Quote:
Originally Posted by old_tv_nut
The grid signals have about 70% DC coupling already. I think this was done to reduce sensitivity to tube drift in the color difference amps.
I think you could get 100% by removing the .0068 cap across the 270K in the diff amp output circuit. This would reduce the AC gain to the same as the DC, and the blue and green brightness pots would still work. I'm guessing the color gain has enough range to make up for the 30% reduction in AC gain.
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Here is a screenshot of the HC chassis schematic to help other readers follow our discussion.
In this screenshot, wouldn't removing C92 cause the G-Y color signal to be attenuated by R103, significantly reducing chroma gain? Also, the output impedance of the color difference amplifier would be very high, significantly reducing the bandwidth of the amplifier.
Instead, if C92 was shorted, the result would be DC coupling, so DC restoration is not necessary, but then the green brightness pot no longer works. Ignoring that pot for a moment, why not just use DC coupling from the plate of the amplifier to the CRT G1 grid? I still do not fully understand that.