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FERNSEH · 19th Apr 2018, 4:44 pm

Reference to my post No. 79. A negative going pulse coincident with the frame blanking period is required for the for the black level sampling.
In the Ultra V17-60 the 500uS pulse present at the anode of the frame blocking oscillator is ideal because the performance of the oscillator is not impaired by the loading of the vision AGC circuits.
The V17-70 and Pilot Spacemaker employ a multivibrator timebase. Today, I hooked up a simple differentiator consisting of a 22Kohm resistor and a 0.047mfd capacitor to the anode of the triode section of the 30PL1 frame timebase valve. The resultant negative going pulse has an amplitude of >100volts and has a sharp leading edge but unfortunately is not broad enough for sampling the black level, the spike is coincident with the broad pulses, not good, we're looking a pulse width of >250microseconds.
The ideal source for the AGC gate pulse is the flyback part of the waveform present at the anode of the frame output valve, but it's positive going so it's no use for the circuit we're considering for V17-63 and PT650.

DFWB.

19th Apr 2018, 4:44 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: DC Restoration questions Reference to my post No. 79. A negative going pulse coincident with the frame blanking period is required for the for the black level sampling. In the Ultra V17-60 the 500uS pulse present at the anode of the frame blocking oscillator is ideal because the performance of the oscillator is not impaired by the loading of the vision AGC circuits. The V17-70 and Pilot Spacemaker employ a multivibrator timebase. Today, I hooked up a simple differentiator consisting of a 22Kohm resistor and a 0.047mfd capacitor to the anode of the triode section of the 30PL1 frame timebase valve. The resultant negative going pulse has an amplitude of >100volts and has a sharp leading edge but unfortunately is not broad enough for sampling the black level, the spike is coincident with the broad pulses, not good, we're looking a pulse width of >250microseconds. The ideal source for the AGC gate pulse is the flyback part of the waveform present at the anode of the frame output valve, but it's positive going so it's no use for the circuit we're considering for V17-63 and PT650. DFWB.