PART FIVE - VIDEO AMPLIFIER CHAIN. Now we boost the tiny signal from the iconoscope mosaic, process it and combine it with sync to make a TV picture.

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Figure 7. Iconoscope Camera Video Amplifier Chain

       Theory of operation. The mosaic input is on the upper left in the diagram. The resistor divider provides a +90V source that is isolated from the second anode. But, exactly the same voltage. The tiny video signal will be 'riding' on this DC voltage. It is AC coupled through C51 to the first amplifier stage. I set the gain of this stage very low to make the input impedance as high as possible and to not load down the mosaic which is at extremely high impedance. Amplifier number 1 boosts the signal by 2. The signal is once again AC coupled to the amplifier 2 with a gain of 20. Q16 is a FET switch that is closed briefly during horizontal blanking and its function is to set the end of C52 at 0 volts. This is called 'clamping' or DC restoration. At this point, the iconoscope video should be sitting on zero volts and going only positive. (should be...) The output of amplifier 2 is coupled through a pot, R35, so we can set the video gain, independant of the sync amplitude. These should operate as non-interacting controls. (they should...) And finally the video and sync are married together at the junction of R37 and R39 and amplified by U8-B before heading for the output jack.

Figure 8. Completed Iconoscope Camera Project

       Your patience has paid off. If you have been wondering what it looks like all put together, here is a photo of the complete camera project as of July 2, 2013.

       This is the initial design of the video amp as I have been able to calculate based on information I can get from Azimov's book Television. The gain looks low to me too. But, we will deal with that as we go. Proper debug will consist of starting at the output jack, far right, and debugging one stage at a time as we go. Logical step one would be to set the output sync amplitude to 286mV or 40IRE of sync, with the output properly terminated in 75 ohms, and the measurement taken at the center pin of the output jack. This is already working. Next I will proceed to inject video at lower and lower amplitudes going down the chain toward the input. This verifies each stage as we go.