‰  NOW 35 WPM  ‰    TEXT IS FROM JULY 2014 QST  PAGE 34‰

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 35 WPM TEXT  ‰  QST DE W1AW  ƒ