John K Jordan
If the lathe still works and if for some reason you can't get the speed display working, an option is to use it without the speed display. Many turners, John Lucas for example, use the PM 3520 with no speed control. In fact, I seldom look at the numbers on my PM3520b or my Jet 1642 - I just turn the speed to what feels right.
If you want some idea of the speed, you can do what I did with my variable speed milling machine with no tach display - I measured the speed with a cheep optical tachometer (from Amazon, https://www.amazon.com/gp/product/B004Q8L894) and made marks on the dial for 250, 500, 1000, etc rpm.
Of course, if it was mine, I'd want to fix it too, just because. Most that I've heard of with this problem were due to exactly the reason that the good Doc mentioned. Also, check the connector and wire to the sensor.