[dwinnert]

I have read conflicting posts stating that the 1.55v batteries do not affect the meter by overexposing and others that say they do. That the circuit is a bridge circuit and voltage within a range works fine.

Just curious.

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Last edited by dwinnert; 09-24-2016 at 05:04 PM.

[wombat2go]

Hi DW,

We discussed that in a thread some years ago.
As I recall the circuit, it is a bridge, but unbalanced.
The unbalance lifts the needle upscale to the null position.
So it is sensitive to battery voltage, but not as much as the simpler single loop LDR circuits of the time.

The circuit is published, we need to ask another electrical engineer on here to load it into a circuit simulator,
and tell us the "EV error-per- battery millivolt"
(I won't have time for a month or so)

[boriscleto]

If you're really worried about it spend the $40 for the C.R.I.S. battery adapter.

MR-9 Mercury Battery Adapter ? CRISCAM Webstore

You can get them on eBay for $25.

[dwinnert]

Solved.....

So I went to Freestyle Photo today and grabbed a Wein Cell replacement @1.35v and set exposure on a light source in my house and lined up the meter as close to center. I then changed to a 1.55v Duracell 392 and checked the meter on the same light source and it was exactly in the same spot. I did this with a few different lighting scenes and tested again with both batteries and observed that even though the voltages were different, the meter acted the same....there was no over or under exposure changes.

So, at least on my own Spotmatic SP II.....1.35v and 1.55v cells meter the same.

A bonus is, if you have a used Wein cell, the plastic sleeve that the Wein cell sits in fits a common Duracell 392 cell. So it fits perfectly in the camera.

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Last edited by dwinnert; 09-24-2016 at 07:35 PM.

[Narks]

Having read various sources saying either that metering is impacted or not impacted using 1.55V batteries, I decided to do my own tests.

Setup was an adjustable power supply, set to either 1.55V or 1.35V, with a 15 ohm resistor to simulate a silver cell or a mercury cell.

Results as follows:

1. Low light, f2.0, 1/8 sec @ 1000 ISO (near limit of low light metering of the Spotmatic):
Metering showed about 1/4 stop (or perhaps slightly less) higher with 1.55V compared with 1.35V

2. Medium light: f5.6, 1/15 sec @ 400ISO:
Metering approx 1/8 stop higher with 1.55V

3. Bright light: f1 @ 1/125sec, 125 ISO (near max daylight):
Metering less than 1/8 stop higher with 1.55V


..So the only real impact is at very low light levels, and the error is minimal.

[egadgetguy]

This seems so logical that it feels like people claiming there is an error have no practical experience with it.

In my opinion a company that would design / build a meter circuit where the results were dependant on the battery voltage seems ludicrous. It would have been easy, even in the 60s, for an engineer to design a circuit that can power a meter that works to power a meter circuit that can be calibrated and works the same within a supply voltage range. The meter would use the output voltage controlled by a light cell which the bias voltage would be powered by the battery but the reading calibrated based on the light cell output rather than the supply voltage. Then the only problem would be as the battery was dieing, it would go below the bias voltage.

Annnnd, I was right. The diagram shows the exact thing with the battery providing a bias voltage and not controlling the meter output, which is dependant on not one but 3 photo cells and not the battery voltage itself.

[vandergus]

In my opinion a company that would design / build a meter circuit where the results were dependant on the battery voltage seems ludicrous.

I mean, a lot of camera companies did it. It's very common in the little fixed lens rangefinders of the 1970's. I tested a Canonet QL17 and it could be off by 2-3 stops in bright sunlight with an LR44/SR44. The error in dimmer situations is much less.

[titrisol]

I mean, a lot of camera companies did it. It's very common in the little fixed lens rangefinders of the 1970's. I tested a Canonet QL17 and it could be off by 2-3 stops in bright sunlight with an LR44/SR44. The error in dimmer situations is much less.

Even some handheld lightmeters can only operate on 1.35V (e.g. Gossen) and some cameras didn;t have voltage regulation.
For those cases hearing aid batteries are cheap and do the job well; just use o-rings to match the size

This question should be part of the FAQ for the Spotmatics, it gets asked a lot and the 1.5V makes hardly a difference

Other resources
FUJICA solving the issue
http://www.pentax-manuals.com/fujica/top1/st701_battery_fix.htm
APUG/PHOTRIO has a lot of discussions

[Bbsteinle]

Just got my Spotmatic F back from Eric H., and he fitted it with a 625A.




---------- Post added 02-12-24 at 09:33 PM ----------

You can see how a Spotmatic meter, where you center the needle in a +/- bracket, is very different from a camera meter where the needle reacts to a light level, and then matched to a “lollipop” follower, like a Rollei 35 (or even a Pentax KX). Battery voltage would make a big difference in the latter.

[egadgetguy]

Just got my Spotmatic F back from Eric H., and he fitted it with a 625A.




---------- Post added 02-12-24 at 09:33 PM ----------

You can see how a Spotmatic meter, where you center the needle in a +/- bracket, is very different from a camera meter where the needle reacts to a light level, and then matched to a “lollipop” follower, like a Rollei 35 (or even a Pentax KX). Battery voltage would make a big difference in the latter.

Good to know. I used a 392 and an o ring.

[titrisol]

Good to know. I used a 392 and an o ring.

That is perfect for the SP and SPII

The SP-F has a larger battery cavity to fit SR76/SR74/AG13