AFTER THE PRIMARY RADIO TUNING UNIT FAILED ON TKOF; A FATIGUED CL65 CAPT EXPERIENCED A SITUATIONAL AWARENESS LOSS; DECLARED AN EMER; AND RETURNED FOR AN OVERWT LNDG.

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THE FO AND I WERE OPERATING AN ACFT WITH MULTIPLE MAINT DEFERRALS; SEVERAL OF WHICH INCREASED OUR NORMAL WORKLOAD. WE WERE OPERATING 1 1/2 HRS LATE DUE TO ADDITIONAL MAINT. THEY HAD BEEN CALLED OUT TO FIX A PROB WITH THE #1 REMOTE TUNING UNIT (THE PRIMARY INTERFACE WITH COM #1; NAV #1; XPONDER #1; TCASII). THE FIX WAS TO REPLACE THE RTU WHICH FAILED AS WE WERE PREPARING TO PUSH BACK. MAINT RESET THE UNIT AND CLRED US TO PUSH AGAIN. AFTER PUSHBACK; ATC TOLD US TO EXPECT RWY 9L; INSTEAD OF RWY 8R (THIS WAS DUE TO NUMEROUS TSTMS AT THE N BOUNDARY OF THE ARPT). I ASKED THE FO TO REPROGRAM THE FMS FOR THE NEW RWY AND ASSOCIATED DEP FIXES. WHEN TWR CLRED US FOR TKOF; WE WERE TOLD TO FLY HDG 065 DEGS (NORMAL HDG IS 095 DEGS TO 110 DEGS). WHEN CLRED TO CONTACT DEP; I REALIZED THAT THE #1 RTU HAD AGAIN FAILED. I SELECTED DEP FREQ ON RTU #2 (NORMALLY USED FOR COMPANY COM) AND CHKED IN. BEFORE I COULD PULL OUT THE CHKLIST FOR THE RTU FAILURE; ATC ASSIGNED US PRESENT HDG DIRECT TO THE FIRST FIX FOR A RWY 8R DEP. I ADVISED THAT WE HAD THE APPROPRIATE FIXES SELECTED FOR RWY 9L AND WE WERE TOLD TO RELOAD THE NORTHERN FIXES AND TO EXPECT VECTORS AROUND WX UNTIL WE WOULD BE ABLE TO GO DIRECT. AT THIS POINT; I WAS OBSERVING A LOT OF LIGHTNING TO OUR L AND BTWN DEALING WITH THE RADAR; RTU FAILURE AND TRYING TO REPROGRAM THE FMS; I LOST THE BIG PICTURE AND ELECTED TO RETURN TO ZZZ. I CONTACTED COMPANY DISPATCH AND ADVISED THEM OF THE DECISION; THEN TOLD ATC THE SAME. ATC PROVIDED VECTORS AROUND WX WHILE I SET THE APCH UP AND PREPARED THE ACFT FOR THE RETURN. I REALIZED THAT WE WOULD BE OVERWT; SO I DECLARED AN EMER TO ALLOW FOR THE OVERWT LNDG. THE FO FLEW THE ILS TO A VISUAL AND AFTER WE RETURNED TO THE GATE; WE TURNED THE ACFT BACK OVER TO MAINT. I BELIEVE THAT THIS RELATIVELY BENIGN FAILURE TRIGGERING ME INTO SUCH A DRASTIC SOLUTION WAS CAUSED BY SEVERAL FACTORS. FIRST; I FEEL THAT THE DEFERRALS PLUS THE PREVIOUS PROBS WITH THE RTU PUT ME INTO A MENTAL STATE OF NOT BELIEVING THE ACFT WAS SAFE TO FLY NEAR WX. SECOND; I THINK THAT THE ABNORMAL DEP PROCS NECESSITATING AN FMS REPROGRAM AFTER DEP PUT ME INTO TUNNEL VISION AND I LOST THE SITUATIONAL AWARENESS NEEDED TO PULL OUT THE APPLICABLE CHKLIST AND BYPASS THE AFFECTED SYS. THIRD; I BELIEVE THAT FATIGUE PLAYED A FACTOR IN THE LOSS OF SITUATIONAL AWARENESS. WE DID NOT HAVE THAT BUSY OF A DUTY DAY; BUT THE FACT THAT THIS HAPPENED LATE IN THE EVENING DID PLAY A PART. IN THE FUTURE; I NEED TO REMEMBER THAT FIRST WE FLY THE AIRPLANE (INCLUDING RUNNING NECESSARY CHKLISTS). WHEN ATC ASKS FOR SOMETHING I CAN'T GIVE; I NEED TO ADVISE THEM AND DO WHAT IS NEEDED. ALSO; I NEED TO BE CAUTIOUS WHEN ABNORMAL SITS (DIFFERENT RWY; DIFFERENT DEP HDG ARE ASSIGNED) CROP UP AND TO CLARIFY ATC INTENTIONS. I FEEL THAT HAD ATC LEFT US ON OUR ORIGINAL RWY WITH ITS RESPECTIVE FIXES OR LET US FLY THE FIXES THAT WERE CORRECT FOR THE DEPARTING RWY THAT I WOULD NOT HAVE GOTTEN OVERLOADED AND AS A RESULT; WE WOULD HAVE GOTTEN THROUGH THE CHKLIST TO RECTIFY THE PROB.