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Old 27th Aug 2019, 10:41 pm   #175
FERNSEH's Avatar
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 6,571
Default Re: Baird T5 restoration project.

The plan was to employ the Baird T23 to test the chassis units from the T5.
But first the Cossor CRT had to be repaired, it will be recalled there was a bad grid connection on the base. That's been put right.
The T23 still doesn't display a picture. A bright raster appears on the screen but the brightness control doesn't have any effect. In these early Baird sets the tube cathode is at chassis potential and the positive going video is supplied to the grid. The brightness control supplies a 0 to 100V negative bias to the tube grid, that's missing.
Pull out the grid CRT plug and connect up the oscilloscope, the video returns. Refit the plug and the video disappears again. Faulty CRT? Hope not, it's more likely the missing grid bias is causing the tube to go into grid current and bypassing the video signal to ground.
So I've got to get the CRT grid bias working before the T23 can be used to test the other set's modules.
The attachment shows the 15" Cossor CRT installed in the T23.

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