Thread: Ct-100
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Old 11-19-2013, 11:53 PM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
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Location: San Jose, CA
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I've learned quite a bit more about what's going on with the unstable focus, and I suspect all of this is related to the flyback being different than the set was designed around (fewer turns on HV winding).

I checked out the connections from the focus supply through the potentiometer and all the way to the CRT pin 6. All looks OK.

Observing the behavior, it was clear that the focus voltage was varying a lot with picture brightness. A little more observation revealed that the focus rectifier filament was rather dim since I had modified the HV regulator to bring the voltage down to 15 kV (which I had done hoping to improve HV stability with too little voltage coming from the flyback).

Right away I was able to observe that by increasing the HV, the focus rectifier would glow much more brightly. In other words, loading down the flyback by regulating the HV down also knocked down the focus filament current a lot. With too little current, the focus supply had too little current to remain stable with changing current draw by the CRT focus electrode.

On the other hand, if the HV regulator was set too high, so that it was basically allowing the HV to be unregulated, there was an equally interesting effect. Now, the filament of the focus rectifier would increase and decrease in brightness along with changes in the scene brightness. In other words, a varying load on the HV also caused focus instability.

It looks like the regulation has to be just right -- low enough to keep a constant load on the flyback with changing scene brightness, but not so low that the flyback is overloaded and the focus rectifier filament runs with too low current.

At the moment, I can't really find a sweet spot where the focus is totally stable, but it is far better than before. It's also a question of how much brightness and contrast this set is really designed to deliver. With relatively low contrast, I could probably get things stable.

Or quite possibly, with a proper flyback, everything would work much better. I'll give some thought to either adding more turns to the HV winding (which will be a bit of a project, requiring much more disassembly and modification of the flyback than I did before), or seeing if a voltage tripler in place of using the HV winding can work.

So far, this is all very educational as far as the HV system of the CT-100 goes!
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