Troubleshooting the Horizontal Multivibrator
(Taken from "Servicing TV - Tuner to Picture Tube" (c) 1959 Howard W Sams & Co.)
The horizontal multivibrator is a relatively simple circuit consisting of a dual triode tube and 12 to 15 component parts. Its purpose is to develop a 15,750hz sawtooth signal of sufficient amplitude to drive the horizontal output stage, which means, of course, that its operation is necessary for the production of high voltage and horizontal sweep.
Before going into a component-by-component analysis of this circuit, let's take a moment to review its operation. In the typical circuit (Fig 1) we find that the multivibrator is essentially a two-stage resistance coupled amplifier with feedback (via a common cathode resistor) to make it oscillate, and a stabilizing network (L15 and C76) which tends to keep it oscillating near the desired frequency. In this type of circuit, tube section A conducts during the trace period and is therefore "on" for a longer time than section B. The complete oscillation or "flip flop" cycle may be explained as follows: The instant that power is applied, assume that section A begins to conduct, developing a bias voltage across R84 and supplying a negative pulse via C77 to hold section B in cutoff. At the same time, C78 charges through R87 and R88. As C77 discharges (due to a sudden drop in plate voltage of section A) the bias voltage it develops across the section B grid load will decrease to a point where section B will conduct. This provides a low-impedance path to ground through which C78 discharges. Also, the bias voltage across R84 will increase and section A will be cut off. This in turn causes the voltage at the plate of section A to rise suddenly, pulsing section B heavily enough to draw grid current and charge C77 to a high potential.
And it goes on like that for a couple O pages...
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That was lame.
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