Quote:
Originally Posted by old_tv_nut
You are not loading down the video when the brightness control is at the end, because the diode only conducts on sync.
However, in the original circuit, the average cathode video voltage is between 0 and 120, but with the diode, the sync peaks are between 0 and 120, and the average is lower (making a brighter picture). You may need to look for a higher voltage for the positive end of the brightness control and put a resistor between the other end of the contol and ground, to get the syncs to be cutoff and normal video to be normal.
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Ok so after looking at it closely, here is my understanding of what is going on. I could well be wrong in some detail:
Looking at the service manual, the AC signal at the plate of the video output tube is shown as being 16V peak to peak, with the big peaks being on the sync pulse. This makes sense from the perspective that during sync, the picture should be blanked. So assuming that there is no DC restore circuit, it looks like at maximum brightness, the signal at the cathode of the CRT ranges between the 0V and +120V, plus the 16V of the video output tube. So at maximum brightness, the AC value of the signal at the CRT cathode is between 0V and 16V, and at minimum value of brightness, the signal ranges from 120V to 136V.
The DC restore function appears to work like this: During the sync pulse, the highest voltage is present, which charges the capacitor. As soon as the sync pulse ends, the voltage at the capacitor is higher than the voltage on the cathode of the CRT, so the diode switches off.
Now, why would elevating the positive end of the brightness control help things? It is usually operated closer to the middle position, or higher. Even if this end were elevated more, how would it affect the DC voltage at the junction of the capacitor and the volume control wiper?