What was found
AFTER PARKING AT THE GATE EXTENAL POWER WAS ATTACHED & AVAILABLE. SELECTED EXTERNAL POWER AND TURNED OFF THE APU. THE AIRCRAFT TOOK EXTERNAL POWER FOR AROUND 5 SECONDS & THEN STARTED FLICKERING. EXTERNAL POWER SWITCH WAS DESELECTED AND THE FLICKERING CONTINUED. MASTER SWITCH WAS THEN TURNED OFF WITH NO CHANGE. CLICKING AND POWER FLICKERING CONTINUED AND A BURNING SMELL WAS COMING FROM CB PANELS 1&2. . GROUND CREW DISCONNECTED EXTERNAL POWER AFTER APPROXIMATELY 1 MINUTE. ALL POWER TURNED OFF AND FLICKERING STOPPED. SUSPECT BAD POWER CART. STARTED APU NORMALLY AND POWER WAS RESTORED WITH NO BURNING SMELL DETECTED.