What you describe does seem odd, but I can't make out exactly what you did to fix it:
I solved it by wiring the series split wires (red and white) of the bridge hb to the front end of the dpdt switch, soldered to the red from the middle pickup as it entered the switch (middle left of the dpdt (position 2 of the dpdt, right?).
Confused me, I don't know what you mean by the "front end" and "as it entered the switch". A diagram of how you had it wired would help.
But hey, if it works now . . .
One thing I could imagine would be some sort of internal defect in the switch itself, or we would have to posit a stray wire making contact from the middle lug of the p/p to the unwired end.
The only other explanation that comes to mind has to do with wire colors. Now, I thought that all SD pickups had the same wire colors, but you posted two SD diagrams which seem to have different colors, so now I'm not so sure. Usually, you are correct, red/white pair is the series junction for SD pickups. But what colors are you dealing with?
Thanks for the reply. I connected the red and white of the bridge to the same spot of the switch that the red from the stacks connected. This is the part of the circuit that enters the switch. On the stacks the red wire is the south and north finish wires, wrapped together as one color. Presumably because the south coils of these pickups are very small and not designed for series parallel, only for coil splitting for a single coil effect. So the red wire on the stacks is the same as the white and red combination on the bridge hb pickup. I'm not able right now to generate a diagram of what I did but maybe I'll try to come up with one that would be worth showing. I do great guitar work but I'm not an engineer nor do I have any skill at technical drawing. Sorry!!!
OK, so bridge red/white pair plus reds from the other two pickups all go to one of the middle lugs on one side of the switch? And, the lug directly below that is grounded, correct?
Still, it makes no sense how this could have happened, absent a short of some kind, either internally in the switch, or perhaps a stray strand of wire making contact- but the stray wire doesn't make sense since all three pickups would then behave similarly. And, I'm still not clear on what you did that fixed it.
BTW, if the Bridge HB, when split to single coil operation, is not hum-cancelling, you can arrange things so that it is. The second SD diagram you posted shows this with the middle pickup, where it's wired "inside out" (i.e., breen/black go to the split switch instead of red/white, and red then becomes the "hot", white to ground). On your set up, you'd have to wire the bridge inside out, as it's the only one with the 4-wire setup.
Rather than have the bridge hb red and white wires connect to the 5-way switch next to the wire from the push-pull, I connected the red and white to the same terminal in the push pull that the red wire from the middle pickup connects to. Clear as mud, I'm sure...