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Guess we're sorta in the same boat here. I'm working with a Magnavox T940 chassis that seems to have its own ideas on how to deal with external video signals. Problem started out when I broke my IF strip board and destroyed a coil so I decided it would be easier to just inject a video signal in behind the detector diode, this introduced some impedance matching issues which were sorta solved with a simple matching network but like in your case the internals of the set don't use the "standard" video levels and the picture was weak & blotchy. Some other members pointed me to a composite input circuit used by an earlier RCA (don't recall the chassis) and I built it up on a perf board tube socket and all but it introduced a new set of challenges and was superseded by an op-amp design using an LT1252 and configured to produce up to 8 volts P-P and it worked... sorta. Other issue I was having involved sync and DC restoration which I tried to inject using a diode based circuit but the Magnavox would not play nice no matter what I did.
I bypassed the 6GH8 sync separator completely and went with an LM1881 sync sep/countdown chip from TI and configured it to feed the horizontal & vertical directly which resulted in a very stable lock no matter how much things varied but the picture quality itself was never 100%. The set has been in the family for 56 years now so I know what it's capable of... well back in the day and now no matter what I do or how I do it I can't get to that level again. Pix tube tests very good on my B&K 467 showing 1.6ma on each cathode and over the past several months I've been slowly replacing resistors and other parts that don't measure up in their tolerances so everything is what's specified on the schematic (Sams 1144 folder 1) at least academically.
So recently I put out a want for a replacement IF board as I wanted to bring everything back to factory stock but with all the new parts and a fellow member pointed me in the right direction, two weeks later a carefully removed IF board and the service manual for that donor set arrived at my door. Just want to say Thanks Again to damen for helping me with this.
That afternoon the IF board was carefully transplanted with the hopes everything would fall into place... then reality hit. I had to go back on all of my mods and restore the sweep board back to original then double-double check all the wiring points, a picture appeared on the screen but it was weak and washed out with the same color issues I was having with the composite input.
Problem I'm having with the color is in the areas of strong or saturated colors they're very bright and saturated on the screen but in areas not as saturated or more like a shading on say a scene with a wall or background of trees the color is missing or only shows the areas that "rise above" some arbitrary threshold like some amplifier is not biased correctly. Think of it like a contrast control is set too high and the gray details are all but lost. I've gone thru the chroma sections replaced resistors and screen bypass caps, all the voltages are correct compared to the schematic but I can't figure this one out.
So what I'm fighting on the IF board is a low video level coming from the detector, it will not go above about 2.5V P-P and the AGC is very aggressive, I have almost no gain no matter how things are adjusted and any thing short of 100% on the AGC pot cuts everything off. Voltage on the IF tubes are within a few points of the schematic and the tubes themselves check good, tried known good subs and I get the same thing. Video/RF modulator is a Blonder Tongue AMCM 860D CATV head end that's padded down to -70 dbm. I'm going to do a raw video injection using my op-amp circuit again this evening right behind the detector diode and look at what's showing up on the grid of the 12BH7 video out tube just to cover this path and I will post my findings once I know more.
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