Pentax Spotmatic Introduction
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SP Series
The 1.35-volt mercury (mercuric oxide) battery originally used in the Spotmatic SP is no longer available due to environmental protection laws.
Replacement batteries available today are 1.55-volt but the Spotmatic uses a "bridge" circuit in its meter, making it insensitive to battery voltage variations. So, despite what you may have read - No meter recalibration is necessary as all Spotmatic SP series cameras can handle the increased voltage.
The alternatives are:
Spend about $30 for an adapter from CRIS Camera Services (Camera Repair by CRIS Camera Services: Digital Camera Repair: Canon, Fuji, Nikon, Pentax, Ricoh, Sigma) and a battery to insert into it. But this would be a waste of money as it is definitely not required.
Spend about $10 for a Wein Products MRB400 cell (modified Zinc-Air cell). They are quite expensive and have a limited life (about 6 to 12 months) unless you remove the battery after each use and seal up its air hole (it needs air to work). This is not a recommended solution.
Spend about $3.00 for one of the many 1.55-volt silver oxide batteries available over the Internet, at Radio Shack, most drugstores, camera stores and even some supermarkets. They will last you for about three years if our experience is typical. Make sure you insert it the right way for proper polarity, and take up the excess space in the battery box with a rubber O-ring from the hardware store, that will cost you about 49 cents. You might have to bend up the battery contact in the battery box to make good contact.
Silver oxide batteries that work are: Mallory PX-400 or RM-400-R, Varta V394, Maxell SR936W or SR936SW, Renata 394, or any type 392 equivalent.
Do not be tempted to use alkaline battery equivalents as they have a much steeper discharge curve. The silver oxide batteries have a flat discharge curve similar to the old mercury batteries.