INSTRUCTOR WITH SPI HAS RADIO PROBLEM; ENTERS ATA PRIOR TO CONTACT. CLOSE ENCOUNTER WITH FGT'S.

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THE PROBLEM AROSE AFTER I RPTED TO DAYTON APCH CTL THAT I HAD MY DEST (SGH) IN SIGHT. I WAS AT THIS TIME APPROX 8 NM SW OF SGH AND WISHED TO CHANGE TO THE TWR FREQ AND BEGIN MY DSCNT. IT WAS A BUSY DAY AND THE APCH CTLR DIDN'T ANSWER ME RIGHT AWAY. I PAUSED ABOUT 10 SECONDS AFTER THE CTLR'S LAST XMISSION (TO OTHER ACFT) AND CALLED AGAIN. THIS TIME I WAS INFORMED (PARAPHRASE): 'IDENT; TFC IS A FLT OF FGT'S DEPARTING RWY 24 AT SPRINGFIELD; THEY WILL BE TURNING SWBOUND; CLBING TO 8000 FT. CONTACT SPRINGFIELD TWR NOW 120.7.' I ACKNOWLEDGED BY SAYING THAT I WAS LOOKING FOR THE TFC AND WAS GOING TO TWR. BY THIS TIME THE LORAN READ 7 MI OUT. SOME TIME EARLIER; I HAD SET 120.7 IN THE #1 COM RADIO. IMMEDIATELY AFTER BEING TOLD TO CONTACT TWR; I SWITCHED TO THE #1 COM; PAUSED ABOUT 5 SECONDS; AND CALLED SGH TWR (PARAPHRASE): 'SPRINGFIELD TWR; XX IDENT; INBOUND FROM THE SW LOOKING FOR TFC.' I RECEIVED NO RESPONSE. AFTER A BRIEF PAUSE; I TRIED AGAIN. BY THIS TIME I HAD THE FGT'S IN SIGHT AND WAS CONSIDERING A L TURN TO AVOID THEIR SW DEP. I WAS ALSO CONCERNED THAT I WAS GETTING CLOSE TO THE ATA. AFTER NO RESPONSE TO THE SECOND CALL; I CHKED THE TWR FREQ ON MY CHART AND DIALED IT IN ON COM 2. I CALLED AGAIN; THIS TIME RPTING THE FGT'S IN SIGHT. TWR ANSWERED WITH A QUERY: 'IS THAT XY IDENT?' I REPLIED; 'NEGATIVE XX IDENT.' THEY REPLIED; 'ENTER R DOWNWIND RWY 15; TFC IS A FLT OF 4 FGT'S DEPARTING RWY 24; SWBOUND CLBING TO 8000 FT.' I STARTED A L TURNING DSCNT TO BOTH AVOID WHAT I THOUGHT WOULD BE THE PATH OF THE FGT'S AND TO ENTER R DOWNWIND FOR RWY 15. I LOWERED THE GEAR AT THIS POINT; ALL THE WHILE KEEPING THE FGT'S IN SIGHT. I HAD ASSUMED CTL OF THE AIRPLANE FROM MY STUDENT BECAUSE OF THE PERCEIVED THREAT AND DELAYED COMS. THE FGT'S CONTINUED ON RWY HDG. THIS WORRIED ME BUT I KNEW THAT ON MY PRESENT COURSE; UNLESS THEY TURNED R WE WOULD MISS. THE FGT'S PASSED BEHIND ME AND SLIGHTLY BELOW. THEY APPEARED TO BE ABOUT A HALF MI AWAY AND ABOUT 200 FT BELOW. THE TWR SAID SOMETHING ABOUT THE FGT'S HAVING TO TURN TO MISS ME AND THAT I NEED TO CONTACT THE TWR 'EARLY ENOUGH TO RECEIVE LNDG INSTRUCTIONS BEFORE ENTERING THE CTL ZONE.' I REPLIED THAT I HAD TROUBLE WITH MY 'TOP RADIO' AND HAD TO CALL A FEW TIMES BEFORE CONTACTING THEM. I THEN LANDED WITHOUT FURTHER INCIDENT. THE RADIO PROBLEM WAS DISCOVERED ON MY RETURN TRIP. THE LIGHT DISPLAY OF FREQ NUMERALS ON THE NAVCOM IS MADE UP OF 7 LIGHT BARS WHICH DISPLAY THE NUMERALS SELECTED. OCCASIONALLY SOME OF THOSE LIGHT BARS EITHER SHORT OUT OR BURN OUT. I HAD APPARENTLY SELECTED 120.9 INADVERTENTLY INSTEAD OF 120.7. ALTHOUGH THE DISPLAY APPEARED AS 120.7. RECYCLING THE FREQ KNOB MADE THE LIGHT BARS IN THE DISPLAY FUNCTION PROPERLY; BUT THE OP OF SOME OF THE LIGHT BARS WAS INTERMITTENT. THIS CAUSED ME TO SELECT THE WRONG FREQ WHEN I BELIEVED IT WAS THE CORRECT ONE. I MIGHT HAVE PREVENTED THIS BY BEING AWARE OF THE PROBLEM AND SELECT FREQS BY 'COUNTING THE CLICKS' WHILE TURNING THE KNOB FROM A KNOWN FREQ. ANOTHER CONTRIBUTING FACTOR MIGHT HAVE BEEN THE WAY THE FGT'S WERE CALLED TO ME AS TFC. 'TURNING SWBOUND' (215 DEGS) GAVE ME THE IMPRESSION THAT THEY WOULD BE TURNING L SHORTLY AFTER TKOF. THIS WAS RIGHT TOWARD MY POS. REALIZING THEIR GREATER SPD I DECIDED THE BEST COURSE OF EVASIVE ACTION WOULD BE A L TURN. BUT THE FGT'S WERE APPARENTLY FLYING RWY HDG (240 DEGS) AND MY L TURN TO AVOID THEM PUT ME IN THEIR PATH MOMENTARILY. HAD I BEEN TOLD THEY WERE RWY HDG; I'D HAVE STAYED E OF RWY 24. FURTHER; MY INITIAL TWR INSTRUCTION TO ENTER R DOWNWIND TO RWY 15 HAD ME TURNING L ACROSS THE EXTENDED CTRLINE OF RWY 24. THIS SIMPLY ADDED VALIDITY TO MY DECISION TO TURN L TO AVOID THE FGT'S. OF COURSE I'M SURE IF THE TWR CTLR REALIZED HOW CLOSE TO THE ATA I WAS; MY INSTRUCTIONS MIGHT HAVE BEEN DIFFERENT.