Problem is the soldering of aged wires. But if it is only the one to the hotshoe maybe I will give it a try. Were you in the camera? Maybe we can guess what could be bloqued or missing. How is the delay between first and second curtain realised?
Sorry, I missed this edit before. You should be OK to desolder and resolder any wires. The wires in the Fujica ST801 (and maybe 701) had Teflon insulation, and should be good. I would expect the 901 to be the same for any wires it might have. I have not been into the ST901 or other Fujicas. I don't know the details of how the curtain distance is established.
I do use a digital device in my photographic pursuits when necessary.
When someone rags on me for using film, I use a middle digit, upraised.