I have a very similar drug store tester. Mine was made by Mercury. Like yours, there isn't a whole lot to the underside. When I first got mine, it was very dirty and the needle would swing slightly negative of zero when the test function was pushed. The short lights would also come up. I found it was just a bent lead shorting to the case of the unit.
It could be possible that your diodes were wired in backwards as well. My crt tester had that done and it would not function at all until that was done.
As for a schematic, this gentleman has posted to Audiokarma in the past saying he would give out any service info for his similar self-service tester if asked. I'd probably get in touch with him for a schematic. They can't be that different:
http://www.tuberadios.com/eico660/