Raster-CRT Bias Generator

The Raster Voltage Generator circuit is designed around transformer T501 and transistors Q501 and Q502. The oscillating output of U201 drives Q501, providing additional current for Q502. The output of Q502 sets the voltage for the primary of T301. The +15V input at T501 pin 4 produces ±15V (30V total) output at pin 5. A tap at T501 pin 6 supplies +24V. The secondary winding on T1 adds +14V to boost the +24V to +40V output at T501 pin 8. This voltage provides a ground reference for biasing G I, G2, and G4.

Raster Generator

Vertical deflection for the raster yoke is provided by the Raster Generator circuit. Because of high current and voltage requirements, the vertical deflection circuit consists of discrete components, forming a resonant circuit. The Raster Generator, like the Raster Voltage Generator, is driven from the Raster Scan Oscillator circuit. The Raster Generator circuit includes transformer T301, which is shared with the CRT Bias Generator and provides the drive signal for the 88mH yoke coil. This creates the vertical scan drive for the CRT.

The Display Gate Array on the Main PCB supplies the synchronizing signal, Raster, which is passed to voltage regulator U201. The output of U201 pin 9 causes Q204 to conduct, providing additional current for Q205. The output of Q205 controls the charge and discharge of C208 through C212. The precise values of these capacitors are essential for maintaining the proper resonance while charging and discharging. Transformer T301 provides high voltage for charging the capacitors.

CRT Bias Generator

The CRT Bias Generator supplies half-wave rectified voltage for the CRT anode and grid from the output of transformer T301. It provides up to -150V for bias of G1, up to +700V for bias of G2 and G4, and between +8.5kV and +11.6kV for anode bias. Master Brightness, R302, adjusts the brightness which largely determines the general background brightness of the CRT. Focus, R306, adjusts the focus.

Section, Operation