[Dave A]

JVC embedded the hi-fi audio (20 Hz to 20 kHz) in to the video track in a scheme called Audio Frequency Modulation (AFM). It was located in a small RF signal gap in the video track in between the luminance and color carrier video signals.

As the audio was in the video track, JVC refined the audio switch to insure continous audio. The video switch would make a sloppy video head switch at the end of each rotation of the head which you can see at the bottom of the screen in the last few lines of video on most VHS tapes as staggered scan lines.

This deck will only play back hi-fi if it was recorded that way. If you have ordinary longitudinal, edge-recorded audio tracks (stereo or mono), the hi-fi will hear nothing and not work. This deck will play both via the audio switch.

Your Christmas 1984 tapes will be old school edge audio. If you can find movie studio tapes from that time and after, they will probably have hi-fi. I don't remember if any home camcorders recorded in hi-fi.

Does the deck have any kind of Time Base Correction? It would probably be a switch-on-switch-off function. They did this later in their high-end VHS decks along with digital correction and those playbacks look great. TBC was the only thing you would get in the 80's. Digital was a few years away.