Heil furnace blowing fuse on board

[pitteach]

I have a 5 yr old Heil forced air 75k btu, 92% furnace that is blowing the 5 amp fuse on the control board. There is a low humming sound coming from the board and the transformer is warm, but when I install a new fuse and throw on the power, it pops immediately. This happens wether or not the t-stat is calling for heat or not. The led trouble code is not showing (obviously). The furnace has worked fine and nothing has been done to it-it just popped all of a sudden. I am suspecting the transformer, but how can I check or are there any other ideas?

Thanks

[jadnashua]

While the transformer might be bad, it is more likely that there is a different failed component somewhere on the board or that the board drives that is overloading the circuit causing the fuse to blow. The fuse, on a well-designed board, will blow before the whole thing burns up. Unless you're lucky and can identify the part that is bad, you'll probably have to replace the board. Now, depending on how good the schematics are that they provide (often not very complete), and the troubleshooting guide is good, and (that's a lot of ands), you might find something off-board that is bad that is overloading the power supply on the board and blowing the fuse.

[pitteach]

I just jumped the t-stat at the board with a new fuse and she fires right up. The t-stat wire is ancient so I will replace and check out the thermostat as well.