Sync Stretcher And Zero Video Gain Sync Adjustments; Frequency Control - AEA VSB-70 Operating Manual

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SYSTEM DESCRiPTION
SYNC STRETCHER AND ZERO VIDEO GAIN SYNC ADJUSTMENTS
Two internal controls adjust the sync signal characteristics. Adjusting R98
CCW (conter-clockwise) will increase the level of the synchronization pulse to
compensate for compression of the signal in external amplifierS. It is recom-
mended that the control be left in the full CW (clockwise) position as operation of
an amplifier at maximum pO,wers~yerelydegrades the linearity of the amplifier.
A circuit has been incorporc;l.ted in the VSB-70 that provides sync pulses even
when the front panel video gain control has been reduced to,zero. The internal
control R39 sets the sync level at zero video gain. 'An oscilloscope is necessary
for adjustment of this control. Monitor the baseband video on pin 2' of U2 with nor-
mal video input. Set the front panel video gain control fully CCW and adjust R39
for the same level of sync pulses.
FREQUENCY CONTROL
TransmiVReceive signaJ~ are mixed with frequenc!es generated in a separate
oscillator section that provides two/three switched selections to match transml,s-
sion or reception frequencies for Amateur Television. YourVSB-70 was delivered
with two crystals (F1
&
F2) which provide operation at 434 and 439.25 MHz,
respectively.
For an example' of the frequency control circuit operations, assume you will
be using channel 3:
• The 93.1875 MHz F1 frequency is doubled, filtered, doubled again
I
and filtered producing 372.75 MHz to the mixer.
• The 372.75 MHz is added to the 61.25 MHz transmit frequency by the
mixer giving a final transmit frequency of 434 MHz. '
• The same 372.75 MHz signal is subtracted from a 434 MHz'receive
signal giving the 61.25 MHz channel 3 frequency to your television
receiver.
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