‰  NOW 40 WPM  ‰  TEXT IS FROM JULY 2014 QST  PAGE 34‰
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.  INSTALLATI

LINEAR IN TERMS OF DB PER VOLT.  BUFFERING THIS SIGNAL WITH A HIGH
IMPEDANCE OPERATIONAL AMPLIFIER GAVE ME A METER DRIVER THAT IS LINEAR AND
INEXPENSIVE TO CONSTRUCT.  WHILE I WAS AT IT, I TOOK THE OPPORTUNITY TO
IMPROVE ON SOME OTHER LESS DESIRABLE S METER CHARACTERISTICS SUCH AS ZERO
DRIFT, AND JAMMING AGAINST THE STOPS WHEN CHANGING MODES.  FIGURE 1 SHOWS
HOW THE NEW METER DRIVER CIRCUIT INTEGRATES INTO A 75S 3B RECEIVER.
COLLINS COLLECTORS WILL BE RELIEVED TO HEAR THAT INSTALLATION OF THE
CIRCUIT BOARD REQUIRES NO WIRE CUTTING THE BOARD MOUNTS DIRECTLY ON THE S
METERS TERMINALS AND THE ORIGINAL METER LEADS ARE PARKED OUT OF THE WAY ON
STORAGE BOLTS AT THE BOTTOM OF THE BOARD.  POWER FOR THE BOARD COMES FROM
THE S METERS PILOT LAMP AND THE AGC VOLTAGE IS AVAILABLE FROM THE WIPER ARM
OF AN ADJACENT ROTARY SWITCH.  CIRCUIT DESCRIPTION THE CIRCUIT DEVELOPED
FOR THE NEW METER DRIVER IS SHOWN IN FIGURE 2.  NO VOLTAGE GAIN IS
NECESSARY.  THE CIRCUIT IS BASICALLY A VERY HIGH INPUT IMPEDANCE VOLTAGE
FOLLOWER THAT DIRECTLY MEASURES THE AGC VOLTAGE AND USES IT TO DRIVE THE S
METER.  POWER IS SUPPLIED TO THE CIRCUIT THROUGH THE V AC AND GND INPUTS

IMPROVING S METER LINEARITY FOR COLLINS S LINE RECEIVERS OP AMPS BUFFER THE
AGC VOLTAGE TO DRIVE THE S METER AND PROVIDE LINEAR DB PER S UNIT READINGS.
DON JACKSON, W5QN  THE S METERS IN VINTAGE RECEIVERS ARE NOTORIOUSLY
INACCURATE AND INCONSISTENT.  MY VENERABLE COLLINS 75S 3B IS NO DIFFERENT
IN THAT AN S UNIT NEAR S 1 REPRESENTS A 3 DB SIGNAL CHANGE AND ONE NEAR S 9
IS CLOSER TO 6 DB.  LIKE MANY OTHER OPERATORS, I WOULD LIKE TO BE ABLE TO
GIVE ACCURATE SIGNAL REPORTS ON THE AIR WHEN LISTENING TO AN ANTENNA OR
AMPLIFIER COMPARISON.  THIS IS INCONVENIENT TO DO WITHOUT AN S METER THAT
HAS A LINEAR SCALE.  HERE, LINEAR IS TAKEN TO MEAN THAT EACH S UNIT
REPRESENTS THE SAME DB CHANGE IN SIGNAL.  AS LONG AS WE ARE REPORTING
COMPARISONS IN DB, CONSISTENCY ACROSS THE SCALE IS FAR MORE IMPORTANT THAN
THE ACTUAL VALUE OF AN S UNIT.  THIS SHOULD NOT BE TAKEN AS A DISPARAGEMENT
OF THE COLLINS S LINE.  ON THE CONTRARY, THE COLLINS DESIGNERS DESERVE
CREDIT FOR GETTING A LOT FROM VERY LITTLE WITH A CLEVER BRIDGE CIRCUIT THAT
USES VARIATIONS IN THE SCREEN CURRENT OF V6 AND THE CATHODE CURRENT OF V7
TO DRIVE THE METER.  ALTHOUGH THE ORIGINAL CIRCUIT IS FAIRLY SIMPLE AND
ELIMINATES THE NEED FOR A VACUUM TUBE DC AMPLIFIER, THE RESULT IS NOT VERY
LINEAR.  LOOKING INTO THE CIRCUIT OF MY 75S 3B AND TAKING SOME
MEASUREMENTS, I FOUND THAT THE AGC AUTOMATIC GAIN CONTROL VOLTAGE WAS QUITE
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ