Wow, okay. That doesn't sound like a HARD failure (no smoke), and the LOW LINE indicator is, to my mind, a very suggestive symptom. My limited experience with the power supply on the 475A suggests that this will be a quick diagnosis.
Looking at the schematic, (<12> Power Supply) we see at the bottom of the page that the LOW LINE indicator is driven by the unregulated +55V rail and the -8V rail. It looks like there's a voltage divider between +55 (unreg) and -8 V rails, but my circuit analysis knowledge breaks down at about that point. I'd check the voltages on the +55 V UNREG (TP1536?) and -8 V (TP1568) rails first, then, if those look good, check Q1582.
I'm a little confused by the schematic (as simple as it is) because they seem inconsistent in whether they label the test points with the expected voltage, and whether they indicate what voltages are unregulated. I can't tell if TP1536 is the same as the +55V UNREG indicated in the partial A8 schematic at the bottom of the page. This would all be much easier to clear up if I had the scope in front of me.
Good hunting
-- Jeff Dutky
