Hello, I was pleased to find this post as, I am having the same problem and the same air handler unit. I blew two transformers. Lucky they were only 9.88 each. So, there is a short going on some where.
I don't know if it may be the circuit board or not, I even had a HVAC guy out and he installed a new thermostat, power switch, a fuse on the line and his transformer blew also, he said it maybe about 500.00 bucks to replace the board , so I was glad to see you found it for 90 bucks on ebay. Here in Atlanta I have a HVAC store I can visit first to see if they have the board. But, was curious about the outcome of your problem. Did it turn out to be the board or was it the "contactor"
I would love to fix this myself since I'm already about 200 bucks in the hole with the HVAC guy..Whom, I haven't been billed by yet, I just asked him for a price on a new unit and was told 4700.00 bucks "ouch" but, still not back with the 1500.00 tax break. So, I'm in a toss up!
Thanks, for any help