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I have had several 7 jp4 crts although different then what you have which had many issues the contrast was lousy as well as the brightness and using a bk440 I used the shorts setting and just pressed the shorts button for a second and seen all the whisker sparks in the gun then I tried it and it was like having a new tube. Although the 440 cannot be used on electrostatic tubes I can , I made an adapter and established a good tube point on the meter and the other functions work except the cutoff because of the lack of proper voltage but it served the purpose. So any old tube could very well have the same status good emission but lousy cutoff due to whiskers that present a slight short within the gun just enough to take from the integrity of the tube. So if you have a shorts remove function try it shouldn't hurt.
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