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Usually, there is a low ohms resistor in the filament line to the CRT and these resistors can open. It would be best if you had an HV probe to see exactly what's getting to the CRT anode.
If I'm not mistaken, these Zenith's with the switching power supply can lose horizontal sweep and still have audio because the voltages for the signal and audio circuits are taken off the switching power supply transformer, not the flyback. I think the only voltages provided by the flyback are for the vertical output circuit, HV, focus voltage, filament voltage, and B+ voltage for the video output stage. If the horizontal circuit is not operating, you won't have any of these voltages.
Since you have 188V at the CRT socket, I'm betting the sweep circuit is working and you have an issue in the filament circuit.
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