I received this known broken KR1AA that had already been replaced by Hank and trying to trouble shoot it.
Does not work with Ni–MH. Varying response to Lio-ion. Has discharged batteries. When I measure a working one with a multimeter, with an anode on the center post, and the cathode on the threads of the head, I get 0. When I do the same on the broken head, I get 25 mV. So does this mean somewhere there is a short to the head and that is causing all of the problems?
I also noticed one of the 3 “spring” soldering points had come loose, what is the best way to reconnect it?
No, it means that you measured in a wrong way. There’s no battery, thus no voltage to measure. You could measure the resistance, but that won’t help you with the problem. Sounds like the boost converter failed. I’d start with a visual inspection, then continue finding shorts and checking diodes / switching semiconductors.
If it’s soldered, solder it. But I’d keep it that way. The soldering just prevents losing the spring.
As you suspect, it could be something bridging a contact somewhere. On a broken kr1aa I have, there were what looked like single filament strands of a high gauge stranded wire (hair like) trapped under a lopsided spring ring, and was shorting one of the little black boxes to the body, (superglued to the board and soldered under the spring ring) causing heavy flickering and the inability to hit turbo while rebooting past a certain level of brightness.