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According to the CBS/Hytron datasheet on the 19VP22, the heater rating is 6.3V/1.8A, which to my thinking suggests a parallel connection of 3 x 6.3V/600mA heaters (a common value), rather than 3 x 2.1V sections in series.
With 3 sections in parallel, a completely open heater is much less likely. An oxidized solder joint inside one of the heater pins is the usual culprit when all 3 guns refuse to light. Back in the day, many shops had a special crimping tool to make this type of repair. Lacking one of these, a vacuum desoldering tool to remove the old solder, followed by the careful resoldering of the pins with fresh solder will usually fix things.
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