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Tom,
Your thoughts on the behavior are correct. The transistor does function "normally" when driven correctly C->E however the breakdown is quite low and agrees with the Keithley 6500 0.75Vdc in the E->C The transistors will also break down with no base drive at about the 0.7 Vdc. I am mildly familiar with avalanche circuits having not needed to work with then out side of 1 or 2 lab circuits, if I am out in left field let me know. But my understanding is that you would avalanche a transistor from C->E to keep it healthy and not shorted. I have not worked with many Ghz + circuits so I am a bit out of my depth here. But isent any E->C current leakage considered a fault. I don’t know of any situations where I would want to avalanche a transistor E->C, but I might have not seen any yet.

This is where the plot thickens in terms of 4xx series scopes even more. In a 475 there are 15 of these of which 13 are on the trigger board in the main triggering path. The logic side of the triggering board gets fed from the +15,-15,-8 and +5 power rails. So depending on exactly where the supplies are fed and how I could see some strange things happening.

The other 2 locations where this transistor is used there is one in the Z axis amp as well as one in the Horizonal drive section if I remember correctly. Do not have many concerns about these to as they don’t need to deal with + and - wave forms but the triggering sections is a bit weird.

Zen