Tach problems

I have a '72 240Z with a 280Z motor in it. It has a breaker-less

distributor installed by a "professional" shop. The tach doesn't

work properly. With the first one, the needle went flying all over the place, then quit. Went to a junk yard and got a replacement

and this one only reads between 0 and around 500 when I'm revving to about 4500. I called the shop to confirm that they

hooked it up correctly and they assured me that they were the professionals and knew what they were doing and I just happened to get two bad tachs. Is there a special way to wire a '72 tach when you are using a newer distributor ? How can I tell what year distributor it is ? I think it is a '78 but I'm not sure.

I pulled the whole dash out today to fix several things, and took the tach apart and noticed one of the little resistors is bubbly and

the outside is cracking off. That is why I think it is wired wrong.

Anybody ?