79 Factory Tach Help

[draimondi]

So, my original factory tach hasn't worked in probably 20+ years. When i finally decided to take it out and try to find out why some diodes on the board seemed to be burned up. I found another 79 truck today with a factory tach and was told it worked fine up until the time it was parked, which may have been quite some time ago. Is there any way to bench test these tachs to ensure they work before installing? Any help is appreciated!