‰  NOW 35 WPM  ‰  TEXT IS FROM JULY 2014 QST  PAGE 36‰

THE MEASUREMENT OF THE AGC VOLTAGE BY U1A, THE INPUT IMPEDANCE OF U1A AND
THE CLAMP CIRCUIT MUST BE VERY HIGH.  U1A HAS AN INPUT CURRENT OF 10 PA,
WHICH TRANSLATES TO AN INPUT IMPEDANCE IN THE NEIGHBORHOOD OF 1010.  THE
COLLECTOR BASE JUNCTION OF TRANSISTOR Q1 IS USED AS A VERY LOW CURRENT
LEAKAGE DIODE.  ALTHOUGH YOU CAN PURCHASE A SPECIAL LOW LEAKAGE DIODE, IT
IS QUITE SATISFACTORY TO USE THE COLLECTOR BASE JUNCTION OF AN INEXPENSIVE
TRANSISTOR FOR THIS FUNCTION.  THE RESULT IS THAT THE OUTPUT VOLTAGE OF
AMPLIFIER U1A IS WITHIN A FEW MILLIVOLTS OF THE AGC INPUT AGC IN, EVEN WITH
R4 IN SERIES WITH THE INPUT.  INSTALLATION THE PRINTED CIRCUIT BOARD MOUNTS
DIRECTLY ONTO THE TERMINALS OF THE S METER USING THE METERS HARDWARE SEE
FIGURE 3R3 RATHER THAN CUTTING THE ORIGINAL METER LEADS, THEY ARE STORED
OUT OF THE WAY ON TWO BOLTS AT THE BOTTOM OF THE CIRCUIT BOARD, WHICH
ALLOWS THE RECEIVER TO BE RESTORED TO ITS UNMODIFIED CONDITION.  THERE ARE
ONLY THREE WIRE CONNECTIONS FOR INSTALLATION OF THE BOARD GND AND V AC,
WHICH CAN BE OBTAINED FROM THE SOLDER LUGS OF THE S METER LAMP SOCKET,
ALONG WITH AGC IN, WHICH IS AVAILABLE FROM THE WIPER OF THE RECEIVERS AGC
SWITCH S12 LOCATED ADJACENT TO THE S METER SEE FIGURES 1 AND 3.  IF YOU
HAVE A STEADY HAND, THE CIRCUIT BOARD CAN BE INSTALLED WITHOUT REMOVING THE
RECEIVER FROM ITS CABINET.  SETUP OPTIONS ONCE THE NEW CIRCUIT IS
‰  END OF 35 WPM TEXT  ‰  QST DE W1AW  ƒ