My unit had a few issues being old and having had corrosion issues. The later problem is not your issue here.
Did you ohm the cable and cable & unit to confirm no grounding issues with the high voltage side of the unit/cable? That can be an issue - also, make certain the high voltage line in the unit is secure and not arcing anywhere (insulation all still good, no other low voltage wires too close or touching that wire?)
Again, did you spray the main switch with its numerous contact points and worked it a few times to be certain that it isn't also coroded? Even a good unit that never had issues with batteries will corrode in time from air/moisture. A good tuner spray can of cleaner/lubrication oil can solve such a problem.
Have you checked some voltages in the unit to trace for where the failure occurs? Trace the three volts and checked the high voltage output? The schematics are required but the main power supply and logic was rather easy to follow on my unit (I ignored the part of the circuit where the complex system of chips 'counts' and/or average incoming signals.)
After a bit of tracing I discovered that I did need to replace one voltage regulator: I de-soldered it, installed a chip holder - that way, one need not solder a chip in place with the issue of heat load destroying the chip. Also, if the same issues arises again, made that new chip removable (Also, I replaced both of those in my unit just to play it safe.) That fixed my third, and to date, last non-battery case related failure issue for my unit. Since then, it has worked perfectly.
This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.