Moving forward, clipped and replaced numerous Tant capacitors,as they failed, throughout.
The last was a challenge, the two-channel plug-in Analogic DC to DC converter five volt input short circuit failed.
Knifing the Analogic module shield PCB from the enclosure, found two electrolytic caps, one of which had failed (short circuit).
Fortunately, the encapsulent is crystal clear and soft enough test and to snip the offending cap lead.
Now have mainframe boards and plug in (all supplies) working but still no display.
Will be testing for deflection yoke scan wave forms and HF/HV supply next.