The sampling strobe pulse generation is initiated when the 2N964 (Q2010) is triggered into avalanche by a pulse coupled into the blocking oscillator transformer.? At this point a rapidly rising current is injected into the snap-off diode, D2002.
D2002 has been resting in a forward biased condition with an adjustable current (from a negative supply) set by R2012, between about 10mA and 40mA.? The current injected by Q2010 is in the reverse direction and much higher than the forward current.? After the storage interval required to "sweep out" the existing charges, the voltage can finally rise in the positive direction.? This rapid rise constitutes the rising edge of the stobe pulse, whose falling edge is set by the "clip line" 2-way transit time.
Tek always had the forward bias of the snap-off diodes adjustable, possibly to account for variation in the Trr and/or to position the rising edge of the strobe into the best portion of the avalanche pulse.? Some earlier instruments (1S1, 4S2) didn't have the clip-line.
Karl G
