i have the same crt
I think it is the pludge pattern test image. (with the -5ire and +5ire square in the edges. But ive read that the correct voltage is 4.0v,(by the voltmeter method). Remember to disable akb card with an jumper first. When i did this the volt get a little lower and the black level was almost perfekt. but iam afraid of lowering the voltage a little more, because the details in the black may dissepear., BTW I have the akb card permanent disable. Sorry for the bad english 