aussiebloke
New Member
G'day all.
I a vintage video camera enthusiast from Australia and I have built from scratch a 2/3" vidicon tube B&W camera using a 1970s vidicon camera schematic, my camera project can be viewed on my website on this page **broken link removed** . The camera is making pictures however there's a problem with the vertical scan. Basically I am not getting a proper sawtooth waveform for vertical scan and as a result I am getting a vertically squashed picture (see attached image).
As you'll see in the attached schematic of the vertical section the problem lies between collector of transistor TR19 and gate of transistor TR21. As I understand TR19's base is fed a 1v pulse every say 20ms and during that pulse period TR19 goes to saturation and shorts the current at the collector to ground and then during the pulse off period the 4.7uF capacitor charges up to about 14.5V, and then when the next pulse occurs the process is repeated which is suppose to create a sawtooth waveform. The problem is however the capacitor charges up exponentially giving a non-linear ramp which results in a picture that's vertically squashed at the top but stretches as the vertical scan progresses down the screen. I have experimented with increasing the capacitance which resulted in a better slope that looks more like a sawtooth but the peak voltage drops as capacitance is increased which is the nature of capacitors as they filter out AC ripple, this results in a better picture with more vertical width but more darker due to drop in amplitude.
I need to find a way to generate a sawtooth waveform from that 20ms pulse coming from the MC14528 chip with a linear ramp, so I was wondering if anyone here knows of what circuitry I could implement between TR19 and TR21 that would give me a linear charge up ramp to a peak voltage of about 14.5v in a period of 20ms? Any help would be MUCH appreciated.
The schematic for this camera came from a book titled "Amateur Television Handbook" which can be downloaded from here **broken link removed** .
I a vintage video camera enthusiast from Australia and I have built from scratch a 2/3" vidicon tube B&W camera using a 1970s vidicon camera schematic, my camera project can be viewed on my website on this page **broken link removed** . The camera is making pictures however there's a problem with the vertical scan. Basically I am not getting a proper sawtooth waveform for vertical scan and as a result I am getting a vertically squashed picture (see attached image).
As you'll see in the attached schematic of the vertical section the problem lies between collector of transistor TR19 and gate of transistor TR21. As I understand TR19's base is fed a 1v pulse every say 20ms and during that pulse period TR19 goes to saturation and shorts the current at the collector to ground and then during the pulse off period the 4.7uF capacitor charges up to about 14.5V, and then when the next pulse occurs the process is repeated which is suppose to create a sawtooth waveform. The problem is however the capacitor charges up exponentially giving a non-linear ramp which results in a picture that's vertically squashed at the top but stretches as the vertical scan progresses down the screen. I have experimented with increasing the capacitance which resulted in a better slope that looks more like a sawtooth but the peak voltage drops as capacitance is increased which is the nature of capacitors as they filter out AC ripple, this results in a better picture with more vertical width but more darker due to drop in amplitude.
I need to find a way to generate a sawtooth waveform from that 20ms pulse coming from the MC14528 chip with a linear ramp, so I was wondering if anyone here knows of what circuitry I could implement between TR19 and TR21 that would give me a linear charge up ramp to a peak voltage of about 14.5v in a period of 20ms? Any help would be MUCH appreciated.
The schematic for this camera came from a book titled "Amateur Television Handbook" which can be downloaded from here **broken link removed** .
