Re: HMV 1804 Television.
The first attachment shows the vision demodulator and amplifier stages of the HMV 1804 and Marconi VT50.
Negative grid bias for the video amplifier valve is derived from the voltage drop across potentiometer VR2 which is connected in series with the HT negative return. Now that the timebases are up and running it is now possible to check the performance of the post detector video amplifier stage. While the timebases were out of action it was not possible to test the video amplifier because insufficient negative bias was developed across video amplifier bias control VR2.
The maximum video developed across the video demodulator load resistor R25 should not be more than 3 volts P - P. As a result of the high sensitivity of the modified receiver an additional vision gain control is required and such a device was fitted in this receiver, the 5Kohm potentiometer was mounted on a bracket and was fitted on the underside of the set, see second attachment. This control will be relocated at the back of the set next to the sensitivity control.
DFWB.
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