BROADCAST TRANSMITTERS                          

DEDICATED TO THE CONTINUED SUPPORT OF RCA BROADCAST TRANSMITTERS

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Video Tray Set-Up

Before attempting to set-up any levels within the video tray it is important to check that the level from the waveform generator or the STL feeding the transmitter is correct.  This should be1 Volt PP with the correct video to sync ratio.  Many waveform generators are set such that the 3.58 burst level and the 3.58 subcarrier level imposed upon the luminance portion of the test signal is the same - 40 IRE units.  For a TV transmitter this is not correct.  The 3.58 subcarrier imposed upon the video portion of a test signal feeding a TV transmitter should be 20 IRE units.

If the subcarrier on the luminance portion of the test signal were 40 IRE units and the luminance level was taken to 100% modulation, the chrominance would over modulate to zero carrier.  On the other hand if the chrominance was taken to the correct level, the luminance modulation level would be low.  With the subcarrier set to 20 IRE units it becomes possible to modulate both the luminance and chrominance portions of the signal correctly.

 

The following adjustments are a step by step process to be carried out in the order shown - from beginning to end.  You cannot jump in at the middle without knowing the previous step was set correctly.

 

Video Input Board
Order

Test signal

Test Point

Adjustment to make
1

Multiburst

TP3

Adjust C7 for flat response
2

Multiburst

TP4

Adjust C9 for flat response
3

Modulated 10 Step or Ramp

TP4

Adjust R21 to center video around ground
4

Modulated 10 Step or Ramp

TP5

Adjust R1 (motorized control) initially for 2.5 V PP.  This control is used later to adjust modulation depth at transmitter output

 

Mother Board
Order Test Signal Test Point Adjustment to make
5 Modulated 10 Step or Ramp TP1 on Video Output Board Adjust R6 so video level remains at the same level when notch equalizer board is switched in or out
6 Modulated 10 Step or Ramp TP1 on Video Output Board Adjust R13 so video level remains at the same level when receiver equalizer board is switched in or out
7 Modulated 10 Step or Ramp TP1 on Video Output Board Adjust R16 for 1 Volt PP
8 Multiburst TP1 on Video Output Board Adjust C1, C8 & C10 for flat response

 

Clamp Board
Order Test Signal Test Point Adjustment to make
9 Modulated 10 Step or Ramp TP2 on Video Output Board Adjust R47 so  tip of sync is -5.8 Volts below ground
10 Modulated 10 Step or Ramp TP6 Adjust C14 for minimum clamp pulse feed through
11 Modulated 10 Step or Ramp TP6 Adjust R28 for minimum sawtooth amplitude

 

Video Output Board
Order Test Signal Test Point Adjustment to make
12 Multiburst TP2 Adjust C1 for flat frequency response
13 Modulated 10 Step or Ramp TP3 Adjust motorized sync stretch control towards maximum until burst disturbance begins to occur.  Adjust R18 to limit sync stretch from going beyond that point.
14 Modulated 10 Step or Ramp TP4 Adjust C10 for maximum subcarrier amplitude
15 Modulated 10 Step or Ramp TP5 Increase luminance to 110 IRE units using motorized control on video input board.  Adjust R40 to clip signal at that point.  Restore luminance to 100 IRE units using motorized control.
16 Modulated 10 Step or Ramp TP6 on Diff. Phase Board Adjust R42 to put tip of sync -0.7 Volts below ground.  For a parallel system this level can be -0.7 to -0.9
Video Output Board adjustments while monitoring transmitter output
  Modulated 10 Step or Ramp Output of demodulator Increase sync above 40 IRE units using the motorized control then adjust R15 to clip sync at about 42 IRE units.  Restore motorized control to 40 IRE units. 
  Modulated 10 Step or Ramp Output of demodulator ** Adjust R29 to minimize Differential Gain in white region.  This adjustment is for a bi-level transmitter, switched to full power
  Modulated 10 Step or Ramp Output of demodulator ** Adjust R23 to minimize Differential Gain in white region.  This adjustment is a bi-level transmitter switched to low power
** R23 becomes the active control on a non bi-level or single ended transmitter running at a single, non-switched, power level.
 

Differential Phase Board
Order Test Signal Test Point Adjustment to make
17 - TP2 Remove cable from J1 on board then adjust the DC offset control R5 for zero volts
18 Multiburst TP2 Adjust C7 for flat response
Differential Phase Board adjustments while monitoring transmitter output
  Modulated 10 Step or Ramp Output of demodulator Adjust R29 to R35 to minimize Differential Phase