A DH8 IN ZBW AIRSPACE AT 14000 FT DSNDS IMMEDIATELY ON RECEIVING A TCASII RA ON TFC; AN ATR43; RPTED AT 13000 FT BUT SHOWING 13900 FT.

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ACR Y; AT 13000 FT; WAS IN THE VICINITY OF THE HNK VOR. ACR X; AT 14000 FT; WAS APPROX DIRECTLY OVER ACR Y. SOMEBODY ASKED 'IS THAT TFC DSNDING?' I ADVISED HIM 'ACR Y IS LEVEL AT 13000 FT.' ACR X ADVISED 'WE GOT A TCASII RESOLUTION...' AND DSNDED DIRECTLY THROUGH ACR Y. ACR X ADVISED 'WE HAVE HIM IN SIGHT.' ACR X WENT FROM 14000 FT TO 12000 FT IN APPROX 12 SECONDS. UPON FURTHER CONVERSATION WITH THE PLT OF ACR X HE ADVISED THE OTHER ACFT WAS 'REAL CLOSE.' MY ESTIMATION OF SEPARATION WAS 0 FT VERT AND LESS THAN 1/4 MI AT THE TIME OF THE INCIDENT. AS A SIDE NOTE; WE HAD PICKED UP A BAD MODE C ALT OF 13900 FT ON ACR Y APPROX 3-4 MINS BEFORE THE INCIDENT. THE PLT OF ACR X ADVISED ME HE BELIEVED THE BAD MODE C FROM ACR Y WAS PROBABLY THE CAUSE OF THE RA. SUPPLEMENTAL INFO FROM ACN 474495: ACR X; LEVEL AT 14000 FT; QUESTIONED IF ACR Y WAS DSNDING. WHEN ADVISED THAT ACR Y WAS LEVEL AT 13000 FT; ACR X RESPONDED THAT HE HAD AN RA. ACR X THEN BEGAN A RAPID DSCNT TO 12000 FT. ACR Y ALSO SAID HE HAD AN RA TO CLB; BUT THIS CLB WAS NOT OBSERVED ON THE RADAR. ACR Y ALSO STATED THAT HE HAD THE TFC IN SIGHT. ACR X DID NOT HAVE THE ACR Y IN SIGHT UNTIL THE LAST MIN. ACR X CLBED BACK TO 14000 FT WHEN ACR Y WAS SEVERAL MI BEHIND HIM. IT IS POSSIBLE THAT THE MODE C ON ACR Y WAS NOT WORKING PROPERLY. THE MODE C ON ACR Y SHOWED 13900 FT WHEN THE 2 AIRPLANES WERE 40 MI APART. ACR Y STATED HE WAS LEVEL AT 13000 FT. THE NEXT COUPLE OF UPDATES REFLECTED THAT ACR Y WAS LEVEL AT 13000 FT. WHEN ACR X ASKED IF ACR Y WAS DSNDING; ACR Y MODE C READ 130 N INSTEAD OF 130 C. CALLBACK CONVERSATION WITH RPTR ACN 474496 REVEALED THE FOLLOWING INFO: RPTR SAID HE HAD ISSUED TFC WHEN THE 2 ACFT WERE APPROX 5 MI APART. HE SAID BOTH ACFT SAW EACH OTHER DURING THE AVOIDANCE MANEUVER. AT THAT TIME THE RADAR CTLR OBSERVED 130 N (N = NOT RECEIVING DATA FROM ACFT) ON THE ACFT'S AUTOMATED DATA BLOCK. HE SAID THE ALT WAS READING CORRECTLY ABOUT 75% OF THE TIME AND HE CONTINUED TO USE THE INFO.