The billboard is a very simple item. Based on the fact that the armature spins freely by hand we can assume there is not a mechanical or binding issue. The 2.5ohm resistance reading between the yellow and green wires tells us there is continuity from the power wire, through the brush springs, brush, commutator segment, armature winding, field and back to the return wire. It also confirms the design value of 2.5ohms for the commutator plus field DC resistance. The only thing it does not tell us is the quality of the contact of the brushes on the commutator. Since an ohm meter uses a very small amount of current for measurement there could be a high resistance contact between the brushes and commutator that only shows up when the motor tries to draw 2A to 4A to begin spinning. After it is spinning it only needs about 1/2A. My next step would be to replace the brushes, springs, and clean the commutator and slots. There is nothing else I can think of that would cause the motor not to run.
Tom, after reading your post I hooked it up and also attached the green wire to the BP. I used a small screw driver to reach in and turn the armature, it started! Tried that over and over and found I always had to give it a 'push start'. However, sometimes the armature would only spin a couple of times and stop. Sometimes, no matter how many times I gave it a 'shove', it wouldn't start. While other times it would run just fine (after a push).
I think that is saying one of the commutator segments mat not be good, but I was surprised it would run at all if one segments was bad. When I had it apart, I checked the brushes and springs -- brushes are less than half used, meaning I don't think they would be a problem. While I was at it, I cleaned the commutator segments. I could try stretching the springs a bit to increase pressure.
What do you think now?
Tom Stoltz
in Maine