Trim problem - Turnigy 9x

[vitormixfer]

Hello!
I was flying yesterday and the elevator trim stopped work stuck going down. I have the telemetry mod, two modules (changing them by a switch) and I'm using OpenTX firmware.
I have unplugged everything from the board and powered it with a power supply, but the trim still goes down. I looked for bad soldering and problems but I haven't found anything.
Can anybody help me?

[jhsa]

Does the trim click when you move it both ways? Check the trim switch is not stuck..

João

[vitormixfer]

Thank you for the answer João!
I've changed the sticks with the trim buttons and the trim still goes down. I think it's something with the board. Even with nothing connected to the board the display and beeper show that the elevator down is being pressed.

[jhsa]

Ok, you will need to do some measurements. You say elevator. Is that mode one or mode 2? It is important to know so we can find out the processor pin it connects to..

It might be there is a short to ground somewhere.

João

[vitormixfer]

Yes, I'm running mode 2. I've tested the resistance between the wires of the trim. On the left stick, throttle, I got 1017 ohms between the center wire and the other two wires. But in the right side, elevator, I got ~1600 ohms with the center wire and one button and 1017 ohms with the center wire and the other button.
Thank you again.

[vitormixfer]

I did the tests again and in both sides I got ~1600 in one button and ~1000ohms in the other.

[vitormixfer]

With the transmitter on, I have ~900 ohms between the wires (gray and purple, gray and white) on the throttle trim. In the elevator's trim I got ~900 ohms between the purple and gray wires but no connection between white and grey wires.

[jhsa]

Please do not measure ohms with the transmitter on. You say you are measuring from the center wire of the stick to the trims? No...
Every trim button has 2 little switches on little PCBs placed next to the gimbals.
Please with the radio OFF, measure the resistance across the switches when pressed and not pressed.

You should expect zero ohms when pressed and open or infinite when not pressed.

João

[vitormixfer]

I was using a multimeter with problems yesterday, I bought a new one and measured the resistance again (I was measuring across the switches). There is a short somewhere, the resistance when the switch is not pressed is around 17kohms.

[jhsa]

Only that trim switch? Elevator down?
If so, please disconnect the 9 pin connector on that side just below the gimbal. Measure the switch again. Does it still read the same?
Please measure with the radio turned OFF.

João

[jhsa]

Even with nothing connected to the board the display and beeper show that the elevator down is being pressed.

I missed this. Please can you confirm that if you disconnect the 9 pin connector the problem is still there?
If it is, then the problem is somewhere on the mainboard.

João

[vitormixfer]

Even with nothing connected to the board the display and beeper show that the elevator down is being pressed.

I missed this. Please can you confirm that if you disconnect the 9 pin connector the problem is still there?
If it is, then the problem is somewhere on the mainboard.

João

Yes João, even with nothing connected to the board, only the power supply and the display.
I measured the resistance between the pins of the 9 pin connector, and there is a short somewhere, connecting the two first pins (which correspond to the trim button). I have checked on the ATMEGA pins too, same thing.
I haven't found where is the connection.
Do you know if the problem can be the Atmega?

Thank you again

[jhsa]

what is the value of the resistance between the atmega pin and ground?

Is that pin 30? If not, it should be pin 28
The schematics I have are a bit confusing because it isn't clear what stick is what.. It should call them left vertical, left horizontal instead of throttle, rudder, etc.. I think the schematics I have are of a mode 1 radio

João

[vitormixfer]

what is the value of the resistance between the atmega pin and ground?

Is that pin 30? If not, it should be pin 28
The schematics I have are a bit confusing because it isn't clear what stick is what.. It should call them left vertical, left horizontal instead of throttle, rudder, etc.. I think the schematics I have are of a mode 1 radio

João

The pin is 30, in my transmitter. I think I was using the same schematics as you.
The value is 17.58kohms between the 30 and ground (pin 53) when measuring on the atmega pins

[vitormixfer]

There in no connection to the ground on all the others trim buttons, measuring on the atmega pins.

[jhsa]

Ok, this is getting complicated. How good are you with a soldering iron?
3 options to find out where the problem is.

1- remove the resistor that connects to that pin.
2- lift the pin by unsoldering it at the same time as you lift it with a needle or something.
3- cut the track between the pin and the resistor. You can repair it later.

I know, I don't like it too..

João

[vitormixfer]

I think I can do this
I will remove the resistor and measure if there is connection from the atmega pins. If I get no connection, I will put some wire, another resistor and solder the atmega pin to the 9 pin connector directly.
What do you think, is it a good idea?

[jhsa]

let's see first where the problem is.. Then we can see what comes next

João

[vitormixfer]

You are right
I removed the resistor and checked the pins. No connection
I'm trying to find where is the connection between the traces, but not very successfully yet.

[jhsa]

There is a capacitor after the resistor that connects it to ground. Check that. If necessary remove it.

João

[vitormixfer]

I have removed the capacitor but there is still connection. I think it's a faulty board, the radio's first owner told me that had happened before and stopped like started. I have changed the capacitor for another new that I've removed from another board, but it didn't work.

[jhsa]

Check for a short between 2 pins under the 9 pin connector on the board. It is strange that the resistance is 17K

If you can't find where the problem is on the board, then maybe try your other idea of connecting the trim switch through a 220 ohm resistor to the processor pin. Or to the pad from the resistor you've removed as it should be easier to solder..

João

[vitormixfer]

Check for a short between 2 pins under the 9 pin connector on the board. It is strange that the resistance is 17K

If you can't find where the problem is on the board, then maybe try your other idea of connecting the trim switch through a 220 ohm resistor to the processor pin. Or to the pad from the resistor you've removed as it should be easier to solder..

João

I did the connection of the trim switch to the processor pin yesterday. Actually, it isn't the solution to the "real problem", because I didn't find where is the short. At least the radio is working well.
Thank you for your attention and support João.

[jhsa]

It is a pleasure to help..
That is a very weird problem. There is this 17k short somewhere between the processor and 9 pin connector.
If it was a complete short I would consider it a more normal problem. But 17k? Maybe the answer is on the other side of the board..
Anyway, good that is working now. At least you can use the radio. Well done.

João