The other thing to know about these transformers with the turret and handle (both dual and single controls) is that when lifting off the cover (after first unscrewing the on/off switch), the wiper stays with the transformer coil (with the 30B, there are two wipers). So it is important to know what the position is of both the turret and the wiper when the cover comes off so when reassembling, the arm connected to the bottom of the turret captures the wiper correctly. I usually set the turret at the mid-point on the speed dial and then try to remember to set the wiper at the mid-point of the coil when I put it back together. There's a sort of U-shaped piece on the arm that comes down on either side of the wiper - you'll see it in the drawings on the factory manual on myflyertrains, as noted above. Worth noting also that the so-called 'deadman' feature of the handle (or the on/off to track power) is due to the spring-loading of that wiper. The aforementioned arm on the bottom of the turret pushes down on the wiper when the handle is pushed into the 'run' position.
While the transformer is apart, its worth checking the status of the wiper - which is typically a small carbon wheel that runs along the top of the coil. Sometimes those are broken or no longer rotate so there is a flat spot on one side -- all of which can lead to the transformer not working correctly.
Its also worth pointing out that while it is tempting to look at the turrets and the arms underneath as a possible problem source, if they are intact they are entirely passive regarding any of the electrical circuitry - other than providing the mechanical connection between the transformer coil(s) and the wiper contact(s).