I just got a Gossen Luna Pro used from KEH. I went to buy batteries, which are not easy to find. I put the batteries in but the meter gives no readings. It doesn't appear to be working. In the manuals I've seen, they say to get "Mallory" PX 625 batteries. Mine didn't say "Mallory" and, now that I remember, they had a Z at the beginning, as in Z 625 PX, or something like that. The guy at the store saw my meter and said that it took a little while for the batteries to kind of "warm up." Did I get the wrong batteries? Can someone help? Does the "+" sign go down or on top? Thanks.