FLC OF B727 ON VECTOR TO ILS APCH TRYING TO KEEP AHEAD TUNES IN TWR FREQ BUT ACTUALLY HAS GND FREQ. REALIZING THE ERROR THEY SWITCH BACK TO CTR FREQ INSTEAD OF APCH. FINALLY TUNING THE RIGHT FREQ AS THEY FLY THROUGH THE LOC APCH. APCH CTLR WELCOMES THEM BACK AND VECTORS BACK ON COURSE.

Narrative

WE WERE BEING VECTORED BY CHICAGO APCH FOR AN ILS IN VMC FOR RWY 4 AT MDW. FO WAS FLYING. WE WERE AT 4000 FT ON 280 DEG HDG. ATTEMPTING TO STAY AHEAD I SET MDW TWR FREQ IN THE RADIO THAT HAD APCH FREQ SET ON IT. I HEARD A XMISSION FROM AN ACFT ON THE GND AT MDW AND REALIZED THAT WE WERE ON THE WRONG FREQ. THINKING THAT I HAD ACCIDENTALLY HIT THE SWITCH TO THE OTHER RADIO I SWITCHED IT BACK. THIS PUT US ON ZAU WHO WE HAD BEEN TALKING TO BEFORE APCH. WE REALIZED MY MISTAKE AS WE FLEW THROUGH THE RWY 4 LOC AND SAW OTHER TFC TURNING OUT OF OUR WAY. THE FO REMEMBERED THE APCH FREQ; WHICH I QUICKLY TUNED IN AND CHKED ON. THE CTLR WELCOMED US BACK AND TURNED US FOR THE APCH TO MDW. THE FO AND I HAD BOTH WOKE UP EARLY THAT MORNING. BY THIS TIME I WAS HAVING A GREAT DEAL OF TROUBLE STAYING AWAKE. IT WAS NOT A REALLY HARD DAY (MKE- RSW-MDW) BUT JUST ONE OF THOSE DAYS WHEN YOU'RE JUST NOT COMPLETELY UP TO SPD. IN SITS LIKE THIS IT WOULD SEEM AS IF THE SMALL MISTAKES ARE THE ONES THAT SLIP BY UNNOTICED.