Quote:
Originally Posted by ct92404
If the wiper or whatever is in the picture control is worn out and not making contact, so that the variable resistor sometimes just isn't connected , what effect would that have?
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If the control is wired exactly as in the schematic, then if the wiper goes open, there is a very large change in the cathode current of the video driver stage, it jumps to zero. Ideally, to help mask over imperfections, the central or wiper connection should be connected to the free end of the control, that way when the wiper goes open the value can only get to 5k. The higher the resistance placed in the cathode the lower the gain of the video driver stage. Certainly if that control was open, you would have negligible video signal ending up being sent to the CRT even if it was ok out of the detector. Try either shorting out the control, or connecting its center leg to its free end.
The purpose of the small 47uuF capacitor across it is to maintain the video driver stage's high frequency response, otherwise at low contrast setting (the control at its max value or close) the picture could look soft with a loss of high frequency detail.