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Repairing a 4-in-1 radio receiver

Hi. I have a Walkera Mini Lama v4-5 model helicopter, with two brush motors and two servos controlled by a 4-in-1 radio. It lost radio control (my fault, switched off the transmitter), broke a blade and the two rotors got tangled into each other. Engines got stalled, but were trying to spin at full power. Absorbed too much current, and something blew inside the 4-in-1; now it responds to servo movement, but doesn't spin up the motors. A new radio is quite expensive here, so I'm trying to repair the one I have. Here are the pictures.

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(the board is horizontally reversed here)

In a RC forum they suggested I replace the two mosfets you can see in the first image on the right, by the engine contacts. Testing them with the diode function of my multimeter gave incoherent results (extremely brief, high reading, followed by nothing), so I assumed they were toast as per suggestions. I removed them and soldered two new mosfets. Nothing changed. What's more, the new mosfets tested in the exact same incoherent way. I removed them from the circuit and retested them, and they are ok. This means the old mosfets were probably ok and the fault is somewhere else. Also, with the mosfets disconnected and the circuit off, I measured the resistance of the two engine contacts. One tests open, the other has a

130ohm resistance (if I'm reading the multimeter right). This seems weird, as the two connections are supposed to be the same. Any ideas? PS Keep in mind that while not absolutely totally electronic-illiterate, I'm very much new to this stuff. Please be specific about components and what I should do. Thanks.
Reply to
drawyoursmile
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Den Sat, 10 Jun 2006 04:40:34 -0700. skrev drawyoursmile:

Take a look at the resistor marked R82 it looks like it is dead. The value

0.82 ohms looks like a short at your meter. 
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Med venlig hilsen

Christian Treldal, OZ1GNN
"Remember Darwin; building a better mousetrap merely results in smarter mice."
Reply to
Christian Treldal

It doesn't short, but it's not 0.82 ohms either. It goes from 2 to 2.2 (multimeter on "200" position). There's another R82 resistor right under the crystal socket. It looks and tests exactly the same. Are both dead? Should I replace them?

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Reply to
drawyoursmile

Den Sat, 10 Jun 2006 08:57:54 -0700. skrev drawyoursmile:

They are probably ok. They looked like they had a crack in the middle, this was the reason for my guess. Med venlig hilsen Christian Treldal, OZ1GNN "Remember Darwin; building a better mousetrap merely results in smarter mice."

Reply to
Christian Treldal

I read some articles about how mosfets and diodes work, and I've ruled out the high-current part of the circuit. The mosfets get the source current from the battery, and the drain conducts just fine to the negative pin on the motor contacts (the positive pins, in the middle, are linked together). However, the gate voltage is equal to the source voltage, and doesn't change as I move the sticks on the transmitter. This makes me think the fault is somewhere in the receiver circuit, as it appears to be ignoring any command that would cause the motors to run. I tested the transistors that drive the mosfets and they test fine too, so it's got to be somewhere beyond that. I've no idea how a motor overload could have caused this. It doesn't seem likely; too much current would have blown something in the high current part of the circuit, not something in the receiver. Maybe it was the crash, but none of the components seem broken and/or desoldered due to physical shock... and it didn't quite crash hard enough to cause this anyway, after all it was on the bed when the accident happened. I'm all out of ideas.

Reply to
drawyoursmile

Den Sat, 10 Jun 2006 11:34:49 -0700. skrev drawyoursmile:

Mosfets are always shorted when they are dead.

Hmmm... crystal oscillators are sensitive to shock perhaps you should try with a another crystal. It might be helpful to you to borrough an oscilloscope.

--
Med venlig hilsen

Christian Treldal, OZ1GNN
"Remember Darwin; building a better mousetrap merely results in smarter mice."
Reply to
Christian Treldal

You mean the removable crystal that sits in the socket of both the radio and receiver? Or is it soldered on the mainboard?

I have no way to get an oscilloscope, sadly...

Reply to
drawyoursmile

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