I'm sure I found this book scanned online a couple of months ago, but I can't remember whether it was linked from somewhere or I just found it.
It's not ray carlsens stuff and it's not commodore's documentation for their cartridge, but it had a step by step diagnosis using oscilloscopes etc.
It's really bugging me now because I didn't have time to go through it at the time, but now I have time I can't find it.