[drh4683]

Do you have the tube type space command 300 or is it the solid state one? Zenith went solid state on the space command starting in 1962. The tube type ones generally has a 5Y3 rectifier that has boiled away from being left on for years and years. Be sure to check all those tubes out.
Also, if its a tube type unit, it will have those black beauty caps in there. Those have to go, replace with sprague orange drops and you can't go wrong.
This should restore it to life, however, you may notice that its not very sensitive, or one of the functions may not work. This is due to the frequency drifting of the tuned circuits. Since its a 300, there are two large detector coils that will stick up out of the chassis and these are the ONLY coils you'll need to adjust. What I do is simply click and hold the remote control button and slightly turn each slug in the detector coil until the relay clicks down and holds for the longest period of time. You can do this one or two feet away from the microphone. Once you get the relay to hold down for as long as possible with the remote button pressed (to prevent rod dampening) you've tuned the detector to the frequency of the remote transmitter. Its the easiest way to get it just right.

If its a solid state chassis, there will be a dual section low voltage electrolytic cap under the chassis, one that ties to the collector (I believe) of each detector transistor. You may want to change that as that can sometimes cause erratic/non functional remote. You'll still need to tune the detector coils as described though.

A couple years ago, I made a special remote control that transmitted the perfect frequency for tuning space command chassis. In theory it would have worked great, but each aluminum rod for the mechanical remote is not "perfectly" tuned to the ideal frequency, so it didn't work as well as I wanted. Thats why I just stick with the clicker alignment method, it works best and is also quite easy.