ZAU CTLR ATTEMPTS TO AMEND B777 CLB CLRNC AFTER RE-EVALUATING OPPOSITE DIRECTION DSCNT DC9 CLRNC AND IS UNABLE TO ENSURE VERT SEPARATION.

Narrative

A B777 DEPARTED ORD; CLBED TO FL230; CLRED DIRECT BAE. A DC9 DSNDING INTO MSN WAS ISSUED 11000 FT. THE SIT WAS BEING WATCHED. THE SECTOR COMPLEXITY WAS LOW. A COORD CALL FROM MKE APCH WAS ANSWERED AT A TIME IN THE SIT THAT IT SHOULD HAVE BEEN IGNORED SO CTL ACTIONS COULD BE TAKEN TO ENSURE SEPARATION. AFTER THE COORD WITH MKE APCH WAS COMPLETED; THE SIT WAS RE-EVALUATED AND IT WAS DETERMINED THAT INTERIM ALTS SHOULD BE USED. I STOPPED THE DC9 AT FL230; THEN STOPPED THE B777 AT FL220. I THOUGHT IN HIS READBACK HE INCLUDED THE COMMENT THAT HE WAS LEVEL AT FL220. THE MODE C CONTINUED TO INDICATE A CLB TO FL225. I CALLED THE TFC FOR BOTH ACFT AND THE B777 THEN STATED 'WE ARE STARTING DOWN TO FL220.' I ADVISED MY SUPVR BUT INDICATED I HAD THE PREVIOUS RPT OF 'LEVEL FL220.' SEPARATION WAS LOST. THE B777 THEN RPTED LEVEL AGAIN AT FL220. AFTER A REVIEW OF THE TAPES; THE INITIAL RPT OF 'LEVEL FL220' WAS NOT THERE. I HAD MADE THE MISTAKE OF HEARING WHAT I WANTED TO HEAR. OTHER FACTORS WERE THE UNTIMELY COORD WITH MKE APCH THAT DISTR ME FROM MY PRIMARY OBJECT OF SEPARATION. EVEN WHEN IT APPEARS YOU ARE NOT BUSY; IT ONLY TAKES A MOMENTARY DISTR TO CAUSE AN ERROR.