I have found the defect. It was not the video detector but the A.G.C. circuit. The A.G.C. amplifier (pentode of the 6U8) was very weak (tested with Funke W19). At the anode there were only -1v. It has to be appr. -31v. With the new 6U8 (european ECF 82) the set is working properly again. -76v (??) at the anode.
The schematics for the ctc-5 are online here:
http://www.mcmlv.org/Archive/Radio/CTC5A.pdf