We have some horses, and a similar setup. Although I haven't had mine apart, it sounds like you have insulation breakdown in the secondary - and that's where you see the spark. The transformer you speak of is the one under the orange, correct? The other transformer is the low voltage transformer for the logic that gives you that jolt every ~ 1 second. The HV transformer is probably a specialty part. I've seen do-it-yourself circuits use a flyback transformer from a TV set or an ignition coil transformer from a car do the same thing, but it would require a good schematic of your unit if you would want to make the modification. I would suspect it would cost more to repair this unit than to go down to tractor supply and buy a new one - unless you happen upon one of those HV transfomers. Let us know.