A stable 1.35V voltage regulator for vintage cameras and light meters — “Stable625”

Permanent fix for obsolete mercury batteries.

Once soldered inside the camera or light meter, it will allow you to use any battery between 1.5V and 5V. Replaces PX625, MR9 and similar mercury cells which provided 1.35V.

Advantages

• Output voltage does not depend on the type of battery, ie. works with alkaline or any other kind. Silver oxide recommended.
• Output voltage does not depend on temperature and amp draw like in MR9 CRIS adapter or any Schottky diode adapter.
• Each voltage regulator unit is much cheaper than MR9 adapter with 10 cent Schottky diode in it.
• Meant to be installed inside the camera or light meter as an upgrade and permanent solution.
• Can last longer than zinc-air batteries, if your camera does not have a on/off switch, just take your battery out for storage to avoid slow drain. Zinc-air batteries will last only 2–3 months regardless of usage and can be difficult to find in rural areas. LR44 or SR44 are the most common to purchase and have much larger capacity.
• Improved replacement for MR9/PX625 battery adapters with Schottky diode, Schottky diode conversions, Wein cells, alkaline batteries. To install the battery in the battery compartment, you may still need to improvise or use an inert PX625 shape adapter for your battery of choice. For example, you can use a faucet rubber o-ring.

Features of the 1.35V version

• Stable 1.35V output, via linear voltage regulator
• Suitable input: 1.5V — 5V; overcurrent protection
• Size approx. 6.5x6.5mm, less than 2mm thick
• 3 soldering pads: output, input and common ground
• <20uA quiescent current
• Works in extreme temperatures: -40C to 80C

Features of the 2.7V version (replacement of 2x 1.35V batteries)

• Stable 2.7V output, via linear voltage regulator
• Suitable input: 3V — 7V; overcurrent protection
• Size approx. 6.5x6.5mm, less than 2mm thick
• 3 soldering pads: output, input and common ground
• <40uA quiescent current
• Works in extreme temperatures: -40C to 80C

Given the above, the obvious problem would be the quiescent current of voltage regulator. It may slowly drain your battery if your device lacks “on/off” or “measure” switch. It’s difficult to predict real-world time to drain, it may be a few weeks or months. Having kept the battery in my light meter for over a year now, I have not experienced much drain yet. Still works as intended and shows healthy voltage.

Yashica Mat 124G — Stable625 installed in the light meter.

Personally tested and used in Yashica Mat 124G with SR44 battery. To accommodate the smaller battery size, I used a square piece of aluminum foil. Resulting input was measured at 1.6V, output at 1.36V.

Initially, I wired it behind the switch on the focus knob side, which required to take off parts of the leatherette. Then I moved it behind the light meter, which is much easier to do and faster. All you need to do is to remove 4 waist level finder screws (JIS head), remove the finder assembly, remove 2 screws securing the light meter cover (good time to replace the light seal on it) and you are ready. Cut the red wire coming off the meter PCB towards the galvanometer. Connect the galvanometer side to output on Stable625, connect the PCB side to input on Stable625, then attach its ground to common ground as pictured. The Stable625 will be activated only when the waist level finder is up. The meter is spot on with digital Sekonic L-308S Flashmate. You may need to calibrate it though, since many Yashica meters were re-tuned for LR44 batteries.

Upper arrow shows thick Vcc wire coming from the battery compartment that needs to be unsoldered from the underneath of PCB by heating the (+) point indicated by the lower arrow.

I have also installed it in two Gossen Lunasix Pro light meters. The installation was even easier, since a multimeter can tell you in seconds which wire connects to the positive side of the battery compartment, and where is ground. What’s more, the Gossen PCB has markings for positive terminal too. The installation was similar, cut the positive input and splice Stable625 (2.7V version) in series. This meter needed more thorough calibration and without a calibrated light source, it may take an hour or two.

Stable625 is a product intended to be installed by a trained camera repair technician. You can bring it along with your camera to CLA, or if you are a camera repairman, offer it as a service. You can send me an email if you are interested in purchasing one or a few. I’m no longer selling them on eBay.

You can order them emailing me directly, the price is $8/ea + shipping and fees. Use the e-mail below.

Ultimately, I hope this can revive plenty of old equipment and put it back to use.

Stable625 1.35V version