Sorry for the delay.
I found the service manual but it isn't very helpful. There are only two sections that even show the head, and I've uploaded them below. Click each image to expand it, and right-click and select "Save image as ..." (or something similar) to save it to your hard drive so you can view it at 100% size.
View attachment 16767
View attachment 16769
I don't expect these images will be much use.
I've never even seen an 8-track unit so I don't know what to suggest. I would actually have to have it in front of me to be able to try to diagnose the problem. You could make a video demonstrating what it does and doesn't do, with a voice explaining what should happen, and upload it to YouTube and link it here. Without that, at a minimum, I don't think I can help.