I would start with any Transistor or FET that is located in the trigger generator or sweep generator circuits (this is an extensive list, refer to your specific instrument schematic). Start with the components closest to the Tunnel Diodes and work out. Keep in mind that you have a "B" trigger circuit which is almost identical and uses many of the same components. You can substitute the "B" components into the "A" circuit until you get a working "A" Trigger. Process of elimination. I have only repaired one Model 453, but I use that same technique in the 465/475 whenever possible. This assumes that your major components are installed using sockets, which makes the swapping of components pretty simple. Soldered in components are more trouble, especially tunnel diodes.
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Michael Lynch
Dardanelle, AR
Michael W. Lynch