;
• The latest Eachine BAT QX105 Vtail QX110 AIOF3PRO flight controller comes with an upgraded FET that can handle more power. Repair vs Replace 1:2;

If you like to do DIY thinks I would recommend trying to repair your drone by yourself. If you are new in the field and you don’t have the necessary tools you should go with original spare parts.

What burned out my flight controller?

Later I found out that the motor drivers (FETs) seem to be under-rated. When the current consumption is too big (long term full throttle or locked propeller) they give up with smoke. In order to eliminate this design issue, you have 3 options:

1. do not heavily stress the motors;
2. replace the stock MOSFET IC with a higher rated one (I have not found one compatible yet);
3. you can stack more ICs, 2 – 3 pieces in parallel.