Old TV circuits, chapter 43: Sound IF

[AlanInSitges]

I always get thoughtful and helpful answers here, so I figured I'd try to get some insight into how this circuit works, and a few lead-up questions:

I'm working on a Predicta, using it in Spain. Since all of my other sets are CCIR or PAL, I use a HLLY 2W transmitter in my attic, fed with a PAL baseband signal from a DVB tuner and set to transmit on UHF channel 26, video at 511.25MHz and audio at 516.75MHz. This works great on my older CCIR-B sets. I would like to make the Predicta tune the same channel 26 broadcast, with sound. We'll leave the subject of sweep aside for now.

Given my woefully novice understanding of how the audio gets from the tuner to the FM detector, I'd like some help walking through the Predicta's circuit and hope to determine if what I want to do is both possible and feasible.

To wit:


I've been poring over Fink and Gulati to understand what is going on here: what comes from the video detector is, if I understand correctly, a 4.5Mhz audio carrier that somehow made its way through the video IF amplifiers and was then subsequently mixed with the baseband video signal by the video detector diode (somehow again). This much I understand, leaving aside the somehows.

From there, we go to connector L24U in the schematic, where this 4.5MHz carrier is coupled by C16 to the grid of the sound IF amplifier, in parallel with a simple tank circuit which I assume is used to filter the adjacent channel's video carrier? Or something? Fink mentions the purpose of this tank circuit in the section on video IF but glosses over it in the part on audio.

Then our carrier gets amplified by the 3AU6, then coupled to the audio detector by T3, and its associated tank circuits which, again, I am assuming serve to sharpen the dropoff on either side of the 4.5Mhz carrier. And then on we go to make sound.

I'm making a huge assumption that the 5.5MHz audio carrier from my PAL broadcast is going to somehow make it through the three VIF stages just like the 4.5MHz carrier does on an NTSC broadcast. And IF that is the case, would it be possible to tune these above tank circuits to have their hump centered on 5.5MHz?

Regardless of whether it's possible I'm still very interested in understanding how this works.

Thanks everyone for the great answers I'm sure will come.