I'm working on a friend's 675 which he bought last week and has all sorts of issues. Correcting them one at a time, but ran into one I can't figure out. It didn't run well and I suspected it was old lube or an e-unit issue. I was planning to rebuild the e-unit today, but decided to bypass it and hardwire the motor to my test transformer to see if it's a motor issue. Clipped a transformer lead to one brush plate and the other to frame. The other brush to the field wire. It ran in forward. When I reversed the brush plate wires, it would not run in reverse. The motor buzzed loudly (not the e-unit, which was disconnected) and the wheels felt "locked". Only when the wires were connected in that arrangement, otherwise, the wheels turn freely in both directions. The springs and brushes are relatively new and the commutator is clean. Any ideas what is going on here?