MECHANIC NOTICES A (DC) CONVERTER WAS MISSING FROM THE ACFT EMERGENCY POWER SUPPLY JETPACK UNIT ON A SAAB-340B ACFT.

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WHEN PERFORMING TASK ### FAILED TEST. WENT TO REMOVE AND REPLACE POWER SUPPLY AND NOTICED THAT THE CONVERTER WAS MISSING ON POWER SUPPLY. INSTALLED NEW POWER SUPPLY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE SAAB-340B ACFT HAVE A PRE-MOD; OR A POST-MOD EMERGENCY POWER SUPPLY JETPACK; INSTALLED ON THE AFT SIDE OF THE COCKPIT AFT BULKHEAD; ACROSS FROM THE GALLEY. BOTH UNITS HAVE DIFFERENT PART NUMBERS; BUT LOOK SIMILAR AND HAVE THE SAME MOUNT CONNECTOR IN THE ELECTRICAL RACK. IF THE PRE-MOD POWER SUPPLY UNIT REQUIRES REPLACEMENT USING ANOTHER PRE-MOD UNIT; THE (DC) POWER CONVERTER MUST BE TRANSFERRED FROM THE REMOVED UNIT TO THE NEW REPLACEMENT PRE-MOD UNIT. THE POST-MOD UNIT HAS THE (DC) CONVERTER BUILT INTO THE POWER SUPPLY BLACK BOX AND DOES NOT REQUIRE ANY TRANSFER OF THE CONVERTER. REPORTER STATED THE JETPACK PROVIDES EMERGENCY STANDY-BY POWER FOR THEIR COMPASS AND NAVIGATION SYSTEMS AND BACK LIGHTING. REPORTER STATED THE EMERGENCY JETPACK REQUIRES A MAINT TEST BE PERFORMED AFTER A CERTAIN NUMBER OF FLIGHT HOURS. THIS PARTICULAR TEST ALSO REQUIRES THE ACFT RIGHT BATTERY CIRCUIT BREAKER BE PULLED TO SIMULATE RIGHT BATTERY POWER LOSS. WHEN THE STAND-BY COMPASS; NAVIGATION SYSTEMS AND THE BACK LIGHTING DID NOT POWER-UP AND THE TEST INDICATED 'FAILED;' THEY DECIDED TO CHANGE THE POWER SUPPLY UNIT; ONLY TO DISCOVER THE CONVERTER ON THE PRE-MOD SUPPLY UNIT WAS NOT ATTACHED. REPORTER DOES NOT KNOW HOW LONG THE ACFT HAD BEEN FLYING WITHOUT ANY STAND-BY POWER MODE CAPABILITY. THERE DOES NOT APPEAR TO BE ANY PRE-FLIGHT CHECK THAT WOULD GIVE ANY INDICATION TO THE FLIGHT CREW THE STAND-BY SYSTEM POWER SUPPLY WAS NOT FUNCTIONAL; UNLESS THIS PARTICULAR MAINT TEST IS PERFORMED. THE LEFT BATTERY SYSTEM DOES NOT REQUIRE THIS TEST. REPORTER ALSO STATED THERE IS A SEPARATE EMERGENCY BATTERY PACK FOR THE CABIN EMERGENCY LIGHTING.