Revisiting an old topic - Adding DC Restoration to a 1950s TV

[Penthode]

Originally Posted by old_tv_nut

Reading your post again, it is not the capacitor and diode that block the beam current, it is the CRT. If/when the CRT conducts beam current, the diode and capacitor will supply it.

I apologize for the awkward/ miscommunication here. But the CRT DC quiescent beam current used to flow thru the resistor from the brightness control center tap to the crt cathode. With this resistor removed and replaced by the diode, note that the current flow is now blocked by the diode because it is reversed biased. The coupling capacitor cannot pass DC. The circuit is therefore indeterminate and depends on component leakage currents to sustain the CRT current.

The CRT beam current is generally quite small at about 1ma maximum. But removal and replacement of the resistor not only makes the CRT bias determined by undesired component leakage but also will lead to clipping of the video at high amplitude. As there is minimal or no proper DC return path it will behave like a grid leak detector and effectively clip the video sync.