The young blokes RC helicopter has started misbehaving,main rotors going flat out, even with the on/off switch turned off!!!

I have found a wire that was broken & resoldered this (white cable next to the blue cable), but have also found a couple of scorched looking electronic components.

I think one is a diode & one is a heat sink?

diode is marked: SR240 mic

heat sink is marked: NTB (on) 85 No 3G R910

pictures attached the 2 items in question are where the red pencil is pointing.

Are my descriptions correct? Where can I buy them from? What exactly do I ask for? is there w ay of testing them with your ordinary everyday multimeter?





Any help would be apprecaited.

Can get a new Circuit board, but would like to see how many$$$$ it would cost to try & repair what we've got.

Cheers Brendan

The heat sink is more than likely a transistor.
Try jay car or similar mate. Don't know if the part numbers will transfer straight over.
You can test them but not in the circuit

One is a Schotky diode.
The other may be the receiver code or model.

http://www.hobbyking.com/hobbyking/store/__688__190__Radios_Receivers-2_4Ghz_OrangeRX.html

those 2 heat sink looking things you are talking about are likely MOSFETs or at least standard type high current transistor. If I am not mistaken RC toys today use a HF AC motor drivers so if it is going flat out there maybe a fault with HF driving circuity or further down like the receiving side of the controller? If there is a short in the output transistors I don't think the motor will even turn.


I never worked on RC stuff before, so I would be pretty useless without the schematics.

whats the other side of the board like? It looks like heat damage from this side, so is the other side charred? It might have been a bad joint that killed the other components.

Thanks for the input guys,

Other side of the board looks fine, I think it may have been the diconnected wire that caused the intial problem.

I'm begining to think a brand new board for less than $20 will be the easiest way around it......

ntb 85n03g is a mosfet not transistor
there is no heat sink there. it is just the component package.
it is probably driven by a microcontroller of some sort which drive the mosfet with pulse width modulation

just like everyone said if it's $20 just buy a new board. most place will charge atleast $35/ hour
mosfet like that is probably around $5 at your local electronic shop and I don't think that is what's wrong

Finally I can contribute!
First. Go buy the $20 PCB, there may be components on there that look fine but are cactus.
Second. Yes you have a standard diode, as indicated by the white marking directly under the component on the board. It can be tested in circuit with a standard multimeter, just put it on any resistance setting, if it reads short circuit its fried!
Third. You sort of got the other component right, what you have described as a heat sink is a part of the component that is used to transfer heat to a heat sink used to cool this component down. If a heat sink was mounted on the component it may not have burnt out in the first place. As to what the component is, It may be an output FET or it could be a Bridge Rectifier Diode, used in power supplies to provide a constant voltage. Turn the board upside down, 3 legs =FET or transistor.... 4 legs = Bridge rectifier. My guess without seeing a schematic diagram is that the white, blue, red & black wires are from the power supply and the mystery component is a bridge rectifier diode.

For the other techs out there, ntb 85n03g is a N-channel power MOS FET, Typical Applications
• Power Supplies
• Converters
• Power Motor Controls
• Bridge Circuits

its a helicopter, no power supply other than battery. its a MOSFET for driving the 3 phase variable motor, exactly like a router spindle and frequency convertor or it could be a H bridge motor drive.

I just re read the OP, you say it goes flat out either on or off? may likely be an internally shorted output MOSFET, thats real common if its been over heated or static discharge or cheap china component, I'd go with the latter. With the motor disconnected the output devices should measure in the mega ohm range across source and drain, few 100k from gate to the S or D I think?, if they are close to 0 ohms they are stuffed.