NES Zapper not working?

[donaldjr]

So I got in two systems today and tested them out (with an old 32" tube TV)

First one tested fine, but when I tried to use the same zapper in the 2nd system it didn't do anything. I plugged a regular controller in and tried out two player super mario and the second controller worked good.

I have a friend that is a novice gamesmith.. I have had him take a few apart just for cleaning and I was going to have him take a look at it - Is there anything in particular he should be looking for?