Help with 3A fuse blowing on cooling

I recently came back from a trip, and the thermostat was still working. On turning the AC on, the wall-powered thermostat decided to die.

Upon investigating in the attic, I noticed the 3A fuse had shorted. I put a new one in, but this one burned as well, as soon as the cooling kicked in.

Following this, I disconnected the yellow and blue (Y and C) wires that lead to the outside unit from the control board and tested the system. On activation of cooling from the thermostat, the fan turned on, and everything worked fine.

I plugged Y and C that lead to the outside back into the control board, and unplugged the yellow and blue (Y & C) from the contactor coil, outside. I let the system run, and the fuse burned.

I go back inside to investigate, unplug Y & C (now Y & C are connected to nothing) and do a beep test between Y&C - no beep.

I connect outside Y&C back to control board, and now altogether, Y&C from thermostat and Y&C from outside meet with their respective colors. Thermostat is off, but when last powered, was set to cooling. I beep test the Y&C terminals directly on the board, and it produces a beep.

Unplugged everything, directly beep tested board Y&C terminals - nothing.

Did a ton of other beep tests:
tY (thermostat Y) to oY (outside Y) - nothing
tY to oC - nothing
tY to bY (board Y) - nothing
tY to bC - nothing

tC (thermostat C) to oY - nothing
tC to oC - nothing
tC to bY - nothing
tC to bC - nothing

oY to bY - nothing
oY to bC - beep (why?)
oC to bY - nothing
oC to bC - nothing

Any ideas what might be wrong, or what the next steps I should check are? I don't think there is a short in the condenser coil... and I really hope it isn't a board problem.

Help with 3A fuse blowing on cooling