HP 5360A counter is sick!

My favorite counter, if I have earplugs handy, is my HP 5360A. Lots of smarts and digits. Has worked fine for about 30 years. Now I turn it on after a few years off and the "self test" jum ps between 800000000 and 30000000 and 100000000. Putting in a signal does nothing but blink the counting phase lights.

I think I have a extender board and a manual, anybody have any hints where to start or just follow the manual ??? The number of boards and IC's is a bit imposing.

start with a visual internal inspection, see if there's any capacitors bulged, corrosion on the circuit boards, burnt components, loose cable connections, etc...then turn it on and check the power supply voltages. if the thing has mechanical function switches, clean them with switch or potentiometer cleaner.