My son, whom I love dearly, but is sometimes and idiot, as you might see later, bought a used RC monster truck last week (Mega Megalodon) and spent all his pocket money on it. The thing was doing great (despite someone having sat on it, it was crooked as hell but no one but me noticed…) until he and his friend decided it would be a swell idea to spray it with water (which we told him not to do…). Now, three days later, the water seems to have seeped in and the car malfunctions. If you turn it on, the front wheels turn right and keep on doing so. So I disassembled the car and there are only three things that could be the culprit: the mini servo, the mainboard or the little knob that tells the car how the wheels are turned at the moment.
With everything disconnected (all the other cables, the remote) and just the servo plugged in, turning it on it will rotate full speed in one direction and one direction only. That kind of puzzled me. There is one other cable to the front connected to this green rotating knob that tells the car how the wheels are turned at the moment so you can adjust it if it is not going straight. This one seens to have rusted a bit (in the pic I have already cleaned and dried it) and I assume that it kind of told the mainboard that the car is going too far left so the servo turns right to set the wheels staight (which they are, but the mainboard does not seem to know). It also does so if the little knob is not connected, only the red power cable for the servo, so I also assume that it must be saved on the mainboard somehow.
So there are basically two questions:
Can I reset the mainboard somehow without desoldering the batteries? I can’t find a button. Would desoldering them make the mainboard reset? That’s one thing that might be the solution.
or
Would buying a new of those little rotating nobs fix it if I can connect a working one that tells the mainboard that the wheels are actually straight? Can this be the problem/fix?
The thing with the green knob is a potentiometer, is full of water is shows the wrong resistance. Try drying is in the oven at 80 degrees or so. Maybe it helps. Do you have a multimeter? Then you can measure some resistances of this thing, then we can tell if it’s the culprit.
hold off on the oven…
First soak it for a few seconds in 91% Isopropyl alcohol, then give it a good blast of compressed air, (can, or 20psi from a compressor). That will dry it out.
Then hit it with a shot of electrical contact cleaner *(available at auto parts stores).
Finally turn it back and forth, from end-stop to end-stop a few times to make sure the contacts are clean and dry… should be good as new…
Most of these servos are controlled by pulse width modulation. The little potentiometer helps you center the servo where you want. Others are correct, try cleaning and purge with air . You should be able to see resistance on all 3 terminals, and resistance change as you turn it.
Thanks so much for all the help and suggestions. I did all that and nothing worked. Maybe I can reset the board emptying the batteries completely?
That’s why I think/thought it might be the culprit, but even after iso, air etc. it does not work. I have to get the multimeter but might not be able to do this before leaving on holidays.
If it isn’t the potentiometer (which is the only thing that really makes sense) what else could it be? The problem also is: I can buy one for cheap, but it wouldn’t fit, which would mean custom parts. Maybe I’ll just write the manufacturer.
I understand. If you want to do a quick test. find 2 resistors in the range of 5k each. (if the potentionmeter is really 10k. use 3 jumpers if you do not want to unsolder the pot leads. solder the 2 resistors together end to end, clip the middle to the pot center lead. and connect the end of each resistor to each end of the pot. if the pot is open when you power up the servo should go somewhere near center or at least a different spot. If this works , then you might try to disasemble the pot and really clean it.
The mainboard looks fine, it’s pretty well shielded (though I tought that ofthe potentiometer as well).
I found a shop in Oldenburg that sells and repairs RC vehicles nearly exclusively, they might be able to help if I can’t fix it.
I’ll keep you guys posted.
A servo has a position sensor inside it. It may even be a potentiometer. The control board tells the servo, “go to 50%”. The servi compares the position to 50% and goes left or right to reach it.
But 50% may not be straight. So they added a potentiometer. The control board is probably applying ground and Vcc to the outside leads and measurung the voltage on the middle lead. Nominally, that would be around vcc/2. The control board interprets that value and adds it into the control signal to send 48% instead of 50% to go straight (as an example).
If you put a voltage divider in (as Larry suggested), and it still jitters and only turns right, I would suspect the servo. Do you have another servo you can put in to test?
If not those things, the main board could be at fault. I doubt a software reset would do it. But there is a lot of terrible software out there.
I have had really good luck with acf-50 and isopropyl.
I also love trying to fix things before it just gets put in the landfill.
Did the controller get wet by accident? Iirc sometimes when you turn those remote controls on they will use that to tell it that it’s in all neutral.
Have a real good look at the circuit board for anything burnt or popped.
Usually I would clean with iso we get the 99% stuff here at work. Then brush around with a tooth brush.
The iso will leave a white powered residue as well.
Then I will spray and soak it in acf-50 and brush around.
If it does it when you have the remote on and all knobs and trims to center then turn the car on. Try holding full righ on the controller.
Thanks for those suggestions.
