‰  NOW 40 WPM  ‰  TEXT IS FROM JANUARY 2014 QST  PAGE 48‰
OPERATORS HEAR THAT I AM USING A VINTAGE TUBE TRANSMITTER THEY USUALLY
COMMENT POSITIVELY ON THE KEYING AND ON THE VFO STABILITY.  I FIND REAL
PLEASURE PUTTING MY CLASSIC GEAR ON THE AIR WITH A SIGNAL QUALITY RIVALING
THAT OF ANY MODERN TRANSCEIVER.  WHEN GREG LATTA, AA8V, ISNT POUNDING BRASS
ON THE HF AMATEUR BANDS, YOU MIGHT FIND HIM TEACHING PHYSICS AS A PHYSICS
PROFESSOR AT FROSTBURG STATE UNIVERSITY, OR WORKING AT HOME IN HIS
ELECTRONICS/MACHINE SHOP.  HE IS AN AWAR

THE VFO.  THE SWITCHES FOR RIT DISPLAY AND VFO A/B ARE OPTIONAL.  CLOSING
THE RIT DISPLAY SWITCH CAUSES THE SHIFTED KEY UP FREQUENCY TO ALWAYS BE
DISPLAYED.  VFO A/B SWITCH TOGGLES BETWEEN VFO A AND VFO B.  THE
SYNTHESIZER OUTPUT CAN BE SWITCHED BETWEEN A DIRECT UNBUFFERED OUTPUT OR TO
THE INPUT OF THE BUFFER AMPLIFIER.  I USE THE UNBUFFERED OUTPUT AS A SIGNAL
GENERATOR THAT OUTPUT DOES NOT ROLL OFF AT THE HIGHER FREQUENCIES.  WHEN
USED AS A FREQUENCY GENERATOR THE SPOT SWITCH SHOULD BE IN THE UP POSITION,
ENABLING THE SPOT FUNCTION.  THE COMPLETED VFO FIGURE 1 SHOWS THE
SYNTHESIZER ROTARY ENCODER AND UP/DOWN SWITCHES MOUNTED ON THE FRONT PANEL.
THIS ALLOWS THE USE OF LARGE PUSHBUTTON SWITCHES THAT ARE EASIER TO USE AND
RESULTS IN A MUCH CLEANER LAYOUT.  THE BUFFERED OUTPUT AND UNBUFFERED
OUTPUT, RIT KEYING JACKS AND THE OUTPUT SELECT SWITCH ARE MOUNTED ON THE
BACK OF THE VFO AS SHOWN IN FIGURE 8.  I USED RCA JACKS FOR THE INPUTS AND
OUTPUTS.  INITIAL TESTING CONNECT THE VFO TO THE TRANSMITTER AND RUN A
SHIELDED CABLE AN AUDIO CABLE IS FINE FROM THE RIT KEYING OUTPUT ON THE
KEYING CIRCUIT TO THE RIT KEYING INPUT OF THE VFO.  FOLLOWING THE

VFO RIT KEYING AND FINAL WIRING THE RITB INPUT ON THE SYNTHESIZER BOARD IS
PROGRAMMED SO THAT THE VFO IS ON FREQUENCY WHEN THE INPUT IS OPEN, AND OFF
FREQUENCY WHEN THE INPUT IS CONNECTED TO GROUND.  TRANSISTOR Q5 INVERTS THE
RIT KEYING SIGNAL FROM Q4 IN THE KEYING CIRCUIT, AND ALSO PROTECTS THE RITB
INPUT ON THE SYNTHESIZER BOARD FROM EXCESSIVE VOLTAGES THAT MIGHT BE
ACCIDENTALLY CONNECTED TO THE EXTERNAL RIT KEYING INPUT.  FIGURE 2 SHOWS
THE VFO BLOCK DIAGRAM.  TRANSISTOR Q5 IS TURNED ON BY THE BASE CURRENT
THROUGH THE 4R7 K RESISTOR WHEN THE KEY IS UP, SINCE Q4 IS TURNED OFF AND
THE EXTERNAL RIT KEYING INPUT IS EFFECTIVELY OPEN.  DURING KEY DOWN, Q4 IS
TURNED ON AND THE EXTERNAL RIT KEYING INPUT IS SHORTED TO GROUND, CUTTING
OFF THE BASE CURRENT TO Q5, TURNING IT OFF.  THIS OPENS THE RITB LINE ON
THE SYNTHESIZER BOARD AND BRINGS THE SYNTHESIZER ON TO THE TRANSMIT
FREQUENCY.  A SPOT SWITCH IS USED BETWEEN Q5 AND THE RITB INPUT ON THE
SYNTHESIZER BOARD.  WHEN THIS SWITCH IS OPEN, THE RITB LINE IS
DISCONNECTED, THE SYNTHESIZER IS BROUGHT ON FREQUENCY, AND THE RIT KEYING
IS DISABLED.  THIS ALLOWS YOU TO SET THE VFO TO THE FREQUENCY YOU WANT BY
SPOTTING IT ON YOUR RECEIVER.  TUNE THE VFO UNTIL THE FREQUENCY MATCHES THE
RECEIVED SIGNAL.  MOUNT THIS SWITCH SO THAT UP IS OFF, AND DOWN IS ON.  THE
SWITCH WILL NORMALLY BE IN THE DOWN ON POSITION, AND SWITCHED UP TO SPOT
‰  END OF 40 WPM TEXT  ‰  QST DE W1AW  ƒ