OM-1 battery and light meter concerns

I recently was given my Grandmother's OM-1 and had a question about

the battery. Apparently, the original battery was Mercury based and

is no longer manufactured. When I asked the folks at the local

camera shop, I was told that there were several replacements

availible, but they all had some effect on the light meter. However,

no one could tell me what those effects were. Has anyone else run

across this? If so, can you tell me exactly what the effects are?

Thanks for your help!

Hi James:

 

<br><br>

 

The standard replacement battery is the 625A Alkaline, which is readily available. The Alkaline battery has an initial voltage of 1.5 instead of the correct 1.35 volts. In addition, the voltage of the Alkaline battery drops off gradually while that of the Mercury cell maintained a very constant voltage over the life of the battery.

<br><br>

The effect of this is that, with a fresh 625A battery, the OM1 meter will read high and you'll get some underexposure. As the battery ages, the voltage will drop and you'll get good exposures for a fairly long period of time, and then as it continues to drop you will begin to see overexposure. If you aren't paying attention, this can eventually become significant - it can reach as much as 2-3 stops.

<br><br>

There are alternatives. One is the CRIS adapter, which lets you use a 1.5 volt silver cell and corrects the voltage for you; another is to just put a $1, 1.4 volt #675 hearing aid battery in the camera. Either of these will give acceptable results. It's also possible to have the camera modified for a 1.5 volt cell, but since these cells are not the same size as the 625 the CRIS adapter seems to me the logical way to use a silver cell in an OM1.

<br><br>

Personally, I use the hearing aid batteries. <a href="http://rick_oleson.tripod.com/index-111.html"><b>Here's everything I know on the subject.</b></a>

Camtech, http://www.zuiko.com/, offers an adapter that adjusts both the size of the battery chamber and the voltage of a 1.5V battery (386 etc.). I believe the adapter costs around $30.

Tim Hughes:

 

<http://olympus.dementia.org/Hardware/PDFs/

OM1DiodeVer2_1C.pdf>

 

Then there's the MR9 adapter.

 

It has an issue about non-linearity.

 

Chuck Norcutt:

"The non-linearity is small and when calibrated to read correctly

at 16EV the errors in the rest of the range are limited to about 1/

3 stop."

 

John Hermanson:

"I have found some non-linearity at BV9 & 6 with older OM-1

(usually less than 1/4 stop and that may be due to aging cds

cells). Meter is calibrated at BV12, then (again if cells are good)

BV15 drops right in."

 

hth

 

Fernando

The CRIS adapter and the others of its type are not perfectly linear, but should be good enough for practical use. The Zinc/Air hearing aid batteries will give technically the best results.

 

:)=

Oh, yes, there has been the last advice:

 

"I think you're being too hard on the MR9 (and other diode

conversions). You want things too perfect.

The small errors don't matter that much. Lots of people happily

use their cameras modified this way. Including me."

 

Fernando

Well, besides the excellent suggestions above, you could buy a handheld meter or use "Sunny

16" for free.

I would also heartily endorse the 675 hearing aid batteries in an OM-1 or OM-1n. But the best way to adapt the chamber is with a rubber gasket known as a #9 O-ring. They cost about $0.39 at Home Depot or Lowes or any other plumbing supply shop. They will fit snugly in the chamber unless you wish to remove them, in which case, a pin or tweezers easily does the trick. The only downside of these batteries is that they wear out quickly once they are activated by unsealing the anode when you remove the pull-tab. They last forever until the tab is pulled, and from that point, exposure to oxygen wears them out rather than use. In dry climates like Southern California or Arizona, they wear out in 6 weeks or so, while in most other climates, they last 3-4 months. But, since these batteries cost under $1 (I pay $11 for an 18-pack at Costco) replacing them frequently is no big deal.

 

If anyone wishes to e-mail me, I've written up a brief 1-page ap-note on modifying the chambers and using these batteries in cameras desiged to use the 625 Mercury Cell, and I'll send it to anyone who asks for it. But it's a trivial, reversible modification - and, IMHO, these 675 Zinc-Air batteries are the best of all the 625 Mercury alternatives.

Another vote in favor of the readily available and fairly cheap 675 zinc air hearing aid batteries. I've used 'em for years. They work.

<p>The problem with Air hearing aid battery is that they are @ higher amp, which can affect the lightmeter resistor output. OM-1 has 4 resistors that regulate lightmeter function. Also these batteries have voltage surge when used initially to higher voltage and drop when used more than 10 minutes and takes time to bring voltage up to its rated voltage. Voltage is also not consistant at its rated output, and wear out quickly.<br>

Diods, on the hand,are also considered to be voltage regulator besides its use as a rectifire. Another word, if the 1.5v battery act in the same manner as the 1.4v air hearing aid battery do (irregular output, and voltage fluctuation), if connected to a diod, the output voltage will be constant and the diod acts like a small regulator besides droping the voltage, and output is constant even if battery voltage fluctuates or amperage increases or decrease during usage.<br>

Diod method is the best way and most accurate and reliable method in my openion.<br>

<a href="http://www.allaboutcircuits.com/vol_3/chpt_3/11.html">http://www.allaboutcircuits.com/vol_3/chpt_3/11.html</a></p>