I know this is a strange question. The brushes on my J7+ stopped spinning and I found after removing the CHM and applying voltage directly to the motor, it would start spinning at about 7 volts, but with resistance and smoke coming out of its small vents. I didn't think 7v was too much, but I found as I let it smoke (burn), it eventually started spinning faster and the smoke stopped.
Put it back in my J7 and this time the brushes seemed to spin, but the bot eventually stopped this time with error thinking the brushes were jammed. I guess I damaged the motor?
By the way, how do I get the J7 into test mode (BITS) so I can see if it is spinning the brushes?