The first step in rehabilitation is to read the Service Manual, and pay attention to exactly what happens on startup. The behaviour of the lights in particular (time spent flashing, number of flashes, etc.) is often used for diagnostics, along with sounds if you know what you are doing. You can have everything from bad RAM, to motor failure, to stuck rollers or a bad fuser, to a small piece of paper stuck in the paper feed to contend with in a LaserWriter, and any one of them can cause malfunction. Much of the workings of a LaserWriter are purely mechanical, and sometimes all that is needed is lubrication.

No doubt the flashing lights were not designed to tell you that your caps are bad, but as your caps may effectively be fine, flashing lights are the place to begin.

To take a concrete example, my LW4/600 will not work if a single fragment of paper is left in the mechanism after a paper jam, no matter how small. Typically these are to be found hidden under a plate in the left front corner. You have to loosen a screw in order to flick the hidden little bit of paper out with a knife or some other tool and, and all is well again.