Yes, it's clearly been a long time since I've had to work on one of these. I was thinking that pulling the drive dog forward would disengage the drive dogs from the rear drum, but that bothered me, as something in the back of my mind kept saying, "A Sweep in front, B Sweep in back."
So the interlocking is done by the knobs and the drive dogs continue to turn the rear drum even when its shaft is pulled forward.
Thanks, that makes more sense.
For the original poster: If the front knob does not turn the B drum, does it turn the 1/8" B drum's shaft? If it does, but the B shaft does not turn the B drum, then the drive dog has either shattered (most likely) or come loose from the shaft. You'll have to remove that retaining washer from the shaft to get it apart. That can be tricky, or you can destroy the washer and buy a new one at your local hardware store.
Does anyone have a better name for that kind of retaining washer? It looks like an internal star lockwasher, but the inside spikes flare in one direction so that it can be pushed on but can't be easily pulled off. McMaster-Carr calls them Push-On External Retaining Rings.
