I have a vintage Pentax Spotmatic F that I inherited from my grandfather. When I first got it the batter for the light meter was working correctly. This is pretty surprising since the battery was an original PX625 mercury battery. After a good year or two of usage, it died on me. I did some preliminary searches on the internet and found this Wein MRB625 Cell replacement battery on B&H. Unfortunately, when it came in the mail it didn’t work. Google let me know pretty quick that this is common with these types of zinc-air batteries. I had a choice, I could buy more batteries and hope that they hadn’t died, or I could find a different solution.
Google was my friend again. I found someone who had mentioned buying an adapter that would convert the voltage of a common button battery to the voltage needed for the light meter. I was a bit skeptical since the adapter was created and sold in Thailand, but the person claimed that it was worth the price and worked well.
I decided to take the risk, bought the adapter and have had light meter success! The adapter that I purchased converts a normal Energizer 386/301 Silver Oxide (1.55V) button battery to the voltage of the MR-9 PX625 PX13 Mercury battery (1.35V). You can get the adapter on Amazon for about $30 and it arrives in around 2 weeks. I suppose the ultimate proof is in how accurately the light meter itself is registering. I’ll find that out after I get my first roll of film developed.