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I calculated what the brightness control range should be from the schematic:
-8 to + 8 volts. The measured tap voltage for normal brightness is about 4.5 volts, not zero as you'd guess, nor -0.5 as listed on the schematic. I don't know at this point if this higher required voltage is due to rewiring the video output cathode circuit to match the CTC-7; could be. Anyway, when first turned on there's plenty of range for too bright to too dark.
When first turned on (bright picture) the voltage is -1.4 volts at the low end of the pot, which gradually drifts lower, darkening the picture. Eventually it's not quite possible to get normal brightness.
It looks like I could just ground the lower end to make it zero volts and no power supply will be necessary to see if that stops the drift. If that does stop the drift, it could be a long term "temporary" fix until I can pull the chassis to work underneath.
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Last edited by old_tv_nut; 01-05-2026 at 10:21 AM.
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