With extreme difficulty. It's one of those things that gets lumped in with 'MLB failure' as the only remaining (but unspecific) cause of malfunction. When you have eliminated the obvious, as Holmes was wont to say, whatever remains, however improbable, must be the cause. You are on the right course by testing one suspect at a time, although use of a known-good MLB would save a lot of time. Keep in mind that not only removal of a part but also substitution with a known-good part can be revealing. However, it is of some comfort if you get a POST chime at startup.

de