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Palo Alto speedometer. http://www.paspeedo.com/ great work very communicative. Brian
You might be better off to buy a second hand tach, I'm sure there are lots out there.
Seems like I remember Palo Alto being pretty pricey..
You could also attempt a repair yourself.They are a little hard to open but a tach will be quite simple inside, may just need some good lube.Perhaps something has fallen apart, here's a link to how another owner went about the task. https://www.oldjapanesebikes.com/gt750/img/page-6/Speedometer_Repair.pdf