No power when the key is turned

Hey all, new to the forum and just bought my first XS400 Easter Sunday, a 1981 XS400H SOHC. Cleaned the carbs and got it running, but the battery drained out (charging issue). No power when I turned the key, so I assumed battery was dead. Bought a combo reg/rec unit and wired it in while the battery was on the charger, but still have no power when I turn the key.

I get voltage across the battery and main fuse with the key off, I have continuity in all the wires I dealt with on the reg/rec swap. I have power to the ignition switch, and took the ignition switch out (no corrosion, continuity everywhere within) and reinstalled it. When I turn the key, the voltage across the battery drops to near zero and I get no voltage readings at the main fuse. Oh, and I have continuity from the brown ignition wire to all the other fuses. Going crazy wanting to ride it again, but cannot get this figured out!

Any ideas on what could be the cause?

Sounds like you have a major short some place.
Do you have the factory wiring harness?
What reg/rec did you put in (there are two basic types, field coil and magnetic field and they aren't interchangable, you need a field coil type and there are two types of those as well)?

Well, I'd talked to an electrician friend of mine while waiting to hear back on the forum and he believed there was excessive sulfate formation in the battery that PO purchased, so I replaced the battery and problem solved. To answer your question though, I don't know what style it is for sure; $25 eBay combo unit from Caltric. Had to bypass the headlight relay to take it for a ride today, but it left me stranded on the way home from work. Rolled around a corner and it just sputtered and died. Nice person gave me a jump, got me about 2 miles before it died again, power to taillight in park but intermittent when in run. Seems to have a loose connection or bad wire. Broke one of the glass fuse holders trying to get a better bite, but just got a new blade style fuse block in the mail today so hopefully I just have poor fuse contact.

Edit: Yes, stock wiring harness, stock lighting...clean stock bike whose only issues seem to be age and a PO who had no clue what they were doing when working on it (exhaust split rings installed backwards, drive chain with 2" of slack, etc.)