TRex Jr. going in circles, help!

We are using the TRex Jr. along with the Tamiya dual motor gearbox (No. 70097) and 4 AA alkaline batteries. The TRex Jr. is in RC mode and we are using Mix mode. There is a two stick 27 Mhz 2-channel controller and a 2-channel receiver, both verified to work by using RC servos.

The TRex was calibrated for centers and endpoints, according to the instructions.

Our problem is this:
Most of the time, when we try to go forward, one motor runs while the other one does not. This makes the robot go in circles.

When everything is powered on, we get a solid blue and solid green light.

When going full forward, the motor that is on get a bright red light and a medium green light. Meanwhile the motor that just sits there has very dim green and red lights.

Please suggest how we can fix this. :unamused:

The Miller Elementary 4th Grade Desitination Imagination team


I’m sorry to hear about your problems with the TReX Jr. There are several possible causes that immediately come to mind:

  1. Did you solder the capacitors that came with the TReX across the leads of your motors? The Tamiya motors are very noisy, and this in turn can cause a lot of problems with electronics that drive them. Soldering a capacitor across each motor’s leads as close to the motor casing as possible should decrease this noise significantly. If you look at section 3.a of the TReX Jr user’s guide, you can see the possible ways we recommend you connect the capacitors (the capacitors are the little blue rectangles with “0.1” written in them).

You can additionally help decrease motor noise by keeping the motor leads as short as possible and by twisting them around each other in a helix.

  1. You should be using at least five alkaline cells or six NiMH cells. I strongly recommend you switch from alkaline to NiMH batteries. Alkaline batteries are not ideal for applications that draw large amounts of current and could lead to power issues. You should be able to find a rechargeable 6-pack of NiMH batteries at a local hobby store.

  2. Are you sure you calibrated your TReX Jr correctly? If the calibration values are incorrect, you can experience strange behavior similar to what you are describing. When you are trying to teach the TReX Jr the channel extremes, you should move each channel to each extreme and hold it there for at least half a second. If the two channels are learned correctly, you should see the status LEDs flash in the following pattern when you first power your TReX Jr: green, green, red, red, red.

One useful test you could conduct is to see how the motors behave when you remove the mixing jumper. With the mix jumper removed, channel 1 will control motor 1 and channel 2 will control motor 2. One channel at a time, gently move one stick from one extreme to the other and note how the associated motor reacts.

Lastly, do you have access to a USB-to-serial adapter or a computer with a serial port? If so, you can connect your TReX Jr to the computer and run the TReX Configurator utility. This program would let you see exactly what signals the TReX is getting through its channel inputs and how it’s converting those inputs into motor speeds. If the problem is with the transmitter/receiver or with the TReX calibration, it would be easy to tell using the configurator utility. The utility also lets you directly command the motors to move at any speed you want, which could help you determine if the problem is with the motors.

If you need help connecting your TReX Jr to a computer or instructions on how to use the configurator utility, just ask.

- Ben