Thanks for the welcome Steevsee

I didn't suspect the bridge at first because the fuse wasn't blowing until I activated the Flipper test after removing the flipper fuses so if the bridge was bad it would blow a fuse as soon as power was applied. Of course despite being an electrician, I have embarrassed myself because to make the flipper test work, you need to push in the door switches to apply the 50 VAC. Now that I am testing properly, the fuse blows as soon as I apply the 50 VAC. I have tested the bridge and one leg is shorted. Not the first time the door kill switch has caught me out.