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Old 09-10-2016, 08:31 AM
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Zenith6S321 Zenith6S321 is offline
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As you say, the VA62 does not have the sweep and marker functions that provides an image of the bandpass response on an oscilloscope. But the VA62 manual does provide instructions on doing alignments for modern (45.75 MHz) color sets. The procedure uses either an oscilloscope displaying the demodulated video or using the image on the CRT to determine the affect of adjustments. The VA62 MULTIBURST BAR SWEEP test pattern provides 10 frequencies that are displayed as vertical black and white lines on the CRT. The frequencies are 0, .5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, and 4.5 MHz. So instead of a continuous sweep of frequencies like the traditional sweep and marker, you get the 10 discrete frequencies that can be viewed in the demodulated video on a scope or the TV CRT. You view the amplitude of each frequency burst on the scope or the contrast ratio (magnitude of the difference between each bursts black and white vertical bars) of the burst. Viewing the unmodulated VA62 "VCR STANDARD" video output on the scope shows all of the bursts at equal amplitude. Viewing the sets detect video shows what frequencies are being passed through the tuner and IF stages. This is helpful if you have a color set that shows a black and white picture but has poor color performance. You can see if the demodulated video color frequencies (3.0-4.5 MHz) are not getting through the tuner and IF stages. Understanding of the IF frequency that corresponds to the demodulated video frequency allows you to pick the IF stage that needs adjustment. So the sets video carrier frequency (45.75 for modern sets) is 0 MHz (the low frequency spectrum of the sync signals) while the audio carrier frequency is 4.5 MHz (45.75 - 4.5 = 41.25 MHz). Stagger tuned IF stages frequencies fall between the video and audio carrier frequencies. Effectively the stagger tuned IF stages are high gain amplifiers each tuned to different frequencies that in total provide a bandpass response to reject adjacent channel signals and amplify the tuned channel from the antenna input microvolts to the volts input to the video detector. Other than the Sencore VA62 manual, the book "Practical Troubleshooting With the Advanced Video Analyzer by Robert Goodman" is a guide on using the VA62 for alignments. Somewhere (probably here on VK) I downloaded a scan of the Video IF/detector section of the book. The book is still available on Amazon and other places.

Last edited by Zenith6S321; 09-10-2016 at 08:52 AM.
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