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dazzlevision · 6th Jul 2020, 4:09 pm

The local/distant control was only operative on VHF (405) reception - and on VHF/625 communal aerial relay systems.

Its purpose was to set the point at which the vision AGC system started to reduce the gain of the RF amplifier stage in the tuner, rather than the gain of the vision IF amplifier. This was to ensure an optimum signal to noise ratio.

This was set at the factory and adjustment was only necessary when the aerial signals were very strong. Otherwise, on typical aerial signal strengths, you would set it to the point at which the picture just began to show "noise/snow/grain" and then back it off slightly.

In the Thorn 1400 chassis, there is a high value resistor connected to the wiper of each contrast control. It is not unknown for these to go very high value (or even open-circuit), causing very low contrast - with the contrast control having no, or little effect.

6th Jul 2020, 4:09 pm	#55
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,612	Re: Marconi 1400 birthday present The local/distant control was only operative on VHF (405) reception - and on VHF/625 communal aerial relay systems. Its purpose was to set the point at which the vision AGC system started to reduce the gain of the RF amplifier stage in the tuner, rather than the gain of the vision IF amplifier. This was to ensure an optimum signal to noise ratio. This was set at the factory and adjustment was only necessary when the aerial signals were very strong. Otherwise, on typical aerial signal strengths, you would set it to the point at which the picture just began to show "noise/snow/grain" and then back it off slightly. In the Thorn 1400 chassis, there is a high value resistor connected to the wiper of each contrast control. It is not unknown for these to go very high value (or even open-circuit), causing very low contrast - with the contrast control having no, or little effect.