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It is possible you do have negative lumina. Every stage of video amplification inverts video polarity* and IIRC the even VS odd number of IF amps in any given model changes the necessary detector/video output of detector polarity. The CRT can take either polarity as long as you choose the correct element (grid or cathode) to drive.
Been a while since I looked at the schematic of these but you might try injecting one video stage closer to CRT than you are. That will probably invert the polarity. If it doesn't and or video is too weak you may need to add a video amp stage possibly with inversion.
* In tube sets amplifier circuits typically are inverting, but if you choose/encounter a topology other than the stereotypical grid/base in, plate/collector out(IIRC that is called common cathode/emitter) there are non-inverting single stage amp topologies.
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