MLG HAS SPD RESTR DUE TO MEL'ED EQUIP; NEGLECTS TO INFORM ATC.

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SECOND LEG IN A NEWLY PURCHASED (USED) AND OVERHAULED ACFT. FIRST TIME ACFT USED 'ON LINE' WITH THIS AIRLINE. NUMEROUS PROBS AND LOG ENTRIES WITH MAINT TRYING TO GET ACFT ON LINE FOR BUSY PAX LOADS. 4 MEL'S CARRIED. 1 (LEADING EDGE SLAT INDICATOR) REQUIRING OPS LIMIT OF 300 KTS; .65 MACH. NEARLY EMPTY ACFT; CLBING THROUGH 2000 FT MSL AT DEP ARPT. 'PACK TRIP' ANNUNCIATOR. DUCT PRESSURE AT NORMAL PSI. MECHANICAL ANOMALY. ONE OF THESE INDICATORS WAS WRONG. FOLLOWED CHKLIST. FIRST ITEM: 'SELECT WARMER TEMP.' NO CTL OVER MIXING VALVE -- WON'T RESPOND IN 'MANUAL' OR 'AUTO.' SECOND ITEM: 'PUSH RESET.' DID THIS AND 'PACK TRIP' LIGHT EXTINGUISHED. NO CHANGE IN PSI -- STILL NORMAL!? BACK TO MIX VALVE PROB. CHANGED FROM PNF TO PF. HAD FO LOCATE 'MIX VALVE' BREAKER. HARD TO FIND. NOT DISCRETELY MARKED. CIRCUIT BREAKER 'IN.' WE PULLED AND RESET IT ANYWAY. MIX VALVE NOW FUNCTIONAL. BY THIS TIME WE WERE LEVEL AT FL260 AT .65 MACH. EXCHANGED CTLS BACK TO FO. CTR INQUIRED ABOUT AIRSPD. WE HAD NEGLECTED TO ADVISE CTLR OF MACH .65 CHANGE AS OPPOSED TO OUR 'CANNED' STANDARD FLT PLAN OF .72 MACH. CONTRIBUTING FACTORS: INCREASED WORKLOAD WITH 4 MEL'S; THEN ADDITIONAL DISTRS COMPOUNDING WORKLOAD. NEW CAPT -- APPROX 220 HRS PIC IN TYPE. RUSHED AND PRESSURED MAINT TO GET ACFT ON LINE WITHOUT ADEQUATE SYS CHKS FOR RELIABILITY. SOLUTION: FLY ACFT FIRST -- INCLUDING NOTIFYING CTR OF AIRSPD CHANGES. BE LATE. TAKE THE TIME TO THOROUGHLY GO OVER FLT PLAN AND 'FILE' AMENDED AIRSPD.