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The Set: Pete Deksnis's Site about the CT-100

Restoring a Vintage Color Television Set

Blocking --

an idiom in the vocabulary of vacuum tube technology.

"Cutting off the plate current by a high negative bias on the grid."

Langford-Smith, Radiotron Designer's Handbook, P.1398

When I designed and built my 2-inch home-brew back in 1959, I used blocking oscillators in the sweep generators. Blocking oscillator transformers came in two flavors (horizontal and vertical) because they had to be designed and manufactured to oscillate at a rate equal to the horizontal or vertical retrace periods.

A blocking oscillator transformer is wound to realize a 180-degree phase shift between primary and secondary, and it has a turns ratio that is typically two-to-one (2:1) with the low-impedance primary in the plate circuit. At initial turn on, as plate current increases, the polarity of the plate-to-grid feedback is positive, due to the phase reversal at the transformer.

As the grid is driven positive during this process, the flow of plate current rapidly increases, quickly driving the tube into full conduction. The grid is now more positive than the cathode, so grid current is drawn (the grid effectivly acts as a plate), developing "grid-leak" bias as the grid current effectivly flows through the grid-leak capacitor, charging it.

This constitutes the first half-cycle of oscillation, and it's standard stuff for oscillators.

Energy for the second half-cycle of oscillation comes from the transformer. Energy stored in the transformer during the first half-cycle drives the grid negative and cuts off the tube -- still standard stuff for oscillators.

But the beauty of this circuit -- and why its application so neatly fits the requirements of a television sweep generator -- is found in its use of the "blocking" function. The negative side of the now-charged grid-leak capacitor is connected to the grid, thereby holding the grid in its cut-off state.

The charge that developed (during the first half cycle) across the grid-leak capacitor is approximately equal to the feedback voltage, which got boosted to that required amplitude by the 2:1 step-up action of the transformer.

Ordinarily, the circuit would oscillate at the retrace rate, except that the RC grid-leak network is designed for a time constant equal to the
trace period. By the time a given trace is complete, the grid-leak resistor has discharged the grid-leak capacitor (2C236, RCA), so the circuit again generates positive feedback to repeat the process.

 

[Updated 3 September 2000]

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