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I try to repair the receiver board of Syma X8 series quadcopter. One of four DC motors is not drived properly, i. e. does not turn on after the signal from transmitters (the other motors do). The motor works if it's connected to other outputs.

Receiver board

Using an oscilloscope I found out that all big 4 MOSFET at right part of the board are OK but one of four SMD transistors marked Y1 (circled in green) connected to the gate of one MOSFET is dead.

I have no previous experience with SMD parts but I found that the Y1 marking is usually used for Zener diodes or transistors. I think this could be the latter, because I see square signal at the "input" pin 1 (base/gate) of other 3 transistors that is amplified at the "output" pin 2 of the other three.

Signal at pin 1 and 2

Using the diode test at the multimeter I measure 797 mV drop from pin 3 to 1 and 795 mV from 1 to 2. So I think this could be NPN or MOSFET.

I tried a NPN like BC547 but it heated up after the start in a few seconds and the motor still not working. If I tried a PNP like BC556, it did not heated but still not turning on.

How can I find the right replacement for the broken transistor? What should I measure to find out the correct type?

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    \$\begingroup\$ Are you sure the associated power MOSFET is ok? It's common for a failing power transistor to kill its gate driver. \$\endgroup\$ – pericynthion Mar 31 '17 at 7:38
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Your diode test results and operating waveforms indicate that it is an NPN bipolar transistor (if it was a FET the Gate drive voltage would be higher, and the Gate would measure open circuit to both Source and Drain).

The circuit probably looks something like this:-

schematic

simulate this circuit – Schematic created using CircuitLab

The transistor should not run hot because both input and output currents are limited by R1 and R2. So there be a short a short somewhere, either in a component or between tracks on the PCB. The most likely culprit is FET1, which may have burned out and is now a short circuit from Drain to Gate and open circuit from Drain to Source. If this happens then Q1 will try to drive the motor directly, but won't be able to because it can't supply enough current.

Remove the FET from the board and measure resistance between Gate and Drain. It should be infinite. If it reads low resistance then the FET is shorted. You should also test the Schottky flyback diode (D1). If this shorts out the FET usually follows.

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  • \$\begingroup\$ I soldered out the FET (FNK30H150) and checked it again. I measured infinite resistance between Gate and Drain. It also passed a simple test described at utm.edu/staff/leeb/mostest.htm. I didn't mention in my original post that I've already changed D1 Schottky that was cracked. \$\endgroup\$ – Jan Mar 31 '17 at 20:04
  • \$\begingroup\$ Did you check that the replacement diode is the right way around? I notice that one of the 0.01 Ohm resistors has a mark on it - just a scratch or is it burned out? \$\endgroup\$ – Bruce Abbott Mar 31 '17 at 21:02
  • \$\begingroup\$ You're right! One R010 has infinite resistance. Is this just a jumper? Or it is used for current measurement? D1 replacement is OK, I replaced Y1 by BC548 and carefully cleaned surface from iron. Now it has normal temperature. But MOSFET drain still have about 8 V all the time. \$\endgroup\$ – Jan Mar 31 '17 at 21:14
  • \$\begingroup\$ Resistors are between FET Sources and ground so they might be for current sensing. If so then there should be a thin track going from each resistor to the sense amp. IF FET Source is open-cct to ground then Drain will stay high. \$\endgroup\$ – Bruce Abbott Mar 31 '17 at 22:37
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That is a SOT-23 package. Simply google 'sot-23 npn y1'. The part could be an SS8050. Note that Y1 is a small enough code to easily be something else from another manufacturer.

At any rate, compare the maximum rating for Ic for this part with the maximum Ic for the BC547 and you can see why your attempted replacement may have failed.

This doesn't tell you why the original part failed, of course. It may have been a bad part to begin with, or there may be further problems to find with your board.

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protected by Tom Carpenter May 18 '18 at 11:28

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