Heathkit SB-401 - Page 14

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(_) Disconnect the VTVM.
( ) Replace the 15.895 MHz crystal.
CARRIER NULL ADJUSTMENT
NOTE: Leave the VTVM RE probe connected to the bare
wire between pins 5 of V10 and VIL.
(_) Place the BAND switches in the 3.5 position.
( ) Key the Transmitter and adjust the DRIVER control
for a maximum indication on the VTVM.
{ ) Rotate the LEVEL control fully counterclockwise.
( ) Place the MODE switch in the USB position.
NOTE: The object of the following CARRIER NULL
adjustments is to reduce the carrier output as much as
possible. If the following adjustments will not reduce the
YTVM reading to approximately “0”, perform the balanced
_modulator transformer adjustment again (to eliminate
_ possible interaction).
Page 93
( )
Short lug 2 of the MIC connector to the chassis with a
jumper wire, or connect a microphone and press the
push-to-talk button; then alternately adjust the
CARRIER NULL control and capacitor for a minimum
indication on the VTVM.
Place the MODE switch in the LSB position and adjust
the CARRIER NULL capacitor to the same VTVM
reading obtained in the USB position.
( )
Place the MODE switch in the USB position and adjust
the CARRIER NULL control for a
indication on the VTVM.
( )
minimum
(_ ) Place the MODE switch in the LSB position and adjust
the CARRIER NULL capacitor to the same VTVM
reading obtained in the previous step.
( )
Repeat the previous two steps until the same VTVM
reading is obtained in the USB and LSB positions of
the MODE switch.
NOTE: The preceding five adjustments can be checked after
the Transmitter is in the cabinet by using a receiver set to the
operating frequency and observing the “S’” meter for
minimum readings as the adjustments are made.
(_ ) Return the FUNCTION switch to STBY.