Speedometer stops working just past 30MPH

I saw a thread discussing checking the cable for the speedometer but I am unsure if this issue would be related to just the cable or if it is something more internal on the actual speedometer's internal assembly. I unfortunately dropped my bike before I replaced the battery.
Fortunately it was a soft drop and in the grass next to my driveway, nothing broke and it ran fine for a week before the battery died out. I swapped it for the same one and it runs fine again, just the speedometer is acting funky. 
The bike is an '80 KZ650
My main question is if it is related to the cable or otherwise, if anyone knows i would appreciate the insight before I start taking things apart. I am still new to fixing vehicles, specifically bikes. Thanks in advance!