Old TV circuits, chapter 43: Sound IF

[Notimetolooz]

Originally Posted by AlanInSitges

I'm wondering about the best method to verify the passing of the 5.5MHz carrier through the VIF section, and then peaking those inductors in the SIF section. I don't have a sweep generator but I wonder if I could just inject an unmodulated 5.5MHz signal at J4, and look for its presence using a scope on the output? If this is viable then I can also presumably then inject a 4.5MHz signal to the SIF and gradually increase the frequency, adjusting the inductors with each increase, so as to walk the center frequency up to 5.5MHz?

Learning is fun. Thanks very much guys.

Nope. 4.5MHz or 5.5MHz does not pass through the video IF as such. Those frequencies are AM modulation on the video IF frequency. It is as if they are in the baseband. Using 4.5 or 5.5MHz after the video detector would work. You would have to be careful you did not load the circuit when you inject the signal. To correct what you said originally, the video detector does not mix a signal.
As far as getting the signal through the video IF, you would have to figure the frequency that the 5.5 MHz sound carrier occupies when it is in the video IF passband.