Certain DisplayPort-HDMI cables can disable HDMI CEC communication
With a setup including a TV and AV receiver where HDMI Consumer Electronics Control (CEC) and Audio Return Channel (ARC) functions were working, these two functions stopped working after changing some connections (not the one between the AV receiver and TV), so I believed it was a corrupt setting in the AV receiver or TV, but when I disconnected a passive DisplayPort-HDMI cable (which a Multimode DisplayPort connector supports HDMI with a low cost passive cable or adaptor) connected to a home theatre PC, HDMI CEC and ARC functionality was restored.
An examination of the HDMI CEC wiring in a number of TV service manuals revealed that the CEC line (a single open collector bidirectional signal) is common to all HDMI inputs which meant that the particular DisplayPort-HDMI cable was sinking the CEC line thus disabling CEC functionality.
So I used a variable angle HDMI adaptor (which the housing can be easily opened) with the CEC line cut for the home theatre PC which fixed this problem.
An examination of the HDMI CEC wiring in a number of TV service manuals revealed that the CEC line (a single open collector bidirectional signal) is common to all HDMI inputs which meant that the particular DisplayPort-HDMI cable was sinking the CEC line thus disabling CEC functionality.
So I used a variable angle HDMI adaptor (which the housing can be easily opened) with the CEC line cut for the home theatre PC which fixed this problem.