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

[Penthode]

I noticed the resurrection of this thread and would like to add my own comments. The RCA 630TS employed 1/2 a 6AL5 diode as DC restorer in the circuit depicted below). The RCA sets from chassis KCS28 thru and up to I think KCS47 utilized DC coupling and no DC restorer diode. From about 1953 RCA abandoned DC restoration.

In every case, RCA did not employ 100% DC clamping and compromised with only partial clamping. You can examine the degree of how well black level clamping is achieved by watching active video cutting between dark to bright scenes while examining the vertical blanking bar on the screen (by misadjusting the vertical hold control so the bar is central on the screen). The bar should remain the same shade of grey or black when cutting between scenes.

The later sets without any DC clamping the bar changes with the scene cuts. The earlier RCA sets the bar shade remaiens "reasonably" constant in shade. This doesn't means that there is no noticeable shift in the bar's brightness.

I think the RCA engineers compromised a little and did allow some AC component. Perhaps this was because of normal temperature drift of components which would result in having to constantly adjust the brightness control. Nevertheless, look at the 630 TS and compare it with your original modification. It looks as if R148 was added to isolate the diode capacitance from the video path. Also note R149 the 1Meg ohm resistor across the clamping diode which is necessary as a DC return.

Also note that in the earlier sets, the clamping diode is across the video path to the CRT grid input. Later sets applied the signal to the cathode. The grid circuit does not draw current whereas the beam current passes through the brightness control and thru and around the clamping circuit. A fundamental error in your earlier circuit is that there is no DC path to allow beam current to flow!! So you are going to have to place a lower resistance across the diode which will compromise the clamping. In other words, not to sound to pessimistic, I do not think it will be a simple task getting the cathode driven circuit to work.

Also note that C143 0.25ufd is charged thru R150 which effectively further dilutes the clamping effect.

The bottom line is that it is not a perfect world and engineering is all about compromises. Especially when dealing with analog circuits! I would suggest you carefully think thru this as there are a few gotchas that need to be addressed.