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Originally Posted by old_tv_nut
1) There is some loss of DC at the coupling between the first and second video amps, but not completely.
You are correct. For proof, I just inserted a diode before the video output peaking coil and ground. It made no difference at all in the picture, so I can assume DC restoration will have to be done elsewhere. Drat, there goes my hope of a simple solution!
Sorry about that!
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2) There is also significant loss of DC coupling in the video output due to the capacitor-coupled contrast control in the cathode
Looks like it's set up as a cathode bypass cap of sorts. Any way to improve this part of the circuit, or am I right in assuming that cap has to stay there? My guess is that any changes would require a complete redesign of the video output...
Probably. In the CTC-5 , it's a DC control, which unfortunately runs the brightness up and down along with the contrast because the cathode (and plate) current is not zero at black. Several people with CTC-5's have modified their sets to be like your CTC-7 to fix that problem, but it reduces the DC coupling
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3) DC needs to be restored at the output of the video output - but doing it there may negate the brightness control action, so something may need to be done to provide a brightness control - I can't remember how DC restoration was added in the Porta-color fix that was posted to Videokarma, but that may provide the solution - suggest looking it up.
I am under the impression that can't happen, because the output and CRT are directly coupled? I can't find the porta color thread.Keep looking and I will too -it's around here (or maybe audiokarma?) somewhere. The brightness could still work if it changes the height of the blanking interval and you end up clamping to the bottom of that - but not sure if that is the case.
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4) Back porch clamps are much more complex and require pulse drivers and very accurate timing to make sure they do not drift into clamping either partially on sync tips or partially on active video. The sync tip clamping is automatically self-gated on the most negative part of the signal.
That's why I got excited by that integrated circuit I first posted, but it does not come in a high enough voltage rating for use on something like my CTC-9. Highest rated version I think was 220v or something like that, and the cathodes of the -9 sit at somewhere around 350...
Interesting - maybe doing something to clamp at the grid of the video output and changing the cathode circuit back to DC coupled like the CTC-5 could work. But I would find the Portacolor thing first, as I recall it being very simple as well as effective.
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