No offence, but you cannot check anything by unplugging certain sections of the PCB's as they are all up to a point interconnected, and its quite impossible for anyone to fault find on equipment like this without a basic test meter or having the knowledge of knowing what readings to expect and from where. Anyone fault finding on this device would start with the most obvious place for the fault, that being its power supply. I suspect that what you were told by whoever you purchased it from was said more so to suggest that the fault was trivial rather than any semblance to the truth.