Scope – Blanking Ckt

To understand the blanking circuit, we need to go back and have another look at the CRT circuits.  When the 555 is triggered, it’s output goes high and stays high as the sweep output voltages increases and the trace moves from left to right across the CRT screen.  When sweep output voltage reaches the threshold voltage, the 555 output goes low, the sweep output voltage drops quickly to -5V and the trace moves rapidly back from right to left across the CRT screen.  So what we need, is for the CRT beam to be on when the 555 output is high (sweeping left to right) and off, or blanked, when the 555 output is low.  This is the job of the blanking circuit.  The 555 output, blank, goes through the 1.5K resistor to the emitter of Qb.  When blank is low, Qb is off.









Current flows from ground, through the 51V Zener, the 1N4148 diode, the zener “stack” and the 47K resistor to the-1200V supply.  This sets the grid of the CRT to -1050Volts.  Since the cathode is somewhat above -1000V, the beam is turned off and the trace disappears.  When blank goes high, Qb turns on and pulls it’s collector up to 0V, setting the CRT grid voltage to -1000Vg and turning the beam on.  The intensity control varies the voltage on the cathode.  This controls the beam current and the brightness of the trace.  This little circuit gave me more headaches than most of the rest of this project.  Getting it to shut the beam off completely, work fast enough at high sweep speeds and keeping it simple was a challenge.

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