Diagnose problem with fm2 lightmeter

[pallo]

The lightmeter on my Nikon FM2 (an FM2/T I bought new in 1998 or so) is showing erratic behaviour and wrong metering results, but I cannot link the issue to any of the commonly reported problems.

The leds work, which suggests that there is no battery issue and the AI tab does not seem to be stuck either.

But: when metering it moves from + to - in a totally erratic way and never settles on a 0. This happens even when I meter on an evenly illuminated surface. The metering results also do not match what I get with a handheld meter (both reading reflection on same surface).

What else could I check to narrow down the possible issues?

 

[Chan Tran]

Does the meter stay mostly at "-" and only flashes "+" real quick sometimes?

 

[pallo]

Does the meter stay mostly at "-" and only flashes "+" real quick sometimes?

Yes! That seems to capture the problem quite accurately.

 

[Chan Tran]

Yes! That seems to capture the problem quite accurately.

So it seems to me that the sensor receives no light or its output is bad. The camera acts as if the lens cap is on so most of the time it shows "-" but when you adjust the shutter speed/ ASA/ aperture combination out of its metering range it flashes + and 0 quicky.

 

[pallo]

An additional check shows that it is AI tab in combination with one speicific lens is the culprit after all.

Metering is fine with nikkor lenses, but my recently acquired Voigtlander 40mm in sl mount (the nikon f mount) does appear to cause some problems with the ai-tab. And this leads the camera to believe that that the diafragm is entirely closed, I guess

 

[Chan Tran]

I am glad you found that out. I found if the AI coupling is pushed beyond the supposedly maximum position that would happen.

 

[pallo]

This issue actually continues to bother me, as it turns out that the problem is really the combination of this particular lens with this body. With a different lens on the FM2, all is fine. With the lens on my F90, all is fine as well.

Should I just give up on this lens and replace it by a Nikkor AI(s) instead?

 

[FotoD]

How strange. Isn't the position of the Al tab the only communication between lens and camera? I'm trying to understand how just the one lens is causing problems.

Hope you solve the problem.

Edit:
I could add that I've had problems with a jumping meter needle on other Nikon cameras. Then the cause was a short circuit inside the camera.

 

[pallo]

Thanks. This prompted me to try to reproduce my earlier findings. I got the same flaky response using another lens as well.

And then I noticed that the problem disappeared after a few fast back and forth movements (from open to close and back) of the aperture ring. And this may explain why I initially got different results with different lenses. Switching lenses was probably sufficient to unstuck the AI tab.