Nikon FM light meter not working

[UgoF]

Hi, I have a Nikon FM. The camera has always belonged to my dad and I have been using it regularly in the last 10 years with no issues at all. The only service it had in almost 50 years was the replacement of the small rubber bands nearby the mirror (they were falling apart).
For the first time in a long while, I have not used the camera for over a month. Maybe 5 weeks.
And since then the light meter just won't work:

• the light is stuck on minus most times;
• if setting the camera to ridiculous settings, eg ISO 3200, time 1/15, aperture 2.8 the light meter reacts - giving very wrong readings anyway: the three red lights blinks and sometimes they are on at the same time.

Based on what I have found on different topics in this and other forums, I have tried:

• replacing the batteries with two very fresh ones (expiry date 2025);
• swap lenses;
• try to read the light with no lens attached;
• move the iso/time ring back and forth several times;
• check and move the little lever on the camera body that gets the aperture setting from the lens;
• clean the battery compartment (which was clean anyway).

Nothing worked.
Did anyone of you have a similar issue? I have read of similar things, but noone reported three red light on at times.
Thank you so much for your attention.

 

[shutterfinger]

Go to https://learncamerarepair.com/productlist.php?category=2&secondary=8 , scroll about 2/3 down the page and download the FM service manual and wiring diagram.
Get a VOM and do some circuit tracing/testing.

 

[Ariston]

I have the same problem with one of my FMs. Shutterfinger is a pro, so "circuit tracing" probably comes easy for him - but it is outside my skill set. I let the gentleman in Camera Service Co. in Atlanta look at mine, and he said he had seen it often but didn't have the part to fix it. I think it was some disc under the ISO dial, but I am not sure.

Luckily, the camera will still shoot just fine. Download a free metering app.

 

[Mamiya_Repair]

In most cases with symptoms like you are experiencing, the issue on a Nikon FM is a damaged FRE assembly. The FRE (functional resistor) is on a brittle substrate and can be easily cracked. The part is no longer available through normal channels.

 

[UgoF]

Hi everyone, first of all many thanks to all of you for your replies.
@shutterfinger: I have downloaded the manual and wiring diagram. I am afraid that I will do more harm than good just disassembling the body for testing. Furthermore, I do not own the right equipment. Servicing my FM is something I would love to do (I like to work on mechanical things) but I would do that with an experienced person at my side the first time showing to me step by step what to do.
@everyone: looking on the web, I have found a small laboratory not too far away from my hometown (about 80km), where a guy in his 80s still repairs Nikon cameras. I dropped him an email stating my problem, let's see if he has the necessary spare parts and can do it. In the meanwhile I will shoot with my external light meter (an old lunasix)

 

[UgoF]

Sorry I have missed to say a thing that might be important. I had just completely forgotten about that.
The last time I had used the camera, I used an extension ring on it. I had never used it before. My dad found it and gave it to me and I wanted to try it just for the sake of it.
Could it have done some kind of damage to the camera?

 

[reddesert]

The aperture sensing ring on the FM moves with a tab on the back of the lens, and has a little button to flip it up for use with non-AI lenses where the aperture ring would jam the sensing ring.

Often, non-AI extension tubes don't project far back enough to interact with the aperture sensor. But if they did, they could jam it.

You mentioned testing that the ring moves freely, so this probably isn't the problem (also, make sure the little sensor is actually flipped down). Even if it were, the meter would probably read wrong rather than fail to function.

The FRE variable resistor track that Mamiya_Repair refers to, on the FM, is under the ISO dial. There could be some small possibility that it's just dirty, but I don't know the odds.

 

[shutterfinger]

Unlikely that a extension ring for Nikon F mount would do any harm. It would have to be damaged and difficult to attach first.

The disk under the ISO dial is likely a resistor like a volume control on a analog radio, the ISO dial has a wiper set of contacts (FRE 570) and the wiper contact sets the metering system for the speed of the film in use.
Variable resistors (potentiometers) are made in linear and logarithmic types. If one can find a potentiometer with the same electrical and physical characteristics as the Nikon original then substitute it into the camera should be possible.

I am afraid that I will do more harm than good just disassembling the body for testing.

I do not assume ones abilities when providing links to service manuals or providing service information. I do assume you, the reader, to have the ability to decide if the repair is in your capabilities.

 

[John Koehrer]

Nikon had at least one camera that used glass as a substrate for the resistor under the ISO dial.
I don't think it was one of their better ideas, but I had one occurrence where the glass had cracked.

 

[UgoF]

Once again, thank you all for the information and support.
I have decided not to bring the camera to the laboratory I had found due to too many very negative reviews I had read about them. People were mainly complaining about having their cameras not assembled properly after the intervention. As my camera is working fine - except for the light meter - I would rather use it as it is. I am not usually sentimental with things, but this is the camera that my father used to take the first shot of me as a new born, and the first camera I had taken a shot with. So I would rather use it with my lunasix.
There is a possibility that Nital (Nikon Italy) will do the job. Let's see.

 

[UgoF]

Hi all, one more post on this as something odd happened.
I was playing around with the camera - changing speed and asa - and noticed that now the light meter is behaving differently. The leds stopped blinking all together randomly as they did before.
The reading is still wrong, but consistently by two stops. EG same lens on a different camera (Nikkormat) goes with 400asa f5.6 1/15 while on the FM goes with 400asa f5.6 1/4
So setting the asa to 1600 gives what seems to be the right reading.
As far as it is consistent I am happy enough. Any recommendation from your side?

 

[Ariston]

I wouldn't rely on it too heavily. As they say, even a clock is right twice a day. It may be just making a somewhat decent connection at the moment, and a good bump may change that... who knows.

 

[UgoF]

@Ariston you are so right. I have just developed the last film shot with the FM. Most photos - from about 1 third of the film - were way overexposed. Meaning that the light meter was already giving wrong indications before become obviously faulty.
@reddesert The interesting thing is that pictures are overexposed after the shot I took with the tele extender (a TC-200). Probably it is just coincidence, but before that the shots were fine.

 

[shutterfinger]

Being you started having trouble after using the TC200 I suspect a problem in the aperture coupling or the switch (lever you flip up for non AI lens). The disassembly begins on pdf page 183 of the manual https://learncamerarepair.com/downloads/pdf/Nikon-FM-Revised-Service-Manual.pdf
The lens mount ring screws have a thread locker on the threads that is dissolved with Acetone. Some nail polish removers are Acetone. Put a small amount around the screw head and allow to sit for a few minutes.
A loose electrical contact or one touching something it shouldn't will cause metering problems.
Removing the lens mount and reassembly is relatively easy.

 

[reddesert]

The TC-200 should be AI, so it shouldn't have damaged the AI coupling, but perhaps something happened during mounting or dismounting.

Obviously something is wrong with your meter, given the odd behavior of the LEDs, and the incorrect readings. One thing to check is that the aperture sensing ring rotates freely and springs back as it should.

But since you mention overexposure, another thing, just to be sure, is to check that the aperture stop down lever is moving when you fire the camera, and that the aperture of the lens is closing down. Sometimes apertures get stuck from disuse and stay open.