For an Owners manual=
http://www.managemyhome.com/mmh/lis_pdf/OWNM/L0606052.pdf

It doesn't seem to show any adjustment though.

Which pulley does the belt drop off? And are you using the correct belt 197253? If you can no longer find the # on the back of the belt -get the proper belt. This style has the dual front guides near the engine pulley and a cable adjustment where the cable clips onto the deck. Probably not likely that the adjustment would help, but no harm in trying. If you clean that area off, you will see a slot and 2 hex headed bolts that you can loosen and slide back to increase tension. It should only let you increase the tension , but not to the point that the spindle brake releases.

Tom the belt falls off the front pulley when the blade is disengaged after mowing fine for awhile. If you throttle back prior to disengagement it will sometimes stay on but its three fingers loose. It has run for a month prior to this with no problem; that is why I do not think it needs a belt. I will try the tension adjustment you suggest.
Bill thank you for the manual. The Sears service parts web site has a nice blow up of the parts as well. I do not want to just throw money at this but I guess I will get new idler springs and a belt.

doesn't the manual say that belt with 100+ hrs may need replacing (stretching,etc.)?

The adjustment mentioned by Tom above worked.
The previous owner had apparently replaced the cable at some time but not torqued the heads and the plate had shifted just enough to make the belt slip. Thanks