WILCOX CORVETTE is probably the most knowledgable about electronic Corvette tachs.
They have some tutorials, videos and reading material about this subject.
I would start at the source... see if there is a signal at the tach...then at the condensor/capacitor mounted on the intake. Then in the dash....
good luck.
I’ll be watching this thread as my ‘75 tach quit working a while ago.
I just shift when the motor is louder than my screaming!!!
ha ha