DRAKE VM2551 Instruction Manual - Page 4

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Front Panel Controls and Indicators

F1 - A/V RATIO Control

This screwdriver adjustment varies the level of the aural

carrier over a range from 12 to 18 dB below the visual

carrier. The aural carrier should be adjusted to

approximately 15 dB below the visual carrier (normal

operation).

F2, F3 - AUDIO LEVEL Control &

Audio Overmodulation LED

The setting of this screwdriver adjustment determines the

peak aural carrier deviation. Clockwise rotation in-

creases the carrier deviation. After installing the unit, and

with an audio source connected, adjust for 25 kHz

deviation by advancing the Audio Level control, F2, until

the Audio Overmodulation LED, F3, begins to illuminate.

Monitor for a few minutes to assure that maximum volume

of the audio source program does not cause an

overmodulation condition.

F4, F5 - VIDEO LEVEL Control &

Video Overmodulation LED

The setting of this screwdriver adjustment determines the

video modulation level. Clockwise rotation increases the

modulation depth. After installing the unit, and with the

desired nominal 1 Volt P-P video source connected,

adjust the VIDEO LEVEL control F4 for the correct

percentage of modulation (87.5%). This setting will be

achieved when the Video Overmodulation LED, F5, just

barely illuminates. Advance video level until F5

illuminates and then back off until F5 just extinguishes.

F6 - CHANNEL Display

This is the 3 digit display which displays the selected

output channel. If the modulator is set to output off-air

broadcast frequencies (the ANT setting) the channel

number is preceded by an A. Off-air channels will display

as A2 through A27. When the CATV channel plan is

selected, a numeric 3 digit EIA channel number is

displayed.

F7 - STD, HRC, IRC

These are LED indicators.

A / V

AUDIO

VIDEO

RATIO

LEVEL

OVER DEV

LEVEL

OVER MOD

FIGURE 1

F8- ANT/CATV

Press to switch between the CATV or ANT (off-air

channels) output.

F9 - SEL (select)

When set to the CATV channel frequency plan, use

button F9 to further select the type of system channel map

that is required. Select among STD, HRC, or IRC. The

indicators, F7, will display the selected setting.

F10, F11 - CHannel Select Buttons

Press F10 to decrement the selected channel settings.

Press F11 to advance to the next highest numbered

channel.

F12 - RF OUTPUT LEVEL

This screwdriver adjustment permits decreasing the RF

output level a minimum of 10 dB below its specified

output level as the control is rotated counterclockwise.

The maximum output level of +55 dBmV is set with the

adjustment approximately fully clockwise.

NOTE: If an output level of less than +45 dBmV is

required, add an attenuator of the appropriate value at

the modulator output and operate the modulator at the

highest practical level (not exceeding 55 dBmV).

Example: For an output level of +35 dBmV, add a 12 dB

or 18 dB attenuator pad to the modulator output and set

the output level.

F13- OUTPUT

This type "F" connector provides a test monitor output that

is nominally 30 dB below the main rear panel RF output

level.

F11

CHANNEL

RF OUT

LEVEL

ANT /

STD

CATV

HRC

SEL

IRC

F10

F12

OUTPUT

TEST

F13