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Old 28th Aug 2019, 1:31 pm   #176
beery's Avatar
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 872
Default Re: Baird T5 restoration project.

Hi David,
how does the brightness circuit work?

In the T5 the negative end of the EHT was used to develop a negative voltage across the brightness control. The grid was connected to the wiper of the brightness control via a 500K resistor that enabled the grid to modulated from the output of the vision receiver (via a coupling capacitor). The CRT cathode is connected to one side of the heater and that is then connected to ground via a low resistance pot in the HT PSU, which is used to set the heater current. See the attached circuit.

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?
I've attached the circuit of the T20 PSU, note the CRTs heater/cathode is connected to brightness control VR4 which is connected to HT via another R18.
Also your set has a DC restoration diode as well...

All this might well mean that you have to test the T5 EHT, HT PSU and vision receiver all together rather than mixing them with bits of T23...

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