Confused About Batteries for MR4

[ktmrider2]

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Can someone explain the thing with replacement batteries for the old Mercury 625 meter?  I am seeing Wein MRB625 cells with a MR9 adapter, the MR9 adapter which uses the Silver Oxide 386 battery which is 1.35 volts, and the Varta 625 battery which is 1.5 volts.  Do the 1.35 voltage batteries require the meter to be adjusted for the different voltage?  Can you simply use a different ISO (ie some fraction of manufacturers suggested ISO, double it, triple it etc).  I had a M5 about four years ago and Sherry had adjusted the meters working voltage.  What about the Varta 625 which is rated at 1.5 volts.

I have a mint 50th anniversary M4 which came with a working MR-4 meter.  I would just like to figure out which battery to use and what I need to do to the meter.  B&H and Amazon have the adapters and batteries.  It is hard to get a clear answer.

Thanks!!!

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[TomB_tx]

Unless the meter has been calibrated for 1.5V it won't be linear if you use a 1.5V battery. That means you could set ISO to get it correct at one light level, and it would be off at the other extreme levels. However, if you are using B&W or color negative film you should still get usable exposures. The Wein cells should work fine, but have limited lifetime if you don't use the camera for a while. I normally use an MR9 (with diode), but the "center" contact on the MR meters is actually 3 prongs rather widely spaced, so may not contact the 386 center contact properly, It can take some fiddling to get correct contact. That said, I've just been using my MR4 with MR9/386 on the M4 I bought in 1968, and it's been working fine.

[Anbaric]

6 hours ago, ktmrider2 said:

Can someone explain the thing with replacement batteries for the old Mercury 625 meter?  I am seeing Wein MRB625 cells with a MR9 adapter, the MR9 adapter which uses the Silver Oxide 386 battery which is 1.35 volts, and the Varta 625 battery which is 1.5 volts.  Do the 1.35 voltage batteries require the meter to be adjusted for the different voltage?  Can you simply use a different ISO (ie some fraction of manufacturers suggested ISO, double it, triple it etc).  I had a M5 about four years ago and Sherry had adjusted the meters working voltage.  What about the Varta 625 which is rated at 1.5 volts.

I have a mint 50th anniversary M4 which came with a working MR-4 meter.  I would just like to figure out which battery to use and what I need to do to the meter.  B&H and Amazon have the adapters and batteries.  It is hard to get a clear answer.

Thanks!!!

There are two types of MR-9 adapter. The more expensive one has a voltage converter, so that you can use it with a 1.55V silver oxide cell and get the correct voltage of 1.35V:

https://www.smallbattery.company.org.uk/sbc_mr9_adapter.htm

http://www.kantocamera.com/english/adapter/adapter_en.html

The cheap 'MR-9' adapters (like most of the ones you'll see on ebay) are simple physical adapters. They are intended to be used with zinc-air cells (hearing aid batteries), which I think are 1.4V.

Another alternative is the Wein cell, which is also a zinc-air cell (advertised as 1.35V) and designed to be used as a drop-in replacement for the PX-625 (without an adapter). Both this and the hearing aid batteries have pretty short service lives.

Finally, you can have some meters designed for the PX-625 recalibrated as you mention. A few camera meters (those with bridge circuits) don't need to be adjusted and are happy with higher voltage cells, but many others will give you the wrong reading when used directly with a 1.5V or 1.55V cell.

As for the MR-4 specifically, there's an interesting comment in this thread that suggests it may not be voltage-sensitive after all, but I don't have this meter and can't verify the claim. It's usually assumed that this meter needs the original 1.35V it was designed for.

Edited March 11, 2024 by Anbaric

[ktmrider2]

I may be selling my M4 since I just purchased a new MP.  The M4 came with a working MR4 and I want to make sure it works as it will be with the M4 if I decide to sell it.  I purchased the M4 about 8 years ago for $3000 and the vendor offered to buy it back at that price.  It is a mint 50th anniversary black chrome M4 which I normally use with an incident meter.  Thus the confusion on batteries as using the MR4 is not a high priority