IC402


IC-402: S-METER (AGC)
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A  MODIFICATION BY G8LRH IS SHOWN IN DUBUS VHF UHF SHF  TECHNIQUE 
HANDBOOK 1984. MOST OF THIS MODIFICATION IS IMPORTANT AND  SHOULD 
BE  DONE.  ONE  PROBLEM SEEMS TO BE LEFT,  S-METER  READING.  ONE 
CRUCIAL  IMPORTANT  POINT,  SHOWN  ON  G8LRH  CIRCUIT,  BUT   NOT 
DISCUSSED  IN  TEXT, IS TO CONNECT A 1K  (2K2)  RESISTOR  BETWEEN 
EMITTERS  OF  Q5 AND Q7. THE 1K RESISTOR TO GROUND  FROM  EMITTER 
(Q7)  I  DID  NOT FIND IMPORTANT, NOR DID I CONNECT  THE  47  OHM 
RESISTOR AT THE OUTPUT FROM IC3. C22 WAS NOT CHANGED SINCE  100PF 
IS  QUITE  LOW  IMPEDANCE  WHERE IT IS  USED.  C35  WAS  LEFT  AS 
ORIGINAL.
TO AVOID TOO HIGH IMPEDANCE IN S-METER CIRCUIT, TWO MODIFICATIONS 
ARE MADE:
1)   D111SS53  IS  LOSENED,  AND TWO SILICON  DIODES,  1SS53  OR 
     1N4148 IS INSERTED IN SERIES, DO NOT SOLDER WHEN DIODE IN ON 
     PCB.
2)   CUT  THE  TRACE  GOING TO R31. CONNECT  R31  TO  EMITTER/Q7. 
     INSTALL A 10K RESISTORFROM BASE/Q7 TO GROUND (OTHERWISE  AGC 
     WILL NOT FUNCTION PROPERLY)
NOW  THE S-METER WILL WORK ON FULL SCALE AND READING SEEMS TO  BE 
NORMAL  FOR A SIMPLE S-METER. ONLY SIMPLE TESTS WERE CARRIED  OUT 
USING  FT-902/MMT432/28  WITH VARIABLE DRIVE  (CARRIER  LEVEL/DE-
TUNING  OF PRE-SELECTOR). SOME FURTHER IMPROVEMENTS MAY  BE  MADE 
LATER WHEN THE RIG IS CONTROLLED.
73 DE JAN-MARTIN ... LA8AK@LA5IV