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EW-Correction

  

We have now solved the problem that electrons are deflected too much to the left and to the right. But we have the same problem at the top and at the bottom of the screen, although not as obvious to the viewer. Especially TVs with large screens will have an East-West correction. To solve the problem, we could vary the source voltage, but this will cause another problem. Lp is actually the primary winding of a transformer. The secundary voltages must remain stable, so the voltage across Lp must also remain the same. This can be accomplished by putting a 'dummy' deflection circuit is series with the 'real' one. Both transistors switch on and off at the same time, so Lp still receives 150V pulses. By varying the voltage across Cmod, we can change the voltage across Cs, because VCs=VB-VCmod. The maximum voltage across Cmod is usually 0.2VB, which equals 30V. Cfb and Cfb2 make a voltage devider. So the 30 volts can be obtained by making Cfb2 4 times larger than Cfb. In that case VCfb2 will be 1∙VB/(1+4) = 30V. Because the period time of both circuits must be the same, Cfb∙Ld = Cmod∙Lmod. So Lmod must be Ld/4. Nowadays, the voltage across Cmod is generated by a dedicated chip. Since both transistors must swich on and off at the same time, they can be replaced by one transistor. We now have a so called diode modulator used in most large TV sets.

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