Basic VCR Servicing 

VHS Hi-Fi is an interesting format when you consider how it's recorded. The lower frequencies required for audio (1.4 MHz for left and 1.8 MHz for right) together with a larger head gap allow the Hi-Fi audio RF signal to penetrate deeper into the oxide layers of the tape. The video heads then record their signal on top of the audio, partially erasing it. But the higher frequencies and the smaller head gap mean that the video signals are only recorded on the top layer of oxide, so enough of the Hi-Fi signal remains in the deeper layers of oxide, and the loss of audio RF is quoted to be around 12dB. This is what is known as Depth Multiplex Recording. The Hi-Fi tracks are also narrower than the Video tracks. For PAL-SP the Hi-Fi tracks are 23μm and the video tracks are 49μm, so there are guard bands in the Hi-Fi audio but not in the video. The azimuth offset between the heads is also increased, where it's +/- 6 degrees for the video it's +/- 30 degrees for the Hi-Fi audio, further preventing any cross-talk. As a very rough analogy it's a bit like dual layer DVDs, the Hi-Fi audio heads are focused on a deeper layer of the tape oxide and the video heads focus on the top layer of oxide. This also explains why it's not possible to "audio dub" over the Hi-Fi audio, as it's buried underneath the video. There was a deck, probably a professional one, that could record just the Hi-Fi audio on it's own, then edit the video on top of the audio. But most editing decks insert Video/Hi-Fi together and audio inserts can only be done on the linear tracks.

Great video thanks, on the head are the two audio heads the two thicker bands top and bottom?

Thanks for the video, in regards to the head cleaner foam auto cleaner. is removing just the foam enough or do you need to disable the arm somehow?