GA-200 SERVICE NOTES
Figure 24 (lower part of diagram) shows the parabolic voltage at line frequency on terminals of C617 (do not confuse with E/W modulation which operates at 100Hz) which, as can be seen in figure 25, is modulated at vertical frequency, due to the modulation brought about by the E/W. The parabolic form of the signal (31 KHz) allows for controlling the gate of transistor Q612, so that when the signal reaches the highest point, it saturates Q609, which cuts out Q612. Control of the gate is achieved by means of Q609 and the
2
divider formed by R631 and R630. Figure 25 shows the same signals in vertical frequency, where the attack on the gate of Q612 can be observed.
D
R
A
T
F
Figure 24
Figure 25
In this circuit, a safeguard (Q614) has been incorporated to protect against overvoltage peaks in the Source of Q612 when turning the television set on.
