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Thanks for the insights Albert.

Makes a lot more sense than just a design that relies on just leakage alone.

In the "T.D. Bias Current" portion of the SM Circuit Description it describes: "Q74 reverse biases CR90 and the charge on C90 is held (except for a very small Q90 base current through R90) to control the arming bias current in period 7." That was the basis of my understanding that it was "leakage" that brought the bias level to just below the TD trip point.

However, the description follows that sentence with "Trig Level control R90 sets the arming bias current to a value below the peak current for the Tunnel Diode.", which I took to mean that it's variability allows setting of the arm bias level. But it's also saying what you're saying. One of those sentences that can be read more than one way?

Given your description, and my growing understanding, it seems that the actual operation is a combination of what you've described, including some leakage (and voltage drop from R90 in C90s storage mode) all contribute to the small reduction of the armed bias level to something below the trip point. I think your description is probably the more significant contributor to the reduced level of the bias setting.

Thanks again for the additional insight Albert.
Dave