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Do you have a B&K 1075/76/77 TV analyst? Those units are made for these type of issues. If you have one set it to IF and inject at full blast at the grid of the 3rd IF with the TV's contrast maxed (depending on IF gain you may need to inject at the grid of the second to get a picture or sound). If you don't get picture from the IF, then inject raw video after the detector, and if you then get an image suspect the detector if not the problem is in the video amp or CRT. If you get picture injecting in the IF work your way back through the IF injecting at grids and plates. If all IF stages work then inject IF at the antenna terminals it should pass straight through the tuner....If not the RF amp is having issues or some other open in the signal path has occurred. If IF does pass through the tuner but it will not tune any RF channel then the oscillator in the tuner is likely not working.
IMHO an analyst is essential for these kinds of issues because if you don't have it to isolate the stage the problem is in there are so any stages that could be at fault that at best you will waste your time checking everything in depth to find the problem, or at worst just end up chasing your tail and get nowhere.
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