When your machine will only straight stitch and refuse to zig-zag or do any other stitch … check the gears.
So here it is, the broken elnagraph pinion removed from my machine and replaced. My Elna SU is now zig-zagging up a storm (and every other stitch on the elna disc).
My trusted local machine technician immediately identified the fault on inspection and a new part was the only solution for this problem, luckily he had it in stock and could do it the same day (yay!). Asked if it was something I may have caused and the reply was “another wear-and-tear issue that comes with age”… so thanks elnagraph pinion for lasting 50-odd years, lets hope your replacement can do the same. An expensive exercise but cheaper than having to buy a whole new machine (something I would have loved to do as I have my eye on the Bernina 550 but still saving for that dream). Glad this machine can keep chugging along…time for some sewing.