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Old 28th Aug 2019, 8:01 pm   #179
FERNSEH's Avatar
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 6,330
Default Re: Baird T5 restoration project.

Beery wrote: "Your T23 is a T5/T20 hybrid. I've attached the circuit of the T20's TRF unit. If the grid bias is adjusted in the same way as the T5 then R18 in the TRF circuit would have to be deleted (point 'M' in the TRF circuit is connected to the CRT cathode).
However, does your T23 control the brightness via the cathode as in the T20?"

Hi Andy,
I haven't checked to see if R18 is present in the RF chassis but it's safe to say that it has been removed by the manufacturer. The 15MW1 CRT in the T5 has a directly heated cathode whereas the Cossor 3265 tube has an indirectly heated cathode. The cathode is connected to one of the heater connections so like the T5 it is at chassis potential. And like the T5 the brightness is controlled by varying the grid voltage which is derived from the negative return in the EHT supply. The resistor bleeder chain across the EHT supply provides the constant current for the voltage drop for the negative CR tube grid bias.

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