There are some useful circuits here: g8ajn.eyrg.net using the 'Projects' button. Dave Kenward's website has been archived by the East Yorkshire Repeater Group. Sadly Dave is Silent Key, so note that some links may be broken and the shop that was operated by Dave is not operational.
VIDEO AGC BOARD with BLACK-LEVEL CLAMP
Essential where you may be taking video feeds from more than one source. The board has link options for AGC on/off and gain 0dB/6dB. Input and outputs have optional attenuator pads to ensure good 75 ohm matching is maintained and output can be either resistor or capacitor. The full CQTV 244 article pdf can be download from the link on the G8AJN website.