So yur buddy was trained by Carnac The Magnificent?
Was the "high" voltage to the transformer 220, or 110? Some indoor units can be 110, some 220; and as posted earlier there can be the transformer inside, outside, or two. Did you check for power before removing the panel? If the unit is 20 years old, or less, the chances of electronic failure are pretty high with pops occuring, unless the stat is a Honeywell T87 or one like it. I'd call a reputable tech instead of parts replacement, it's safer and techs do not appreciate owner fixes to nowhere because that can add problems to the original one.