CDTV no display issue

[Castellen]

As there's no RGB video, that suggests something around the Deinse or video hybrid area.

I'd start by checking you have +5V on the +VID line (pin 2 of HY1). If not look at R6 (4.7 Ohms) which may be open circuit.

Also try the obvious of re-seating socketed ICs, though fortunately most of the video stage is soldered in.

The CDTV video section is near identical to an A500, so if required you can also get parts from an old A500 which are usually easy to get.  The actual video schematic for the CDTV is here.

I could repair it for you, although return freight from New Zealand would be getting pricey.

[Castellen]

The video module bypass thing essentially involves linking pins on a connector as it suggests in that photo.

A faulty Denise would probably result in the system not booting at all, but it's not as though you've spent a lot of money and there's a small chance that may fix it.  The next logical step is to do a bit of fault tracing but you'll really need an oscilloscope for this, plus know what you're doing.