10-11-2016, 09:10 PM
With regards to screening and decoupling quite extraordinary care was taken during the development of this unit. It has to be borne in mind that development of this TRF unit started at least one year before production started, this was cutting edge technology in 1935/6.
Not much progress today, the video developed across the video detector load resistor exhibits the usual fault when handling a positive modulated vision signal, that is as the gain is turned up the active video part of the waveform becomes progressively crushed.
With negative modulation TV signal it is the sync pulses that become crushed under the similar conditions.
Back in the sixties there was a similar problem after converting Thorn 700 and 800 series sets to dual standard operation with the add-on 625 IF amplifier unit. The unit employed a PCF80 as the video amplifier. It was discovered the negative going video was driving the video amplifier into cut-off on the sync pulses, the grid base characteristic was not up to the task of handling 625 video. The fault was solved by introducing AC coupling between the detector and the grid of the video amplifier.
Geordie McBoyne.
Not much progress today, the video developed across the video detector load resistor exhibits the usual fault when handling a positive modulated vision signal, that is as the gain is turned up the active video part of the waveform becomes progressively crushed.
With negative modulation TV signal it is the sync pulses that become crushed under the similar conditions.
Back in the sixties there was a similar problem after converting Thorn 700 and 800 series sets to dual standard operation with the add-on 625 IF amplifier unit. The unit employed a PCF80 as the video amplifier. It was discovered the negative going video was driving the video amplifier into cut-off on the sync pulses, the grid base characteristic was not up to the task of handling 625 video. The fault was solved by introducing AC coupling between the detector and the grid of the video amplifier.
Geordie McBoyne.
