fm2n off by 11 stops?

kipkeston

kipkeston

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I came across an fm2n whose meter is consistently over by 11 stops.

sunny-16 test has a correct meter reading with the following settings:

  • iso 200
  • f/2.8
  • shutter: 1 second

And this problem is setting invariant, i.e. it also gives a correct reading for

  • iso 400
  • f/2.8
  • shutter: 1/2 second

or

  • iso 6400
  • f/2.8
  • shutter: 1/30 second

Does anyone have a procedure for verifying how this can be fixed? Thanks

Actually it's over by 200 -> 100: 1 stop, plus 2.8 -> 16: 5 stops, 1 sec -> 125: 7 stops, so more like 13 stops?
 
Not being familiar with the model this is an educated guess, but as it's constant, I wonder if one of the meter cells has failed (ie not reacting to light at all?). Or the attachment of the cells to the prism has been compromised (opaque cement etc) and is preventing most of the light entering the lens from reaching the cells? Assuming the battery is the correct type and in good condition with good cell contacts there's got to be a component failure, yes it's not outside the bounds of possibility there is a galvanometer fault or fault in the wiring circuit, but I'm inclined to think the cells are the most likely culprit. Members with experience of the model ought to be able to offer more informed, specific comments of course.
Cheers,
Brett
 
You do have a lens attached, correct? I know you mentioned f2.8 but I just wanted to be sure. Eleven stops is a lot!
 
I'm not familiar with this model either, but hands down the most common meter fault I've encountered in old cameras is poor battery contact. Even if the contact surfaces look OK, they could probably be improved with thorough cleaning. I've also had a few that used a cap screw as one contact for a button cell where I had to add a brass strip to get good continuity. Get out your multimeter and start checking the circuit (and battery voltage, obviously).
 
Aperture coupling tab damaged or broken off? Assuming a aperture metering range down to f/128, 13 stops would be exactly the value I'd expect if the camera fails to read out the aperture on a lens set to f/2.8...
 
FM2n will not work with NAI, no way to fold up the index tab on the body.

I would check the tab!
 
Interesting tab theory. Does that mean that if there is a tab reading issue, I would expect the blades to fully close during exposure (even if set to say 2.8)?
 
kipkeston said:
Interesting tab theory. Does that mean that if there is a tab reading issue, I would expect the blades to fully close during exposure (even if set to say 2.8)?
Click to expand...

No, the actuation of the aperture mechanism itself is controlled via the coupling hidden within the lens mount when the lens is attached. A coupling tab or ring is a feature found in a number of pre-electronically controlled cameras that reads the position of the aperture ring. This will permit exposure readings to be made with the lens open, (as opposed to the stop down metering systems employed by many of the earlier SLRs to utilise TTL metering), because it communicates the working aperture to the meter system without a need to physically stop down the lens.
Cheers
Brett
 
Go here for a good explanation of Nikon AI issues/conversion.

http://www.aiconversions.com/history.html

As all of the images/info on the FM2n I've found point to it not having the ability to flip up the AI coupling lug I would need more details before I hazard more than a SWAG.

What lens are you using? Is it AI compatible? Does the AI lug on the body move freely and is the spring snappy?

Bill Bingham
 
Thanks for the clarity. The camera has seen a 45 pancake (ai-s not the GN) and a 28-75 tamron A09 (af-d like with an aperture ring).

The aperture tab looks like it's in good shape. It moves a bit when a lens is put on it, and gets pushed (rotates) when the aperture ring gets moved. All as expected visually.

Changing the aperture ring does change meter correctness in the direction you would expect. Things are just very off.

Is the battery the next thing to look into? Removing the bottom plate?
 
Are you getting these readings with the film counter past 1? i.e. after winding on the shutter mechanism several times?
 
Maybe pop the bottom off and check around the battery compartment for corrosion.

The wires are very thin and a little gunk could have a noticeable impact on continuity.

I have a bin of parts from a FM2, if you need a new compartment.
 
wouldn't give up on a possible tap issue - the tap is pulling a string which controls the meter. This string can brake and then you have exactly these readings. You need to check if there is enough resistance in the tap to come back to the original position at about 1 o'clock (it's being brought back to the "zero position" with the help of a spring .
You can also check if/ how the LED moves when you move the tap without a lens attached.

I had a FE, similar camera to the FM apart from the shutter, and this was the issue. Difficult to repair though...
 
I also have an FE2 where the spring that connects the tab to the light metering system inside the camera was broken. The symptom there was the tab would get pushed along by the lens as it was stopped down but would not spring back to its lens unattached position.
 
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