[Penthode]

I see no reply from my posting on this yesterday: perhaps my message was too ponderous and I did not make myself clear? Let me try to launch this from another perspective.

Modern displays represent the dc or background level a lot better than most TV sets from the '50's and '60's. This need not have been since the sets from the late 40's were much superior in this regard. My 1949 RCA TV has a dc coupled video amplifier which represents the dc level amazingly well. And of course the TS630 design from 1946 included a dc clamping circuit which also yielded excellent results. But starting in about 1953, when set bandwidths became narrrower yielding less picture detail, the maintaining of proper dc in the picture was dispensed with.

Now on to color. The earliest RCA color sets (CT100 and 21CT55) had dc clamping after the matrix and just before the tube. I would assume the maintainance of dc in these sets to be pretty good. The CTC4 and CTC5 sets must have been originally fairly good. The CTC4 contrast control progressively shunts the signal with a capacitor to reduce contrast. This means that if the contrast control is kept fairly advanced, the dc should be coupled pretty well through.

The CTC5 however used a variable resistor in the cathode of the second video amplifier to reduce the gain. The problem with this arrangement is the interaction of the contrast control with the brightness control which led to blooming from overcurrent of the CRT. This must have been a problem because subsequent designs (CTC7 thru CTC10) used the variable coupled cathode capacitance which increases AC gain at the expense of losing the dc component. Alas, the CTC5 modification was done to copy the later series receivers hence reducing the dc component.

I have attached the circuit of the unmodified CTC5 video amplifier. I have marked the dc path between the first and second amplifiers in red. The AC path follows the red until where it branches off to the green path after which it recombines with the red. Note that the dc component is divided at the output of the delay line by R182 and R101A (the brightness control). Note also that as the brightness control is reduced, less of the video dc component reaches the next amplifier stage. This implies that the set up of the tube (background controls) should ensure that the brightness control is mostly advanced or as close to maximum as possible to ensure the dc component gets through. And without the electrolytic bypass capacitor used in the mod. and later series chassis's, the second video amplifier will have the same gain for the video ac and dc components. The division of the dc is not a bad thing because even at 50% reduction, only minimal dc shifts are noticable. Hence maintaining a percentage of dc is all that is really needed.


I hope you appreciate what I am trying to say. I am asking if others are aware of the dc level problem? (My test with the blanking bar will reveal this on any set, color and black and white. My 1949 RCA passes this test excellently in this respect).