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Here is what I'd try to localize the hum bars. Insert a signal generator into the antenna terminals, and set it to a VHF channel visual carrier frequency. Connect a bias box to the AGC line. Connect a scope to the video amp. Turn on the AM modulation on the signal generator and adjust the bias to produce a somewhat undistorted sine wave on the scope (I say somewhat because the AM modulation of most signal generators is not that linear). Turn off the modulation. Now connect a detector probe to the scope and work your way back, looking at the plate of each IF tube until you see hum on the scope.
A possible source of hum is the AGC line, by the way.
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