Nordyne E2EB-015HA
No power to tstat. Traced problem back to transformer. Only 3v on secondary. Replaced transformer & consistently have 243v primary & 26v secondary. Fan works fine. Cool works fine. As soon as heat is turned on transformer starts popping & burning up.
What would should I check next?

Either the Heat wire is shorted, or Contactor/Relay Coil is shorted.
Try removing the low voltage wire at the coil of heat contactor, and try again, if it doesn't blow, it is the relays coil, Maybe add a 3 amp blade fuse holder first to protect the transformer. 40 VA? Good Luck