If voltages are correct under USB, that means 5V->3.3V is working fine. The problem is most likely in the 24VAC->5V conversion. The Resistance Between PTC (top leg) and D1 (left leg) doesn’t tell much because these two pins are connected by PCB trace. Here are my suggestions:
– check if you have any missing component
– did you insert IC1 (MC34063)?
– does the PTC fuse feel very hot?
it would definitely help if you can post an image of the lower-left side of your board (the 24VAC->5V conversion part).