Zenit Automat: New but with a Strange light meter

[Gero Giambrone]

I bought a 1991 Zenit Automat a month ago. It looked new and in excellent condition, but the light meter behaves strangely: when checking with an external light meter or in a bright environment, the aperture-shutter ratio is correct, but in low light it overexposes by a stop. Sure, I can use the correction dial, but it's not convenient because I have to judge the lighting conditions myself. I've seen how to disassemble it in some YouTube videos: perhaps there's a potentiometer I can adjust for low light?

 

[runswithsizzers]

The Internet says that camera should have four PX625 batteries (mercury) -- which are no longer available.

What batteries are you using in it?

 

[Gero Giambrone]

First of all, thank you for your reply and for the helpful comments you made. Along with the camera, I found two new 4LR44 batteries (6v) from the previous owner, and I thought they were fine. You're right; I see online that this camera used 4 PX 625 1.35V batteries, for a total of 5.4V. The 1989 manual lists Soviet RC-53 1.25V batteries, for a total of 5V. A difference of just 1V could cause the light meter to malfunction! I'm looking for solutions: aside from the MR-9 adapter, which is too expensive (you need four), do you know of any alternatives?

 

[4season]

1.25 volt is likely a typo: The voltage is determined by the battery's chemistry.

Are your meter readings being influenced by light entering the camera's eyepiece? I've noticed that KMZ-made Zenit cameras can be particularly susceptible to this, including cameras made as late as 2005.

 

[runswithsizzers]

aside from the MR-9 adapter, which is too expensive (you need four), do you know of any alternatives?

First of all, I believe some built-in camera meters are a lot more affected by incorrect battery voltage than others. So without doing more research, it is hard to know if incorrect voltage is in fact likely to be what is causing your problem.

One commonly recommended solution to the mercury battery problem is to use zinc-air batteries which have a voltage very close to the 1.35V mercury cells. There is a brand, "Wein" which is selling zinc-air batteries for camera applications. But some say batteries used in hearing aids are more commonly available and less expensive.

This link discusses several alternatives in GREAT DETAIL, and provides more information than anyone would ever want to know (you have been warned):

https://www.butkus.org/chinon/batt-adapt-us.pdf

 

[Gero Giambrone]

Thanks everyone for your replies. I remembered having four 1.35V zinc-air hearing aid batteries. After the first reply, I connected them in series, obtaining a voltage of 5.4V like the 4 original Mallory PX 625 . Unfortunately, the light meter readings in low light are identical to those with 6V (like a 4RL44 battery) . I think the Zenit Automat light meter has a system that automatically compensates the voltage from 6v to 5.44v and it has two "sectors," one for high-light conditions (which works well), and one for low-light conditions (which overexposes by 1-2 stops) but 5V or 6. I'm afraid I need to open it up...

 

[Gero Giambrone]

In your opinion, when you see the camera open towards the end of the video, can you guess where to adjust the exposure meter?

 

[4season]

In terms of ISO, shutter speed and aperture, where are you observing errors?

 

[Gero Giambrone]

For example, with 100 iso and a value of 15 EV the reading is right, if EV is 8 it overexposes 1,3 stops