The monitor turned on and showed a green screen. However, after I turned it off the screen had a bright spot in the center for at least several seconds before it went off. What might be the cause of that?
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The circuitry for ray clamping at powering off is faulty. But this is not critical, especially for monochrome CRTs.
It's only a concern if the spot is brighter than the normal video display. Otherwise, this is just what a hot cathode "looks" like after deflection has ceased.
The spot seems to be brighter than normal display. Any suggestions on how to troubleshoot and fix it? I had some experience swapping capacitors in the Apple II monitor and I've Sams computerfacts on Apple II monitor that shows the schematics, but I don't really know where should I get started for this issue.
Why? And how bright is the "normal video display"?
I found out that the monitor actually doesn't display any video from my Apple II. I did a bit troubshooting and found that one transistor (Q102) has a short between the base and emitter. Based on Sam's computerfacts on Apple II monitor, the Q102 transistor is part of the video amplifier circuit. Would it possible for the transistor to cause the bright spot at all? I'm still waiting for a replacement transistor so I'm wondering if I should check other transistors, etc.
To get started, look for the part of the circuit connected to G2 (the SCREEN electrode of the tube) or G1.
When the monitor is shut off, the main supply voltage B+ is no longer produced and it falls down to zero. B+ is the supply for the deflection circuitry, so the deflection stops, leaving the beam aimed at the center of the tube face. But the EHT is still high, due to the tube's capacitance, and the cathodes are still hot (thermal inertia). Hot cathodes emit electrons, and when there is accelerating voltage, they will continue to stream towards the tube face and, unless turned back by the grid electrode(s), strike the phosphor at that point. The spot fades away as the cathodes cool and the EHT drops (electrons striking the front of the tube neutralize the + charge on it). For a fairly small tube with a 15 kV accelerating voltage and a 100 pF aquadag, the charge that must be neutralized is on the order of 1.5 µC. If we suppose that the afterglow cathode emission is .5 µA, it will neutralize that charge in a couple of seconds. However, on some displays, the glow lasts for several minutes, so it must be an even tinier current.
Any bright beam spot on the phosphors will cause some damage, even more so when stationary. Partly from simple heating of the phosphor and partly from stress to its crystal lattice. So it's worth fixing this issue.
To prevent the afterglow, either the EHT must have a bleeder resistor, inside the potted flyback transformer assembly; or the grid(s) must be made negative to cutoff the beam, or both. It's tricky to force a grid electrode negative when power is already shut off, so the first option is the easiest. This bleeder resistor must be very high value, multiple gigaohms, so testing for it directly is not that easy. An indirect approach is to use a high-voltage probe at the anode cap, and see if the anode voltage drops to zero within a few seconds of power off: that would prove the presence of a bleeder.
Or there may not be any attempt to prevent the afterglow. On the SAMS schematic for the A2M4090 Monitor IIc, there is no provision for making G1 or G2 negative, and no EHT bleeder, so the designers accepted that bright spot. It also can be worse depending on component values drifting over time. If you can prevent the bright afterglow spot by turning down the brightness knob before switching off power, that would be a good idea—and if it works, it helps narrow down the cause to certain components, using the schematic.