Happy Halloween! NTSC "Pumpkins" slide on CTC-5

[Penthode]

It appears to me the RCA Engineers discovered that there was insufficient DC restoration in the CTC5 luma path. In the CTC7 they made minor modifications to increase the DC coupling on the 0.1uF DC block.

The CTC5 thru CTC11 video amplifiers are similar. CTC5 and CTC7 are almost identical. My first experiment will be to replicate the CTC7 design in the CTC5. It will entail removing two capacitors and changing only two resistors.

Note below I have included three attachments below: the first is the original CTC5 design, the second is the CTC7 and the third the modifications I plan to make to the CTC5.

Note that the brightness control provides in each case a percentage of DC coupling bypassing the 0.1uF capacitor. The CTC5 only has a lower DC coupling percentage and the plan is to increase it. In the CTC5, I will simply remove the two 0.01uF capacitors which appear to be unnecessary decoupling and reduce R407 from 180K to 82K and R409 from 680K to 270K. This will reduce the effective resistance across the 0.1uF coupling capacitor to deliver the black level reference straight from the video detector (through the first stage cathode follower). I may lift R182 4700 (underlined) to see if it makes any appreciable difference to the DC coupling.

Before and after the modification I will take some photos of the vertical blanking bar with test video with a high contrast ratio (which would normally crush the blacks) compared against a black scene to see the effective change in displayed luminance of the blanking bar. I will post the photos here so you too can be the judge.

Note the brightness control bottom leg in both CTC5 and CTC7 receives its negatve potential tapped from the negative grid leak action from the Horizontal output tube grid. The circuits are essentially identical and since apart from the change in resistor values between CTC5 and CTC7 which was carried thru to CTC12 and beyond, I hope this will do the trick!