Spent the last 2 days tracking down why the scope does not display a trace with the service manual and schematics. Beam Finder does show 4 dots one for each channel and vertical position adjustment works. The on-screen text appears and changes when the controls are changed so the general CRT functions ok. Power supply voltages are in spec and clean.
Fairly sure U421 A Trigger Source Selector IC is defective (Tektronix Part # 234023920). Timing signals arrive and no output while U431 B Trigger Source Selector which receives most of the same inputs as U421 outputs the expected trigger signals.
I emailed a request for the cost of the part from Tektronix. Couldn't find the part on eBay just broken units which could be scavenged. QService Electronics lists the part out of stock.
Any other suggestions.