CAPT OF A B727 OVERSHOT ASSIGNED CLB ALT DUE TO THE DISTR OF A PA SYS MIKE PROB. THE MIKE SHORTED AND GAVE OFF A BLUE SPARK WHEN USED BY THE SO FOR AN ANNOUNCEMENT CAUSING ALL FLC MEMBERS TO DIVERT THEIR ATTN TO THAT INCIDENT.

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ON CLBOUT FROM BOSTON; WE WERE GIVEN AN ALT OF 16000 FT. DURING CLBOUT; WE DEVELOPED MULTIPLE PROBS. OUR ALT REMINDER WAS INTERMITTENT AND THE FO ALTIMETER WAS OUT OF TOLERANCE READING HIGH. APCHING 16000 FT; THE SO PICKED UP THE PA MIKE AND THERE WAS A BLUE SPARK FROM THE PA MIKE TO THE MIKE HOLDER AND THE PA 'ON' LIGHT ILLUMINATED WHEN IT SHOULDN'T HAVE. CONCERNED ABOUT AN ELECTRICAL SHORT; WE ALL LOOKED DOWN. I; THE CAPT; WAS FLYING AND I FLEW THROUGH 16000 FT. THE ALT REMINDER; WHICH HAD MANY PREVIOUS WRITE-UPS; HAD FAILED TO WORK; AND SERIOUSLY CONCERNED ABOUT THE ELECTRICAL SHORT; I DIVERTED MY ATTN AWAY JUST LONG ENOUGH TO MISS THE ALT. WHEN I CAUGHT IT; I WAS AT 16500 FT ON THE CAPT'S ALTIMETER AND THE FO ALTIMETER READ 16880 FT. WE IMMEDIATELY DSNDED TO 16000 FT ON THE CAPT'S ALTIMETER. THERE WERE NO OTHER ACFT IN THE AREA AND THERE WAS NO LOSS OF SEPARATION. DURING THESE MULTIPLE DISTRS I FAILED TO MAINTAIN THE MOST IMPORTANT PART OF COMMAND -- TO MAKE SURE SOMEONE IS PAYING FULL ATTN TO FLYING THE ACFT. HOWEVER; GIVEN THE SERIOUSNESS OF A VISIBLE ELECTRICAL SHORT AND THE SURPRISE OF IT; IT CAUSED ME TO BUST THE ALT. THE PA SYS WAS WORKED ON PRIOR TO DEP FOR 2 SEPARATE PROBS AND WAS DISPATCHED WITH THE CABIN PORTION INOP. THE FO ALTIMETER HAD BEEN WRITTEN UP MULTIPLE TIMES FOR OUT OF TOLERANCE PROBS. THE ALT REMINDER HAD MANY PREVIOUS WRITE-UPS. IN ADDITION; AFTER LNDG; THE CAPT'S ALTIMETER READ 100 FT LOW. THE AIRPLANE WE WERE FLYING WAS 28 YRS OLD AND HAS HAD MULTIPLE PROBS THAT APPEAR TO BE THE RESULT OF AGING ACFT AND WIRING. THE REPEATER ITEM PROGRAMS IN PLACE OBVIOUSLY DO NOT WORK. IT IS DIFFICULT ENOUGH TO FLY IN THE NE CORRIDOR WITHOUT HAVING TO ALSO DEAL WITH ACFT THAT SHOULD HAVE BEEN RETIRED LONG AGO. BUT WITH ALL THAT SAID AND ALL THE CONTRIBUTING FACTORS; I MADE A MISTAKE OF AN ALT EXCURSION AND ONLY I; THE CAPT; AM FULLY RESPONSIBLE. I HAVE LEARNED A VALUABLE LESSON ABOUT DISTRS AND LIVE SPARKS IN THE COCKPIT SERIOUSLY CONCERN ME.