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Yaesu FT301 mods
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FT301
FILE OF INFORMATION ON FT301 MODIFICATIONS
DISPLAY LED'S:
THESE RUN HOT AND OFTEN BURN OUT SEGMENTS. CURE IS TO REDUCE OPERATING
VOLTAGE WHICH TYPICALLY RUNS >5V. FROM VOLTAGE REGULATOR Q2 ON THE MAIN
CHASSIS. A 1N400 DIODE IN SERIES WITH THE OUTPUT OF Q2 DROPS NEARLY A
VOLT FROM THE SUPPLY VOLTAGE. IF THE DISPLAYS STILL WORK OK, THEY WILL
RUN COOL. (PREVIOUSLY, OTHERS HAVE REMOVED RED PLASTIC LENS FOR BETTER
VENTILLATION, OR HAVE EVEN DRILLED HOLES THRU TOP OF RIG.)
SPEECH PROC:
OFTEN FT301'S HEARD ON THE AIR HAVE HAD DISTORTED AUDIO. A SINGLE I.C.
CONTAINS ALL THE MIC AMPLIFICATION AND ALSO DRIVES THE RING MODULATOR
IN THE SSB TRANSMIT PATH. THIS AMP IS DRIVEN TO HORRIBLE DISTORTION IF
THE STOCK (HANDHELD) MIC IS USED AT FULL CLOCKWISE MIC GAIN SETTING.
THIS IS THE WRONG PLACE TO HAVE CLIPPING OCCUR. ALSO RESPONSE OF AMP
IS FAR TO "GENEROUS." SUGGEST LIMITING LOW END RESPONSE WITH SERIES 2.
UF CAP (MUST BE MYLAR OR NON-POLAR) AT MICROPHONE CONNECTOR. HAVE
IGNORED THE FACT THAT THE HI END RESPONSE OF THAT AMP GOES WELL BEYOND
50 KHZ. R.F. CLIPPING IN MY FT301 WAS ACTUALLY ONLY ABOUT 6Db. DUE TO THE LACK
OF RF GAIN IN THE CLIPPER STAGES. THIS CURE FOR LOW GAIN WAS MOST
EFFECTIVE: AN EXTRA STAGE OF GAIN IS AVAILABLE FOR THE TAKING ON THE
"NB UNIT" CARD. (Q201 IS NORMALLY USED ONLY WITH THE PROCESSER OFF AS
AN ALTERNATE RF PATH) RIG Q201 IN SERIES Q202, AND KEEP BOTH STAGES
POWERED UP REGARDLESS OF POSITION OF "RF PROC" SWITCH. WITH THE EXTRA
STAGE, IT IS EASILY POSSIBLE TO KEEP THE RING MODULATOR IN THE LINEAR
REGION AND STILL DEVELOP 15 TO 20 Db. OF RF CLIPPING.
TO IMPLEMENT THIS CHANGE: JUMPER PINS 10 & 11 ON THE N.B. CARD
SUPPLIES VOLTAGE). OPEN 1 K R205 AT WIPER TO VR201, LET IT HANG.
OPEN 0.01 C204 AT TRANSFORMER, AND JUMPER TO WIPER OF R205 WITH SHORT
LEAD (UNSHIELDED SEEMS OK). RUN VR201 AT NO MORE THAN ABOUT 3/4 GAIN.
WITH HIGH LEVELS OF RF CLIPPING, IT IS IMPORTANT THAT THE AUDIO BE
RATHER LACKING IN BASS TO AVOID A MUDDY SOUND. PREEMPHASIS ACCROSS THE
FULL AUDIO BANDPASS SOUNDS PRETTY BAD, SO I WOULD UP WITH A .1 CAP IN
SERIES WITH THE WIPER ON THE MIKE GAIN CONTROL AND FINALLY SHUNTED THAT
CAP WITH ABOUT 1000 OHMS (ADJUST THAT VALUE OF RESISTANCE TO GIVE GOOD
ARTICULATION AND STILL BE SEMI-PLEASANT.
INFO VIA K2ZLU @ KD6TH-4
I haven't tried or verified this, proceed at your own risk! WA2ISE
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