Legally Repairing a Cortex+Odd Motor Issue

Hi everyone, Are there ways that we can repair a cortex legally so that we can still compete in VRC with it because cortexes are very expensive and we do not want to spend money on buying more. Specifically, is there a way to fix a broken port that is no longer giving power? Could we maybe send this to VEX for “official” Servicing if we are not allowed to do it ourselves? I am not sure whether any of our ports are dead, but I do suspect one of them because we were driving normally and randomly one of the motors began acting up. It intermittently begins working and stopping. Do you all have any suggestions. We have not tried all of the basic things like redownload firmware or replace the motors yet, but we were wondering whether there are other things we can try if these don’t work so we don’t have to wait at our next meeting. And going back to the original question, do you think this could be an issue with the cortex or not? I am kind of doubting that is a cortex issue because it is intermittent. I would assume that the motor would not work at all if it was the port, but I could be wrong.

Thank you all for your support. I have asked so many questions lately and I really am grateful for all of your prompt and helpful responses.

Typically a bad motor channel is cause by the H Bridge (a chip) on the bottom of the board. It’s is very difficult to repair and would not be a competition legal fix. Do a google search for H bridge.

As I understand, Vex has a very economical but limited out-of-warranty replacement program for organizations. Contact Vex support for details.

A few things:

  1. An H-bridge is a type of motor control circuit. Sometimes it’s a single chip, sometimes it’s discrete switching elements (bipolar transistors, MOSFETS, or even relays with snubber diodes.)
  2. Unless you’ve done enough testing to isolate the problem, I’d say it’s unlikely to be a problem with a Cortex port. It could be, of course, but other problems are more likely. What you describe sounds like: loose wiring, (check that the extension wires/motor controllers are fully plugged in to the Cortex and that each connection from the port to the 2-wire motor leads are fully plugged in); partially retracted connector pin, (happened to my daughter’s team this past weekend); frayed wiring at the motor, (happens all the time); frayed wiring in an extension cable; partially disconnected cable shorting against a metal edge (more rare, but we’ve seen it); bad motor controller. (It happens, but it’s rare as well.)
  3. Only ports 1 and 10 are Cortex H-bridge ports; 2-9 are PWM, and the drive electronics live in the motor controller.
  4. The “bump” in the 3-wire to 2-wire controller contains a circuit board. It interprets the control signal and drives the motor through an H-bridge. Just not the one mentioned above.
  5. If you do have a bad port (which you can determine with testing) you probably can live without it for now. Rewire and reprogram.
  6. People often don’t use ports 1 and 10. If you are controlling a motor on port 4 and determine it is bad, wire it to port 1without a motor controller. (2 wire connection, no motor controller required). Change your configuration to use the new port. You really shouldn’t be talking port numbers in your code, so this should take minutes to implement.
  7. If you’re all full up on motor ports, but you have two motors that are always run together (as happens often on tank-drive chassis and multi-motor lifts) then use a splitter to drive two motors from one port.

If any of these suggestions don’t make sense, ask away.

People don’t often use ports 1 or 10? I was under the impression that the fewer y-cables, the better, within reason.

My rule of thumb is to (a) verify that both ports work bidirectionally and then (b) only put relatively low-stress (and therefore requiring less power) mechanisms on them.

But I do follow the minimizing Y-cables mentality; if nothing else, it makes it easier to troubleshoot issues with motors (running motors individually). My ideal wiring configuration with 12 motors would use ports 1 and 10 and Y-cables going from the Cortex to inputs on the power expander.

It is hard to add to an excellent list posted above by @kypyro.

@The Electrobotz, the first thing you need to do is to determine if the problem is with Cortex, motor controller, or the motor itself. Since you are asking about Cortex repairs, I assume, you already tried to plug the same motor into different controller/port and did not see any problems with this motor. And if the other motor and/or controller plugged into suspect port started to act up too - it clearly proved the case of damaged Cortex.

In my experience, the intermittent motor behavior, most of the time, is caused by the bad connection. It could be:

  • solder point of the black wire breaks loose inside the motor (we had to repair two cases since the last season)

  • one of the wires has a break (we had one of the MC29 wires caught between 2 c-channels - it look ok from the distance, but conductors got damaged)

  • one of the connector pins or sockets is bad (in the worst case a piece of debris could get into the motor socket on the Cortex)

  • the wires exiting motor are damaged (this could cause a short and damage MC29 or H-bridge, so the best prevention is to keep about 1" of wire attached to the motor body with a rubber band or zip-tie)

If you are sure that it is the Cortex problem, then I agree, that the easiest solution would be to simply avoid using this port and have an extra “Y” from another one. I didn’t see the limit of 2 Y-Cables in this year’s game manual as it used to be before, but you may want to ask in the official Q&A to be sure.

Soldering SMD parts yourself is not easy, even with the special tools. People who do it, have at least a few hundred dollars worth of the equipment for this job. Sending it to the official VEX repair service will certainly save you time, money and keep it competition legal.

However, if you really want to try it yourself, you will need at least a basic SMD rework station, including soldering iron with very fine tips, third hand with magnifier, fine tweezers, desoldering braid, smd flux, and some practice to get experience soldering and desoldering SMDs with the old computer boards.

We are not sure that if it is the cortex yet. We have replaced the motor twice and have put on a new integrated motor encoder. We now have also tried downloading firmware (RobotC and MasterCPU) and the program. The issue happens intermittently, so when we switched the ports, the issue did not occur with either. However, we are not sure if the problem will appear again, so we are going to try testing more.

Which port? 1 & 10 are special; diagnosis procedure is different for 1&10 vs 2 through 9.