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Registered Member #8120
Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
I want to draw vector graphics (aka a clock) on a CRT tube (5LO38I). Basically, it works, but i get a weird problem.
Increasing the brightness distorts the image.
In other words, the modulator is going between OFF at -100 V and ON at X V. With X going from about -60 (where it is barely visible) towards 0V, the image distorts like this:
The image appears to contract towards the centre of the screen, and bend in various fashion.
Naturally, i checked the anode voltages, and they both appear stable - no dips or changes at any time scale. So are all other supply voltages.
What could it be? Is that normal, perhaps - the thicker beam requiring more voltage to deflect or something? I'm not quite sure where to look, and these things are not exactly documented.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
You need to up all the voltages the electrons are repelling each other and spreading on their flight to the tube face. More acceleration doesn't give them the time to spread, as they are going faster. downside is that you will need more deflection voltage for the same reason.
This is why PDA tubes were developed (post deflection acceleration)
Registered Member #8120
Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
Figured it out. The deflection plates are not perfectly isolated, there is a leakage current proportional to the beam brightness. While i made very high impedance deflection amplifiers, thinking that they are just voltages with no current.
Thus, the voltage applied to the plates sagged, the beam gravitated towards the centre, producing weird distortions.
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CRT - more brightness, more distortion?
