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Germainum diode? heat sink when installed? Heat from a soldering iron will ruin a germainium diode if not heat sinked.
I would def input a composite video siganl at the grid of the 1st video amp just to clear everything beyond that. Make sure everything is isolated before doing this (a .1uf cap should do it, make sure there is no voltage between the grounds of of the chassis and the video comp ground. I generally use a meter to compare grounds before hooking ANY two things together. A DVD is a good source of a comp video (yellow rca jack). check the grounds 1st of ok hook dvd ground to chassis ground, then connect the center lead from the dvd comp video to thru the cap to the grid of the video amp. Its possible it could be out of phase (neg pic no sync) but most of the time it works.
If this produces a good pic then you know for sure its the RF/IF or a bad video detector diode.
I generally do a quick front to back ratio check on the detector diode in circuit IIRC it should be at least 4 to 1
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