ALT BUST IN GLASS COCKPIT ACFT IN NIGHT OP.

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LAST LEG OF 4 LEGS. FO'S FIRST TRIP ON LINE (3 1/2 YRS IN THE FE SEAT). VERY WEAK FO. CLRED TO 12000 FT ON DSCNT. I CALLED '1000 FT TO GO' AT 13000 FT. AUTOPLT ON AND I OBSERVED 'ALT CAPTURE' ON MY DISPLAY. I LOOKED DOWN ON MY L SIDE TO REACH APCH PLATE. ATC CALLED 'TURN L TO 350 DEGS.' I RESPONDED WATCHING FO TURN HDG SELECTOR. AT THAT TIME; THE ALT ALERTER VOCAL AND LIGHT CAME ON (250 FT LOW). I LOOKED AT THE ALT AND THEN REALIZING WE WERE LOW; DISCONNECTED AUTOPLT AND STOPPED THE DSCNT 350 FT LOW (11650 FT) AND BEGAN CLB BACK TO 12000 FT. NO ACFT OBSERVED ON TCASII. 30 SECONDS AFTER LEVEL AT 12000 CTLR GAVE FREQ CHANGE TO APCH CTL. I MISSED THE ACTUAL LEVEL OFF AT 12000 FT. FACTORS CONTRIBUTING TO THIS ARE 1) FATIGUE (LONG DAY); 2) ADDITIONAL STRESS OF FLYING WITH WEAK FO ALL DAY LONG IS VERY FATIGUING. 3) FO WAS TRAINED ON MLG FO AND RETURNED TO SO WDB FOR 2 MONTHS AND THEN RETURNED TO MLG. I WILL BE EXTREMELY VIGILANT AND AM FLYING WITH THIS FO FOR SEVERAL TRIPS. IF HE HAS NOT IMPROVED TO A SAFE LEVEL I WILL TAKE THIS TO SUPVR TO PREVENT FURTHER POSSIBLE PROBS.