The CTT option operation and self test seems to be dependent upon
1/ CTT board, EEPROM on board
2/ Trigger Hybrid U500, only certain ones work, even though they may work fine in a non CTT scope.
3/ successful CTT calibration, stored on NVRAM of A5
At the moment I have all three of these factors in question, as the original A5 board is in the hands of a friend to do the SMD leaky cap clean-up and NVRAM replacement. So, I am using another scopes A5.
I shall get the original A5 back and see if that changes anything after a CAL attempt.
The design of the CTT option seems rather flaky, to depend on unspecified U500 properties.