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on a DVD or converter box there is a jack that says 'video out" this is the composite video, it is the same as what comes out of the video detector (if the tuner and IF strip is aligned correctly). it is the video signal with the sync pulses but no audio.
the idea is to couple that video out directly to the grid of that video amp tube.
To be safe you should use a large value film cap, like a 1 uf. take the cap and put a alligator clip on one side of it, then connect the cap to a shielded lead center tap. to be extra safe, connect the shield to another cap again a 1uf and another clip, connect that to the metal chassis. the caps are there to protect against a hot chassis and a voltage difference at the grid (some grids will be biased to something other than 0 volts). The caps prevent damage to the convert box or DVD incase there is a DC potential, they will pass the video signal. there may be some losss of video signal (may look like smear) but should present a decent pic.
If so then you know the problem is in the tuner or IF. if still a bad pic then its prob in the video amp or after.
I dont recal if you checked the AGC voltage, you could also try clamping a variable DC power supply (or even a couple D cells and a pot to make up a varialbe DC power supply). too much AGC will kill contrast.
One thing, a long shot, but would not hurt to check is the detector diode, I did not look to see if you can check in circuit but I would try it.
I would try the video composite signal 1st, if you get good conrast then move on to the AGC, and check the video detector diode with a DMM in diode check mode.
Remember the diodes are VERY heat sensitive so the must be heat sinked when soldering. Often a diode is destroyed when working on a set and replacing a part that shares a term stirp lug with a diode. (germainium diodes anyway, modern diodes are silicon based and are very heat resistant).
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