B767 CREW HAD A MALFUNCTIONING PACK. DURING TROUBLESHOOTING; THE CREW HAD A CABIN ALT WARNING HORN AND THE FLT ATTENDANT DEPLOYED THE PAX EMER OXYGEN MASKS.

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ON TKOF ROLL; THE L PACK STARTED TO CYCLE ON AND OFF STARTING ABOUT 80 KTS. WE CONTINUED THE TKOF AND THE PACK CONTINUED TO CYCLE EVERY FEW SECONDS. WE SHUT THE PACK OFF AT ABOUT 3000 FT TO KEEP FROM GETTING THE EICAS MESSAGE EACH TIME IT CYCLED. WE CONTINUED THE CLB WITH THE CAPT FLYING AND LOOKED FOR THE CHKLIST AFTER WE WERE CLBING OUT OF 7000 FT. WE RAN THE CHKLIST FOR BOTH THE L PACK OFF LIGHT AND THE L ENG BLEED LIGHT. NEITHER ONE FIXED THE PROB. THE CAPT CONTINUED TO FLY AND I CONTACTED DISPATCH AND WE PATCHED TO ACR MAINT CTLR. HE HAD ME RUN THE SAME CHKLIST AND THEN HE GAVE ME 5 CIRCUIT BREAKERS TO PULL AND RESET. WE WERE THEN AT FL270 AND THE ALT WARNING HORN WENT OFF AND THE CABIN ALT SHOWED 10000-11000 FT. THE CAPT DECLARED AN EMER AND WE STARTED A DSCNT. THE L PACK STARTED TO WORK AGAIN; WE RAN THE RAPID DEPRESSURIZATION QRC AND BY FL190 WE HAD THE CABIN ALT BACK DOWN AND UNDER CTL. WE LEVELED AT FL190 AND THE CAPT GAVE THE ACFT TO ME AND HE TALKED TO DISPATCH. OXYGEN MASKS IN BACK WERE MANUALLY DEPLOYED BY THE FLT ATTENDANT AT THE CAPT'S ORDER DURING THE DSCNT. WE RETURNED TO ORD AND CONTINUED TO HAVE SOME PROBS WITH THE CABIN PRESSURE. WITH A FLT IDLE DSCNT OUT OF 17000 FT; BOTH PACKS ON THE CABIN CLB AT 400 FPM.