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.

http://img83.imageshack.us/img83/6867/lamacent48lw.jpg
http://img84.imageshack.us/img84/703/lamacent31tv.jpg (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.

Repairing a 4-in-1 radio receiver
 

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.
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?

http://img106.imageshack.us/img106/1...aresist6xd.jpg

Repairing a 4-in-1 radio receiver
 

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


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?

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."

Repairing a 4-in-1 radio receiver
 

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.

Repairing a 4-in-1 radio receiver
 

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.
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...

Repairing a 4-in-1 radio receiver