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Update:
Removed the control plate and cleaned all the controls... removed the tuner assembly and cleaned the tuner. Am surprised to see a wafer style tuner instead of turrets like on a Zenith, it seems kind of cheap.
Tested most of the tubes. Found one weak pentode section in a 6GH8 - replaced it. This TV has very low hours, a handful of tubes were replaced in 1970 and all of them still test almost new. All of the tubes the TV came with are RCA brand.
The only dust in the entire cabinet is stuck to the tubes in the high voltage section (focus rectifier, etc)
So on to the problem - the TV warms up and makes a beautiful picture... my Zenith of similar age could never make such good colour. However after it has been on a little while, it loses sync and the picture gets scrambled, looking like someone has turned the fine tuning control too far. Adjusting the fine tuning will bring it back, but not for very long. Sound is perfect and almost completely free of buzz. I am thinking it could be AGC related somehow, but don't really know how to go forward aside from getting the correct sams service data and starting to check voltages and waveforms.
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