|
Jonathan:
There are many causes of buzz in intercarrier sound IFs. The video carrier is AM modulated and contains high amplitude vertical sync pulses. Intercarrier sound is basically a dual superhet receiver using the video carrier for the second oscillator.
The 4.5 difference becomes the sound IF frequency. In a properly aligned receiver, the sound carrier is down to about 10% of the maximum video gain to minimize beats and hash in the final picture. The necessary gain is made up in the 4.5 mHz sound IF stages. The sound stages must have necessary gain to drive into limiting to eliminate the sync buzz. Any improper alignment or poor stage gain can result in the buzz.
The video IF must be properly aligned to assure enough sound gain as well as video bandwidth.
Sometime around the early 60s, broadcasters reduced sound transmitter power from the arbitrary 50% of video to ranges of 10%-18%. It was then necessary for the manufacturers to increase sound IF gain to compensate for the reduced power. That compounds the problem for early TVs with intercarrier sound.
In conclusion, the buzz can usually be eliminated, but it can take some methodical troubleshooting. Understanding the source is the first step.
Don
|