‰  NOW 35 WPM  ‰  TEXT IS FROM NOVEMBER 2012 QST  PAGE 55‰

KEYBOARD OR COMPUTER INTERFACE NO BLOWN SWITCHING TRANSISTORS.  THE
POLARITY OF THE VOLTAGE BEING SWITCHED DOES NOT MATTER, SO MODERN GEAR 
POSITIVE LOW VOLTAGE KEYING CIRCUITS, VINTAGE CATHODE KEYED POSITIVE HIGH
VOLTAGE, OR VINTAGE GRID BLOCK KEYING NEGATIVE HIGH VOLTAGE RIGS CAN ALL BE
KEYED.  K3B DOES HAVE MAXIMUM VOLTAGE 350 V AND CURRENT 120 MA LIMITS.  TO
ALLOW FOR A REASONABLE SAFETY MARGIN, IF YOU PLAN TO KEY A CIRCUIT OF MORE
THAN 300 V AND/OR 100 MA, YOU SHOULD USE THE KEYED XMIT LINE TO CONTROL AN
EXTERNAL RELAY THAT CAN HANDLE THE VOLTAGE AND CURRENT DEMANDS OF YOUR RIGS
KEYING CIRCUIT.  THE REMAINING CIRCUITRY IS ASSOCIATED WITH THE SIDETONE
FUNCTION.  Q6 AND Q7 FORM AN AUDIO OSCILLATOR WITH A FREQUENCY OF
APPROXIMATELY 650 HZ.  THE OSCILLATOR RUNS CONSTANTLY AND, WHEN THE UNIT IS
KEYED, THE SOLID STATE SWITCH OF K3A ROUTES THE SIDETONE TO THE CONTACTS OF
K2 AND FROM THERE TO THE HEADPHONES.  ESSENTIALLY THE UNIT AUTOMATICALLY
MONITORS YOUR KEYING WITH THE SIDE TONE, BUT WHEN THE SYSTEM SWITCHES BACK
TO THE RECEIVE MODE, YOU WILL HEAR THE OUTPUT OF THE RECEIVER.  VINTAGE
VACUUM TUBE TRANSMITTERS NEED TO BE TUNED A SIMPLE MATTER WITH A STRAIGHT
KEY BUT MORE OF AN ISSUE WITH A KEYER.  THE TUNE S2 SWITCH IS A NORMALLY
OPEN, MOMENTARY CONTACT PUSHBUTTON SWITCH.  PRESSED AND HELD, THE SWITCH
FUNCTIONS JUST LIKE THE KEY, TRIGGERING THE SYSTEM INTO TRANSMIT, KEYING
‰  END OF 35 WPM TEXT  ‰  QST DE W1AW  ƒ