What was found
CREW DESCENDED TO FL240 AND TURNED OFF RT PACK. WHEN OPERATING JUST ON LT PACK, AIRCRAFT WOULD NOT MAINTAIN PRESSURIZATION. CABIN ALT WARNING HORN WENT OFF AND DIVERTED. SUPPLEMENTAL DATA UPDATE CAUSE: E/E RAIN CURTAIN HAD BECOME LOOSE AND FELL ON TOP OF AUTO PRESSURE CONTROL CAUSING HEAT AND BLOCKAGE OF AMBIENT AIR. SECURED RAIN CURTAIN AND REPLACED THE AUTO PRESS CONTROLLER AND REPLACED RT MIX VALVE AS PRECAUTIONARY. NO REPEATS SINCE ORIGINAL.