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If you aren't sure that the CRT is okay a quick and dirty check I usually do in these cases is to briefly short the suspect gun to ground turning it full on. If the CRT is good this will give you a bright raster of the respective color. Otherwise you can measure the voltage at each gun. A gun that is completely cut off will be at around 200V.
Check the driver transistors using the diode test range on your multimeter. You can do this with them still in circuit comparing the readings from the red transistor with those from the others. If they are all basically the same the transistor is probably okay, if not replace it.
If the transistor is okay and there is still no red it's time to start working backwards. You may also want to check R1, R2 and R3. They should be around 2.2k.
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