FLC OF A DEHAVILLAND DH8 EXPERIENCED PROB WITH THEIR XPONDER ENRTE AND WAS NOT UNTIL APCH CTLR ADVISED THAT HE HAD A PRIMARY TARGET ONLY THAT THEY WERE AWARE OF THE PROB. THEY CYCLED THROUGH THE MODES AND IT THEN STARTED WORKING OK.

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DEPARTED KENAI ON IFR CLRNC V456; 5000 FT TO ANC. CHKED IN WITH CTR; LEVELED OFF AT 5000 FT MSL. 15 NM S OF ANC; REALIZED WE SHOULD HAVE BEEN HANDED OFF; SO CALLED APCH ON COM #2 WHILE MONITORING CTR ON COM #1. (CTR WAS TALKING CONSTANTLY; COULDN'T GET A WORD IN EDGEWISE.) APCH CTLR SAID HE HAD A PRIMARY TARGET IN MY POS. CHKED XPONDER CTL HEAD; NOTED IT FLASHING 'STANDBY' INDICATING THAT THE XPONDER HAD LOST PWR AT SOME POINT (FLASHING) AND WAS NOT XMITTING (STANDBY). TURNED CTL KNOB SLOWLY THROUGH 5 CLICKS AND CHANGED XPONDERS AS A PRECAUTION. APCH THEN VECTORED AND SEQUENCED US FOR ILS TO RWY 6R AT ANC WITHOUT FURTHER PROBS. I SUSPECT ONE OF 2 THINGS HAPPENED -- EITHER THE FO MOVED THROUGH THE 5 CLICKS TOO FAST FROM 'STANDBY' TO 'TA/RA' (IN WHICH CASE THE XPONDER INITIALLY INDICATES TA/RA; BUT AFTER A TIME DELAY; GOES BACK TO ITS PREVIOUS MODE) ON THE BEFORE TKOF CHKLIST AT KENAI; OR THE XPONDER LOST PWR ENRTE AND WENT TO STANDBY; WHICH WOULD EXPLAIN THE FLASHING ANNUNCIATION. I DON'T REMEMBER WHETHER CTR EVER POSITIVELY HAD US IDENTED; BUT WE DIDN'T HEAR FROM THEM AS FAR AS WE KNOW -- THE LEG IS VERY SHORT (43 NM) -- ABOUT NOT GETTING OUR XPONDER. THIS PARTICULAR XPONDER HEAD IS PECULIAR ABOUT HOW FAST YOU MOVE THE FUNCTION SWITCH -- OTHER CREWS HAVE HAD SIMILAR PROBS.