Three lug M5 - internal voltage regulator or not

[wlaidlaw]

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I have been told by someone who has one, that the later three lug M5 has an internal voltage regulator and will meter accurately with 1.55V alkaline 625 type cells without recalibration. I am far from sure that this is the case because I  thought the metering circuit for both early 2 lug and later 3 lug M5's was the same as the CL, all of which in the absence of the original mercury cells, need a 1.35V zinc-air Wein cell. These are pain in the proverbial, as they run down even when not being used and Murphy's law being what it is, will always be flat when you come to use the camera. At least on the M5, you don't need to open the camera back to change the battery, like you do on the CL. The alternatives are to use the MR-9 regulated adapter which model dependent, use an SR43 or 44 silver oxide cells or get the camera re-calibrated by an expert. I have had a number of the MR-9 adapters and have not found them too reliable. I find they tend to need "waking up" by pressing the voltage check button on my CL or taking a whole lot of readings on my MR-4 Leicameter, after which it will either settle down or you know the MR-9 adapter has failed. I have one which works properly, one with a "wake up" and two dead ones.

I am thinking of adding an "ugly bug ball" M5 to my collection of 28 Leicas, as I have been offered a near mint boxed black three lug example  but not sure I can be bothered, if I  have to use an MR-9 or get it recalibrated. 

Wilson

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[Lux50]

I purchased my 3 lug M5 #135xxxx new in 1974. Sent it to Leica USA for a CLA in 2005. At that time, besides going over the camera, they recalibrated the meter for the newer 625 batteries as part of the service. My M5 continues to work well and is a great camera!

Leica User since 1970      Leica Owner since 1971      LHSA Member since 1973    

[TomB_tx]

DAG did a CLA fixing minor issues with my 3-lug M5 several years ago, and asked if I wanted it re-calibrated for 1.5V batteries. I kept it original, and use an MR9 without problems (so far).

My M5 is a "50 JAHRE" sample, and a couple years ago I added a 50 JAHRE SL2, which came with a 1.5 V cell installed. It seems to meter fine, so I don't know if it was previously adapted, or just lucky.

[jerzy]

in all CL, Leicameter, M5 the lightmeter my be trimmed in a way that the needle will show proper value at battery test when using 1,5V battery without reducing the voltage. But metering will be off (Ohm rule, passive resistors and different current flowing through curcuit when bright and dark light). I have bad experience with adapters with built in diode for reducing voltage, so now I always mount diode within the circutry, in most of cases recalibration is not needed (if the lightmeter was properly calibrated on 1,35V).

Adapters without the diode, adapting the battery size only are reliable, they have much more contact surface than those with diode.
And alkaline 625 are available as well, but they are alkaline

[ironringer]

Good advice Jerzy, and many users report problems with the regulated adapters (with tiny diode embedded).

I believe that modern silver oxide batteries have a less pronounced (slower) discharge curve (voltage versus time and load) from their nominal 1.55 volts. Some meter circuits can be adjusted to tolerate the 1.55 volts versus the PX625/PX13 mercury cell's 1.35 volts, and since that voltage is relatively constant over time, the meter will remain accurate, compared to using modern alkaline cells that have a much steeper discharge curve over time. As an alternate and easier approach, if the camera can tolerate the 1.55 volts, adjusting the ASA/ISO setting can compensate for the higher (and constant) voltage. Physical adapters (shells) are available to "upsize" smaller solver oxide batteries to the PX625 size (15.6mm diameter x 5.95mm high).

When I had a Leicameter MR-4 serviced by Metrawatt (now GMC-I Service GmbH ) in Nuremberg four years ago, they installed a silver oxide battery and calibrated the meter for its 1.55 volts.

[cnguyen]

Best solution for me is to buy this - Loopacell 625A PX625A LR9 V625U PX625 1.5V 10 Batteries from Eboy and set the ASA/ISO to match with the true film ASA using my gossen luna pro and I'm done.

[wlaidlaw]

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7 hours ago, cnguyen said:

Best solution for me is to buy this - Loopacell 625A PX625A LR9 V625U PX625 1.5V 10 Batteries from Eboy and set the ASA/ISO to match with the true film ASA using my gossen luna pro and I'm done.

Not the solution I would or do choose. IMHO the best solution is to buy an adapter for PR44 zinc air batteries. These are very cheap and seem to outlast the very expensive PX625 Wein Zinc Air batteries. If you use silver oxide or even worse alkaline batteries in a location originally designed to use mercury batteries, not only will the meter reading be wrong but the error will change as the battery voltage declines. Zinc air maintain the correct 1.35V very steady voltage until they are near exhaustion and the voltage then drops off very rapidly. I use the PX625/PR44 adapters in my M5, CL, Leicaflex SL2 and one of my MR-4 meters. I had two of my MR-4 meters recalibrated to use silver oxide cells, one in Germany and one in the USA. Neither are as accurate as my last MR-4 meter which I kept on Zinc Air cells. The only location I use an alkaline 625 cell is the accessory battery location in the Leicaflex, which provides the illumination for the meter needle, where steady and accurate voltage is unnecessary. 

Wilson

[Matlock]

7 hours ago, cnguyen said:

Best solution for me is to buy this - Loopacell 625A PX625A LR9 V625U PX625 1.5V 10 Batteries from Eboy and set the ASA/ISO to match with the true film ASA using my gossen luna pro and I'm done.

The problem with that are those batteries are Alkaline which do not produce a consistent voltage and can leak or distort. The best bet is a MR-9 adaptor. There is no point in having the meter recalibrated as Silver Oxide batteries are no longer freely available in 625 size.

[wlaidlaw]

54 minutes ago, Matlock said:

The problem with that are those batteries are Alkaline which do not produce a consistent voltage and can leak or distort. The best bet is a MR-9 adaptor. There is no point in having the meter recalibrated as Silver Oxide batteries are no longer freely available in 625 size.

Over the years I have had a number of these MR9 regulated voltage adapters, usually from The Small Battery Company. They use a reverse biased Schottky diode as their regulation device. My personal experience is that the diode only lasts about 5 to 6 years before it fails, so not a cheap solution. The only place I am still using one of these adapters is in my film CL as in a poor bit of design, the battery for the meter is inside the film chamber, so if the battery or adapter fails mid film, you are left with the choice of sunny 16 metering (good enough for B&W and colour negative but not for slide/reversal), using an external meter or wasting film. The MR-9 currently in my CL is on its last legs and needs to be "woken up" by doing two battery checks before you meter for the first time. It will then stay awake for the rest of the day. Again if using an MR9 adapter, it is preferable to use a silver oxide button cell with more stable voltage and longer life than the cheaper alkaline button cells. 

Wilson

[Matlock]

1 hour ago, wlaidlaw said:

Over the years I have had a number of these MR9 regulated voltage adapters, usually from The Small Battery Company. They use a reverse biased Schottky diode as their regulation device. My personal experience is that the diode only lasts about 5 to 6 years before it fails, so not a cheap solution. The only place I am still using one of these adapters is in my film CL as in a poor bit of design, the battery for the meter is inside the film chamber, so if the battery or adapter fails mid film, you are left with the choice of sunny 16 metering (good enough for B&W and colour negative but not for slide/reversal), using an external meter or wasting film. The MR-9 currently in my CL is on its last legs and needs to be "woken up" by doing two battery checks before you meter for the first time. It will then stay awake for the rest of the day. Again if using an MR9 adapter, it is preferable to use a silver oxide button cell with more stable voltage and longer life than the cheaper alkaline button cells. 

Wilson

I use 3 MR-9s and have had no problems, one has been in use for about 8 years. I use that on in my Rollei 35 which, like the CL, requires the camera to be opened to change the battery. Your point about using Silver Oxide rather than Alkaline is well made.