FA10 WITH XPONDER PROBS EXPERIENCES TCASII TA WHILE UNDER NON RADAR PROCS WITH ZAU AND ZOB.

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DEPARTED ORD ENRTE TO GKJ. EVERYTHING NORMAL WITH DEP CTL. HANDED OFF TO ZAU; AGAIN NO PROBS. PASSED TO NEXT CTLR WITH CHICAGO. WE WERE CLRED TO CLB TO AND MAINTAIN FL240 AND CLRED DIRECT TO OUR DEST. RESPONDED AND COMPLIED. AT THIS POINT WE BEGAN TO EXPERIENCE XPONDER DIFFICULTIES. WE WERE UNAWARE OF THIS UNTIL THE CTLR ASKED US WHAT OUR XPONDER CODE WAS. REPLIED 'XYZW.' ADVISED THAT WE WERE XMITTING 'VYZW.' RECYCLED TO ANOTHER UNIT. ADVISED THAT NOW WE WERE NOT XMITTING AT ALL. RESELECTED THE FIRST XPONDER. NOTHING WAS SAID UNTIL THE CTLR CALLED WITH 'SAY POS.' REPLIED WITH RADIAL/DME USING THE WATERVILLE VOR (VWV). AT THIS POINT I QUESTIONED THE CTLR AS TO THE PROB SINCE HE HAD BEEN WORKING US FOR QUITE A WHILE. HIS RESPONSE WAS 'RADAR CONTACT LOST.' AT THIS POINT I CONTINUED TO RELAY RADIAL/DME FIXES IN AN EFFORT TO AID THIS CTLR. DURING THIS PERIOD OF LOSS OF RADAR CONTACT; BOTH THE CAPT AND I SAW A TARGET ON TCASII APCHING AT THE SAME ALT. PERFORMED DSCNT MANEUVER (CAPT PF) AS PER TCASII AND ADVISED ATC. CTLR REPLIED 'ROGER.' ADVISED ATC THAT WE WOULD REMAIN AT FL230 UNLESS ADVISED OTHERWISE. AGAIN REPLY OF 'ROGER.' EVENTUALLY WE WERE HANDED OVER TO ZOB AND AGAIN RELAYED RADIAL/DME POS USING THE CARLTON VOR. THIS FREQ. XPONDER REACQUISITION WAS ESTABLISHED AND REMAINDER OF FLT OPERATED NORMALLY. WE STILL DO NOT KNOW WHAT CAUSED THE PROB WITH OUR XPONDER. BOTH UNITS HAVE BEEN REMOVED AND ARE BEING INSPECTED. AS TO THE LOSS OF RADAR CONTACT. I BELIEVE THAT DUE TO THE WORKLOAD OF THE CTLR AND OUR LOSS OF XPONDER SIGNAL CAUSED THE CLOSE CALL WITH ANOTHER ACFT. HAD IT NOT BEEN FOR TCASII; THE OUTCOME OF THIS EVENT WOULD HAVE BEEN TRAGICALLY DIFFERENT. AUTOMATION IS A WONDERFUL TOOL BUT ONE CAN BECOME TOO ATTACHED TO THE HIGHEST LEVEL. SOMETIMES USING THE LOWEST LEVEL HELPS TO MAINTAIN A BETTER SITUATIONAL AWARENESS OF EVENTS SURROUNDING THEM -- PLTS AS WELL AS CTLRS.