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Old 30th Aug 2019, 10:37 pm   #197
beery's Avatar
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 850
Default Re: Baird T5 restoration project.

Originally Posted by FERNSEH View Post
Well now I know the reason why the brightness control doesn't work in the T23. The CRT has leakage between the anode and grid, possibly a piece of cathode material has got lodged between the electrodes.
When the brightness control plug is pulled out sparks are seen to jump across the grid and earth connections of the socket.
Hi David,
I was just wondering if there is actually a spark going across the ends of the brightness control. What I mean is, what if the high potential across the brightness control socket comes from the 1M EHT return resistor being high... And if that is the case, what if then one end of the brightness control was o/c? Would that not also give you unajustable brightness?

It does seem odd that although there seems to be a high leakage in the tube, it can still be modulated easily by the video drive.

Maybe croc clipping a meter between the wiper connection of the pot and chassis (I would not want to dab a probe onto it live if there is any EHT around) will give you more of a clue. Then you could run the same test again with the grid and heater connections of the CRT removed to see if the voltage range of the pot wiper is the same.

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