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maybe the tuner is not working. If you have a schematic and can find the input of the video amp, then I would try injecting a composite signal there.
you can just take the video out put jack from a dvd and use that (not the RF but the video lead). To be safe I would put a meter on the test point to make sure there is no high potential (I messed up and "thought" I had the input of the video amp, when in fact I had a B+, fried my DVD). this will tell you if the problem is before or after the IF/detector stage.
Not knowing the chassis you should make sure its not hot (one side connected to the mains) before attempting working on it to avoid any nasty shocks. back in the 60's there was a lot of ways of hurting yourself (not as many product liabliity lawyers I guess).
I use an isolation transformer on any set that does not have a power transformer, and even then you have to be careful. A motorola I worked on had a big transformer for the filaments but none for the B+, just a voltage doubler and a floating ground for the B-
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