AN MD-80 CAPTAIN AND MECHANIC DISCOVERED THAT A VOLTAGE GAUGE WAS INADVERTENTLY REPLACED THREE DAYS EARLIER WITH A LOAD METER GAUGE. THIS CAUSED THE SELECTOR SWITCH TO BURN OUT; TRIPPING TWO CIRCUIT BREAKERS.

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DURING PUSHBACK; WE STARTED THE L ENG. I NOTED THE SOUND OF THE L GENERATOR COMING ON LINE; YET THE VOLT AND FREQ METERS WERE FULL SCALE L (NO INDICATION). SINCE I HEARD THE GENERATOR COME ON LINE; I MOVED THE SELECTOR SWITCH TO APU TO SEE IF THE FREQ AND VOLTAGE GAUGES INDICATED FOR THE APU. THEY DID NOT. WE CALLED MAINT AND THEY WANTED ME TO CHK THE MEL FOR GENERATOR INDICATION. I TOLD THEM THE GAUGES DIDN'T INDICATE FOR THE ENG GENERATOR OR THE APU GENERATOR AND ALSO THAT SOMETHING DIDN'T LOOK RIGHT WITH THE GAUGES (MORE ON THIS LATER). DURING THIS DISCUSSION WE WERE ASKED TO TAXI FORWARD FOR TFC IN THE ALLEY. EVENTUALLY; I CONVINCED MAINT WE SHOULD RETURN TO THE GATE. WE TAXIED ACROSS THE ALLEY. THERE WAS NO EXTERNAL PWR AND NO CART AVAILABLE. AFTER ABOUT 1 HR OF TROUBLESHOOTING; THE FLT WAS CANCELED. OUR NEXT LEG WAS LATER THAT DAY. WE WERE ASSIGNED THE SAME ACFT FROM ABOVE. AFTER TALKING TO ONE OF THE MECHS THAT HAD WORKED THE PROB; AND AFTER READING THE LOGBOOK; WE DISCOVERED THAT THE VOLTAGE GAUGE HAD BEEN PREVIOUSLY REPLACED WITH A LOAD METER GAUGE BY MISTAKE. THIS IS WHY SOMETHING DIDN'T LOOK RIGHT WITH THE GAUGES. ON ENG START; THE WRONG GAUGE CAUSED THE SELECTOR SWITCH TO BURN OUT WHICH CAUSED CIRCUIT BREAKER L-22 (L VOLTMETER) AND CIRCUIT BREAKER L-24 (APU VOLTMETER) TO TRIP. THIS GAUGE HAD BEEN PREVIOUSLY REPLACED 3 DAYS PRIOR TO THIS EVENT DUE TO AN INTEGRAL LIGHTING WRITE-UP. THE NEXT LEG IN THIS ACFT WAS COMPLETED WITH NO ABNORMALITIES.