Bigtrak doesn't drive anymore

This sounds like the motors are stalled. I would turn the drive wheels by had a bit and see if it frees up. Make sure you have fresh batteries in it. These things were battery hogs.

I had to remove the motors and remove the end that holds the brushes. The grease that was used is old and fouls up the contacts between the brushes and armature. I just used the point of an x-acto knife to remove the clumps of grease. You can clean the rest of the grease with denatured alcohol and a Q-tip. Be careful not to bend the brushes. ( that's what I used.) If you have something else other than denatured alcohol or Q-tips, be my guest. Now about the rest of the gears. Most of the time, you may see the glue came undone that hold the magnets in place. The magnets help Big Trak to stay on a straight line when going forward or backward. I'm sure the glue worked well back in the day, but that was 46 years ago! GOD....I'M OLD! You can use a drop or two of thick CA glue on the magnet holders to hold the magnets in place. Let the magnets dry on their own without accelerator. When I used accelerator, the glue didn't stick and the magnets fell out of their holders and locked the gears up. So I had to take it apart again. I found that applying thin CA glue helps the thick CA stick a whole lot better. But let it dry on it's own. I use Labelle 102+ptfe gear lubrication on the the gear rods. Just a drop is plenty on each rod. DO NOT USE GEAR GREASE!! The grease might get near the emitter and the detector. The emitter and detector tell Big Trak how far to travel when you program it. In others words, Big Trak will not know when to stop or stop turning if grease gets on the bulbs of the emitter and detector. So just use very, very little oil on the gear rods. The gears move freely very well after all these years. I know, I've redone about 18 of them so far. Enjoy! And I hope this post helps.