How can I tighten the drive belt(s) on my Frosted /Rossman64 sewing machine?

I acquired a second hand F/R 64, model no' 507, serial no' 5009293. An electrician friend helped oil, grease all parts, but said I'd need to tension the belts soon. That time has come, I think, as I've been sewing denim without any problems, but since sewing some thin leather, the belts appear looser, and when starting sewing or slowing down I can hear the motor whirring, but the needle struggles to come back up after going through the leather, after the first time I removed the thread ( by cutting it and lifted the needle with difficulty, only to find, that it was sewing as it should, and I didn't have any nots of thread below the leather; unlike previously prior to lubrication) I'm using a leather needle and bonded nylon thread, suitable for the needle. It's practically the same size as the polyester thread as I was using before. I've adjusted the upper tension and bobbin tension, so it doesn't bind. I haven't got the manual, but searching online only shows how to oil all points, and use the differing feet for sewing.
Can you advise what needs to be done, show pics or email me pics. If I need to change the belts, I don't know how to loosen the belts to remove, or how to remove the side panel with handle wheel to gain access.
Hope to hear soon, as I don't fancy hand sewing this project.