The CTC-5 Contrast Control Modification

[Penthode]

Originally Posted by NewVista

What is C402 doing coupling signal back to the screen divider?

Curious circuit. The signal on the unbypassed screen grid will be anti-phase to the cathode coupled signal. The RC network off the screen grid is also interesting: perhaps the circuit was to improve the frequency/phase response of the amplifier? I shall give this more thought

Originally Posted by NewVista

Other end of brightness goes all the way to H-out grid just to find -35v?
Both Brightness & Contrast controls are weird, the Contrast Mod is
a good start but there's scope for more improvement as DC performance
is poor

The other end of the brightness control is through a resistor to a negative voltage source. The size of the resistance defines how much of the dc component is fed to the grid of the second video amplifier. i see on later chassis's (CTC10 for example) the resitance is larger and the end is tied to a more negative potential. This facilitates mor e dc component getting back.