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Old 27th Feb 2021, 11:09 pm   #339
FERNSEH
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Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Re: Baird T5 restoration project.

The rewound line output transformer will be refitted in the T5 timebase unit and testing procedures will be carried out in the T23. But first the T23 must be returned to working order. It will recalled the set is having problems with the Cossor CRT. The plan is to modify the CRT brightness control circuit to the arrangement employed in the Baird T18 and T20.
In the original T5 circuit the brightness control is in the video DC restorer. The negative voltage present at the cold end of the EHT transformer secondary is adjusted by the brightness control to provide the sync tip reference.
In the T18 the brightness control varies the cathode voltage of the CRT. The positive going video is AC coupled to the grid of the CRT. The DC component of the video is lost. It should be possible to retain the DC restorer diode with the negative going sync tips clamped to chassis potential.
The attachment shows the T18 CRT circuit.

DFWB.
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