I wanted to post a quick tip that I don't think I've seen mentioned here. I've had some problems with intermittent contact in Woodstocks with modified battery packs replaced with primary cells. This intermittent contact is due to the slight misalignment of the positive pole of the battery. The contact in the calculator ends up rubbing against the side of the postive end of the battery. I've started inserting a small piece of double sided mounting tape under the positive cell. This corrects the alignment, resulting in perfect and reliable contact every time. A very simple modification that has made a big difference in all of my Woodstocks.
Edit: make sure you only uncover one side of the tape so that the battery will slide on the other.
Edited: 25 Aug 2010, 11:03 a.m.