‰  NOW 40 WPM  ‰  TEXT IS FROM JULY 2014 QST  PAGE 36‰
THE SENS POT FOR A METER READING OF S 9.  6.  REPEAT STEPS 4 AND 5 UNTIL
BOTH READINGS CAN BE ACHIEVED WITHOUT FURTHER ADJUSTMENT.  7.  SET THE
RECEIVER RF GAIN FULLY CCW AND ADJUST THE CLAMP POT FOR A FULL SCALE
READING ON THE S METER.  TEST RESULTS FIGURE 4 IS A GRAPH SHOWING THE VALUE
OF AN S UNIT OVER THE ENTIRE S METER RANGE.  THE PLOTS COMPARE THE
LINEARITY OF THE S METER OF MY 75S 3B, WITH ITS ORIGINAL METER DRIVER
CIRCUIT, TO THE RECEIVER MODIFIED WITH T

INSTALLED, THE RECEIVER CAN BE SET UP IN SEVERAL DIFFERENT WAYS.  THERE ARE
THREE PARAMETERS WE ARE CONCERNED WITH PT THE AGC THRESHOLD, WHICH IS THE
RF LEVEL IN DBM AT THE RECEIVERS 50 ANTENNA INPUT WHERE THE S METER BEGINS
TO MOVE EFFECTIVELY THE DB PER S UNIT INDICATED BY THE S METER AND P9 THE
RF LEVEL IN DBM AT THE RECEIVERS 50 FOR A METER READING OF S 9.  ASSUMING
THE METER HAS A CONSTANT SENSITIVITY S THE THREE PARAMETERS ARE RELATED BY
THE EQUATION NOTE THAT YOU MAY ONLY CHOOSE TWO OF THE THREE PARAMETERS AS
THE THIRD WILL BE DETERMINED BY THE EQUATION.  ALTHOUGH THERE ARE AN
INFINITE NUMBER OF PARAMETER CHOICES, THEY CAN BE NARROWED DOWN
CONSIDERABLY WITH A FEW COMMON SENSE ASSUMPTIONS.  FOR ON AIR COMPARATIVE
MEASUREMENTS, WE WANT S TO BE A WHOLE NUMBER, SUCH AS 3, 4, 5, OR 6 DB/S
UNIT.  TYPICALLY YOU WOULD CHOOSE A DESIRED S AND P9 AND THEN CALCULATE PT
FROM THE ABOVE EQUATION.  FOLLOWING THE ALIGNMENT PROCEDURE, YOU WILL THEN
DETERMINE WHETHER THE RECEIVER HAS ENOUGH GAIN TO ACHIEVE THE PT YOU HAVE
CALCULATED.  SETTING S TO 3 DB/S UNIT RESULTS IN A METER RANGE FROM S 0 TO
S 9 OF ONLY 27 DB, WHICH IS A BIT LIMITED.  CHOOSING 6 DB/S UNIT WOULD BE

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 40 WPM TEXT  ‰  QST DE W1AW  ƒ