Monday, June 12, 2006

Spotmatic Mercury Battery Replacements

The match-needle light meters found in Pentax Spotmatics are undeniably cool. I'm not talking about the TTL center-weighted averaging, which made the Spotmatic terribly popular. I'm talking about the experience of using an M42 screw mount lens in its native environment and adjusting the aperture according to the needle meter. It just feels right. But as the years go by, fewer and fewer Spotmatics are likely to be found with decent CdS cells that continue to function as sensitively as they did thirty years ago. And to compound insult to injury, the mercury batteries for these cameras are no longer being sold in the U.S. (mind you, with good reason for environmental concerns). What's a Spotmatic fan got to go through just keep their light meter working these days? Well, first you have to find the right battery replacement.

Photo: Pentax Spotmatic ESII, circa. 1973.

The Spotmatic, Spotmatic II (SPII), Spotmatic IIa, SP500, and SP1000 all used a 1.35v '400' mercury cell, also known as a PX400. The Spotmatic 'F' is the only Spotmatic that used a 625 mercury battery. To replace these batteries, you'll need a zinc-air cell equivalent or an approximate silver-oxide cell. Both types of new batteries use zinc instead of mercury, and have an advantage in that they keep their voltage right up until they're ready to conk out, where as mercury batteries tended to reduce their voltage as they drained. There are also adapters to change the voltage of a common battery to match the required voltage. And if you're really up the creek, you can try an alkaline version.

Zinc Air Cells
Exell and Wein, makers of the Safe Sync, both produce zinc air cell replacements. These batteries come with a little plastic wrap seal over their contact, which you peel off, and then you keep the battery exposed to the air for about half an hour before you put it in your camera. Under their original seal, the batteries will keep for up to 10 years, but then once you open them it's a short ride. The electrolyte in the battery works as long it's exposed to the air, until the electrolyte is entirely dissipated, after about 2-6 months. Some people try to reseal them between uses to get them to last longer, but it doesn't always work. Thankfully, these aren't too difficult to get a hold of now, for between $5 to $10 each at all sorts of battery stores online, let alone the old standbys B&H or Adorama.

If you have the Spotmatic 'F' model, you'll want either the 1.35v Wein Cell PX625 or the Exell Z625PX.

For all the other Spotmatics, you'll want either the 1.35v Wein Cell RM400 / V400PX or the Exell Z400PX.

Silver-Oxide Zinc Cells
If you're up to trying, a silver-oxide watch battery can also make a good replacement in a pinch. Karen Nakumura of put it succinctly when she wrote that Pentax "built in a bridge circuit in the metering which makes it battery voltage independent," which is to say that a silver-oxide battery of about 1.5v tends to work just fine for the light meter in most Spotmatics without effecting the sensitivity (your mileage may vary, but most people are happy to report less +- 1/3 EV). Gene Poon notes that the Spotmatic 'F' was the exception, because it has a dual core meter that requires a precise voltage, so it's not too good to substitute the wrong voltage.

If you you're going to try a 1.5v silver-oxide battery in your Spotmatic, you'll want to find one that's about the same size. The 1.5v PX400S (aka. 387S) fits just about right. As does the Energizer E387S, which Radio Shack sells as the "387S" - special order only. Otherwise, you can take a 1.5v battery that almost fits (like a 395S) and snug up the difference with something that preserves the contact. Karen suggests a finding a mini rubber o-ring at a hardware store, some people use tin foil. All the silver batteries should cost you under $5.

C.R.I.S. Adaptors
C.R.I.S. Camera Services have been manufacturing neat little adapters that piggy-back common batteries like the 377 or 386 cell and use a custom diode to put out the proper voltage. If you have a Spotmatic 'F', you'll use their MR-9 adapter and a silver-oxide 386 cell. If you have any other type of Spotmatic, you'll use the H-B adapter and a silver-oxide 377. The adapters cost $30 each and will last you through dozens of batteries (more than the life of your camera, probably).

They don't last as long, but if you're desperate and not too particular, you can try a general brand 387 or 400 alkaline watch battery. But if you have a Spotmatic 'F', you'll be looking for a 625, which actually works surprisingly well.

But what if none of these options appeal to you? Are you a hearty soul with knowledge of bridge circuits and diodes? You might want to consider joining the Yahoo! Spotmatic List and getting a hold of some schematics to go in and tailor the camera to fit the battery of your choice. Otherwise, you'll need the help of a repair professional. But whatever you do, good luck keeping those Spotmatics working!

Links: AOHC on Spotmatic Batteries, Karen's Battery Page, Exell Zinc Air Batteries, PX400S at Battery Guys, Wein Cells at My Old Camera, CRIS Mercury Adaptors, Yahoo! Spotmatic Mailing List


Sune said...

Thanks for info. Recently received a Spottie, so found out that batteries were an issue. The one it came with had dissolved somewhat in the compartment.

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