I have a Kenwood KR 4140 stereo receiver that has been sitting on a shelf for 20 years. What should I do if it isn't working?

Take the cover off (10-12 screws). Get a can of compressed air, blow dust and cobwebs off each board (from one side to the other so as to not blow dust one way, then back the other way). Now inspect each capacitor (hard wired batteries) for acid leakage. If there is any, remove the capacitor, clean up the acid with a cotton swab and rubbing alcohol, and replace the capacitor. Next, look at solder points on everything (I know, tedious, but you don't want a short!), you want to make sure each transistor, resistor, and capacitor wire is secure. Also, look to see that solder is only touching one trace per wire, or you could have a short. If everything looks okay, assemble, plug in, and turn it on (assuming it's not wet from alcohol). Repeat if necessary.