What was found
ON PREFLIGHT THE WATER SYS CIRCUIT BREAKER WAS BLOWN. WHEN PUSHED BACK IN SMOKE BILLOWED FROM THE LAV SINK IMMEDIATELY. BREAKER WAS PULLED OUT WITHIN 5 SECONDS AFTER SMOKE STARTED. INSPECTED WATER TANK, SWITCH, CIRCUIT BREAKER AND SOLENOID. NO EVIDENCE OF ARCHING OR SHORTED WIRING. FOUND WATER TANK OVERHEATED AND WHEN POWER APPLIED TRIPPED BREAKER. R & R WATER TANK. SERVICED WATER TANK AND POWERED UP ACFT. LET RUN FOR 30 MINUTES WITH NO SMOKE OR POPPED BREAKER. SYS OPS CHECKS SATISFACTORY AT THIS TIME. ALL WORK DONE IAW MM 38-20-01.