A short between A & B means that either a) somehow the primary and secondary wires of T501 are shorted together, or b) there was a short on the board before you put T501 in.? Most likely a) but check both.
A short between primary & secondary is easy to accidentally do if you scrape too much of the lacquer off the ends of the wires.? Also, sometimes the core itself has some sharp edges just inside the holes on either end - these can scrape the insulation off the wires as you pull them through the core, potentially causing shorts.? You can use a craft knife or a small conical grinding stone from a Dremel tool to carefully 'sand' those edges of the core to make them smooth, so they don't scrape the wires.
