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I'm interested in a more general solution like some kind of filter or blanking pulse shaper. It seems like the days of analog broadcasting are numbered, and the problem may reappear with digital TV receivers. Who knows how they will decide to f..ck up that blanking intervals in the future... I've aldeady messed with visibility of teletext signals in vertical retrace. I developed a retrace blanking circuit and installed some sort of it in each antique TV. It's just a simple differentiating circuit which forms rectangular pulses from the vertical sawtooth fall and injects them into the brightness control. Of course, I made it out of vintage components so they are almost invisible in the original circuits. Last edited by Gleb; 03-25-2015 at 02:43 AM. Reason: correcting |
#2
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Just curious. Phil Nelson Phil's Old Radios http://antiqueradio.org/index.html |
#3
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Phil, better late than never, sorry for delay. If a CRT is driven the 'classical' way (the signal to the cathode, the brightness control to the 'grid'), it comes pretty simple:
Just cut up the wire from the CRT's grid, and connect a 150-200k potentiometer to the gap. Then take a signal from the vertical oscillator tube's plate, and feed it to the CRT's grid through a capacitor of a few thousand picofarads. Balance the potentiometer to blank the retrace lines completely and not to let the picture get distorted. Then measure the actual resistance of the potentiometer, and install a resistor of the closest value instead of it. It gets more tricky if the CRT is driven the 'odd' way (the signal to the grid, the brightness control to the cathode). In such a case, the flyback pulses from the vertical oscillator have the 'wrong' polarity to feed them to the cathode of the CRT. So you have to get them from the plate of the vertical output tube where the flyback pulses are dirty and distorted by the reactance of the vertical deflection coils and/or transformer, and need to be extra cleaned and shaped before feeding them to the CRT. Last edited by Gleb; 02-05-2017 at 05:17 PM. Reason: addition |
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