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H-B Adapter - Voltage Reducing Converter

H-B PX400 Adapter

Type: Voltage reducing adapter kit

Contents: Kit contains 1 x H-B Adapter, 1 x 377 cell. Output Voltage: 1.35 V.

Diameter: 14.6 mm. Height: 3.7 mm. 

Price: 29.43 each. Postage and packing included.

Emulates: PX400 / EPX400 / V400PX / RM400 / H-B / E400N / PM400R / KX400 / T400N / 387/ HB.

The "Rolls Royce" solution to problem of the discontinued Mercuric Oxide PX400 / EPX400 / V400PX in the Pentax Spotmatic. Our H-B Adapter has the outer dimensions of the Spotmatic battery compartment cover with a recess that takes the inexpensive and easily obtained silver oxide 377 cell. Buried within the H-B Adapter are micro electronics that drop the voltage from the 1.55 Volts put out by the silver oxide cell to the 1.35 Volts that your  camera was designed for.

This H-B Adapter is a suitable for the following Spotmatic cameras made and marketed by Pentax, Asahi and Honywell......

This adapter is NOT suitable for the Spotmatic F or SP F.

No adjustment or modification to your camera is required. The H-B Adapter does not require activation or a supply of air. The H-B Adapter and silver oxide 377 cell has a significantly longer battery life than the zinc air WeinCELL MRB400 batteries.

H-B Adapters are a one time purchase, and will last a lifetime. Simply replace the expired silver oxide 377 cell with a fresh one.



The H-B Adapter is designed for low current and cannot be used in applications that exceed 200 micro amps. Fortunately, there are no Spotmatic cameras that require this high level of current.

If you wish to measure the voltage from the H-B Adapter and silver oxide cell with a digital voltmeter, put a 10 KiloOhm resistance between the measurement terminals. This will allow sufficient current to flow to activate the voltage reducing circuitry. An analogue voltmeter will draw sufficient voltage to activate the voltage reducing circuitry.

Click here to purchase extra 377 cells.

There are other possible mercury free replacements for a PX400 cell, click on the links to find out more:

There are no alkaline cells manufactured of this size and shape. 

The S400PX cells produce 1.55V which is different from the 1.35V produced by the original PX400 mercury cell. This doesn't normally cause a problem because today's films are very tolerant to under-exposure and because the under-exposure is compensated for during printing. (so you would have greater problems if you were using slides). If you do find you need to compensate, because the S400PX silver oxide cell produces a steady 1.55V, you are able to compensate for the higher voltage by a  fixed number of stops or by adjusting the ASA setting. There may be as many as two f-stops underexposure by a camera that uses 1.55v instead of the original 1.35v, but the compensation you require is constant.

Another PX400 replacement is a zinc air battery. The WeinCELL MRB400 is a custom zinc/air battery designed to replace banned mercury batteries.  WeinCELLs deliver the same 1.35 V voltage and stability of output as did the mercury PX400 batteries so there is no need to adjust your exposure. The WeinCELL MRB400 batteries last much longer than hearing aid batteries, some times up to a year. To achieve this longer life, the MRB400 used a proprietary electrolyte and has only two small air holes, instead of the 7 larger holes found in standard hearing aid batteries.

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