I am new to this site and appreciate all your help. I tried to replace a light fixture in one bedroom with a newer, heavier one. The house was built in 1971 and has 2 circuits for all the lights. After turning off the power to the circuit that had the bedroom lights on it, I set about to replace one of the fixtures in a bedroom with a newer, heaver one. I installed a brace and new box and then went up to wire it.
The old fixture box had 3 cables of 12/2 coming into it. With power on, I used my voltage tester on each wire coming into the old box and discovered that all neutrals were hot as well as the blacks in the other cables. However, after I cut the soldered wires off and then checked again each wire with the voltage tester, I discovered that on one of the 3 cables coming in, the black wire was hot, the neutral was not hot (I'll refer to this as "sparky". This neutral had been "made hot" by the soldering of it with a hot wire.
So, I went about to connect the 3 cables again, making all three of them all hot like the electrician before me did. To do this it didn't seem to me to matter which wires were wired together since all were going to be hot anyway, right?
So I created two groups, each with one (originally hot) wires and an equal number of white wires and grounded all grounds together to the new metallic box, then made sure to pigtail between the 2 groups. I wasn't particular which wires from which cable went to the 2 groups, since on the old box, each wire was hot anyway. I then tied all grounds to the metalic box.
When I threw the power, the breaker tripped immediately. I am not sure what I failed to understand. But I obviously did something wrong. Any advice would be appreciated.
Here’s 3 photos of the switch loop that I cut out.