Schwinn Johnny Pro Stationary Loud Noise Near Sprocket I have an older Johnny Pro.  All of a sudden it began making a loud noise, like a bang, from the area near the sprocket. this occurs approximately every 1.5-2 revolutions of the pedals.

I have investigated, but cannot pinpoint the problem.  It may be that the bearings/washers/whatever between the sprocket and the body of the bike are worn.  I tightened the bolts at the base of the pedal, hoping that would do the trick, but no.  I then removed the bolts, trying to disassemble the sprocket connection, but I can't figure out how to take it apart; nothing moves.   

Looking at it from above, it appears that the sprocket wobbles a bit and that there is an uneven space where the bearings are.  I still cannot tell exactly from where the noise originates; my daughter thought it was when the chain was trying to right itself on the crooked sprocket.
From your description it sounds like the 'bottom bracket' bearings have become damaged. These are the bearings that the crank axle runs in in the frame of the bicycle. 

To confirm, turn the cranks by hand, if you feel an area of extra resistance, any 'grinding' or a 'notch' or 'clunk' that the cranks pass through the bearings need replacing.

I found a lo-res exploded drawing here of the Johnny Pro, it indicates that the design uses a 'three-piece' crank (two separate crank arms, axle and bearings in a cartridge type unit). The cranks typically fit onto a square taper or splined interface on the axle. If the bolts at the  base of the cranks are removed the  cranks do not come of the axle, a special crank puller-tool is needed for that. The cartridge axle and bearings thread into the frame but a special tool is needed to interface to them to unscrew them.

Here's a video that shows how these things come apart.

The bearing cartridges themselves are not very expensive. You'll have to make a determination if you want to try this on your own or get a bike repair store to help you.