As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PREFLIGHT, RT HSI WAS FLICKERING ON AND OFF THEN SMOKE STARTED BELLOWING OUT FROM UNDER THE WHOLE INSTRUMENT PANEL. COCKPIT FILLED WITH SMOKE IN SECONDS. FOUND THE RT EHSI TO BE THE CAUSE OF THE SMOKE. INSTALLED A KNOWN GOOD UNIT AND FUNCTIONALLY CHECKED WITH NO FURTHER DISCREPANCIES.