Nikon EM: Check and clean, issue with automatic shutdown of the exposure meter

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Andreas Thaler

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Erratum

The gear for driving the aperture simulator ring needs four full revolutions and not one corresponding to the service manual.

Otherwise it has too little power and cannot bring the ring into its starting position.
 
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Andreas Thaler

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This is what the EM looked like at the beginning.




I think it has also improved visually.


+++

All information provided without guarantee and use at your own risk.
 
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Andreas Thaler

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Fixing the piezo beeper

When I checked the EM today (day one after service/repair is always exciting) the piezo beeper in the top cover remained silent.

It signals overexposure or underexposure when you tap the shutter button, starting at 1/30 second, which is here the set limit for blur-free handheld shots.






I took off the top cover, checked the solder connections, and soldered on a second beeper, which worked. I was relieved because that proofed that the electronic signal is OK and I didn't have to worry about the circuit.

So I soldered the piezo beeper back on and now it delievers sound. Apparently I had soldered poorly yesterday.






I measured the beeper's input with the small oscilloscope.

There is a square wave signal at about 1.7 Hertz, which the top cover acoustically amplifies as a resonance body.








Glued on with Pliobond, the small sound generator returned to its place.
 
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Andreas Thaler

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I don't know much about audio and frequency technology, but this seems to be an alternating voltage signal, with the negative amplitude generating the sound? (Scope set on DC).
 
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Andreas Thaler

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Readjust the spring driven gear for moving the aperture simulator ring (Nikon AI coupling system)

As shown above, the aperture simulator ring is kept under tension via a spring-driven wheel to transfer the set aperture values to the camera.






The aperture simulator ring is rotated via a tab on the lens's aperture ring as well as a second ring and is pushed towards the largest aperture by the spring drive.

It turns out that the set spring tension is not sufficient, the ring gets stuck and no longer follows the aperture ring on the lens.








At first I assumed that the ring above it and the edge on the bottom of the bayonet needed a little bit of lubrication, but that wasn't the case. So I removed the greasy film.






After I turned the spring-driven gear wheel to about eight turns instead of four, the aperture ring worked.

It now willingly follows the ring on the lens again.
 
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Andreas Thaler

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Andreas Thaler

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When I checked the EM today, I noticed that the battery was drained to almost 0 volts.

This is obviously related to the light meter, which no longer switches off automatically.

I assumed that the M90 setting (mechanically generated time) would switch it off, as the measuring needle would then go to the off position.

Let's see what can be done.
 
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Andreas Thaler

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Automatic shutdown of the exposure meter depending on the set aperture



When testing the EM, I noticed that the light meter only switches off automatically after about 25 seconds if the set aperture is not smaller than f = 4 (measuring needle in off position).

The arrow indicates the corresponding position of the AI aperture coupling tab for orientation.




I therefore cleaned and aligned the wipers of the potentiometer for the aperture simulator …





… and cleaned the associated contacts.

But that didn't solve the problem. If the wipers are to the left of the red line in the image, the light meter stays on.


I've had some experience with these potentiometers. Sometimes they go crazy without it being clear why.

I'm therefore going to replace the ring with the wipers in the first step and in the second step the contact track with original parts from an abandoned EM.

If that doesn't solve the problem, I'll move on to the circuit diagram. At least I now have a solution to turn off the light meter.

I'll know if that's the case after checking the battery voltage today - I left the camera overnight with the batteries inserted.
 
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mshchem

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Oh my. I have never appreciated these cameras. I was always an F2 guy, F3 then F5. I have big hands, these small Nikons never fit. Too plasticy.
I recently picked up a nice FM3A, nice camera but again on the small side.
One thing nice about cameras like the EM is they are light. Definitely a plus. No way I could still lug a F5 around all day.
 
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Andreas Thaler

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The batteries do not show any unusual reduction in the open circuit voltage overnight.

The resistance between the positive connection in the battery compartment and ground is around 3.6 kOhm, regardless of whether the mode selector is set to AUTO, B or M90.

Without taking into account the internal resistance of the battery, at 3 volts open circuit voltage there would be 800 uA quiescent current:

I = U/R
I = 3 volts/3.6 kOhm ~ 0.0008 A = 800 uA

With internal resistance taken into account, a lower current flows.

I will measure the actual current.

It is important to note that there is no increased load on the battery when the camera is in standby mode.

Therefore no short circuit in the camera's electronics.
 

forest bagger

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This happens sometimes, when the potentiometer breaks by a hard impact - usually as a result of a fall of the camera with the lens attached.
 
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Andreas Thaler

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Replacing the potentiometer for the aperture simulator



This EM (with damaged rewind crank) is intended for spare parts and must hand over its potentiometer for the aperture simulator including ring with wipers.




The camera works, but the meter needle in the viewfinder jumps around like crazy.

This indicates that the potentiometer is dirty causing contact problems.

But the exposure meter switches off after about 25 seconds, even at small apertures.

That's how I want it to be with the repair candidate as well.




First I replace the ring with the wipers, they are in perfect condition. Clean with isopropyl alcohol.

But this doesn't change the behavior of my EM for the repair. The light meter only switches off up to aperture f = 4.




So I remove the plate with the resistance track.

You can see it here.




The plates differ externally, but hopefully not in their electrical values.

Above is the spare part EM, below is the repair candidate.

Both cameras differ in details, e.g. in the leatherette, the lens release button and the backlight button (blue - silver).




The ceramic plate has been removed and I cleaned the dirty contact tracks with isopropyl alcohol.

For this I had to remove the top cover as well as the bayonet cover.




The two versions.




I'm extending one of the two connection cables. Unfortunately, I don't have one in brown.

These are fine cables from other cameras that I collect for repair purposes.




The contact plate has been replaced, screwed on and both cables have been soldered.




Test - and unfortunately replacing the potentiometer did not solve the problem, the EM behaves as before, from aperture f = 5.6 the exposure meter no longer switches off.

If I increase the ASA number on the camera, it works with f = 5.6.

There is obviously a relationship between these factors.

But the measuring needle stands completely still in the viewfinder and no longer jumps back and forth due to the cleaning of the potentiometer.


Next

I'll take a look at the circuit diagram to see if there's an explanation.

But first, I'll check whether the EM with the replacement potentiometer still delivers plausible exposure values.

The camera can be used, although the aperture ring must always be set to f = 4 or larger apertures after taking photos so that the light meter does not drain the battery.
 
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Andreas Thaler

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The reference for the light meter is my Nikon F4E, which I work with.

It is one of the two F4s whose aperture control I repaired. The camera has worked perfectly since then.

Both exposure measurements on the yellow door of our refrigerator produces identical values. The replacement potentiometer can therefore remain in the EM.
 
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Andreas Thaler

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And while I'm sitting here in the kitchen writing, the EM surprises me.

When I set an aperture smaller than f = 4, after a while there was a click and the exposure meter switched off, the meter needle went up in the viewfinder.

This also works with the smallest aperture f = 22, but it just takes longer.

On the other hand, aperture f = 4 now significantly exceeds the 25 second switch-off time.

It looks like the light meter shuts off faster when there is more light.

The only reason I can think of for this behavior in the EM's analog control system is a faulty capacitor that doesn't charge and discharge reliably. This means that the timing is no longer correct. But that's just a guess.

Perhaps the circuit diagram will reveal something.
 
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Andreas Thaler

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Nikon EM Service Manual

Factory service manual
learncamerarepair.com

The circuit diagram shows that part of the circuit is housed in an IC (rectangle in the middle). What happens there is not documented.

I cannot tell from the circuit diagram how the time control for the light meter works, except that transistors, resistors and capacitors control the IC or are controlled by it.

As is often the case with such cameras, the electronics remain a mystery.

But maybe someone can recognize typical circuit parts?
 
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Andreas Thaler

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It is likely

that most electronic faults in these cameras can only be fixed by replacing the entire circuit board.

This in turn creates new problems during assembly if you are not familiar with the camera in question.

Replacing the electronics is a highly challenging task, including readjustment.

Who can do this and where can you get a fault-free replacement?


But that's probably how

the service technicians did it back then, because it was also the more economical option for individual troubleshooting. Except for known errors that could be resolved by replacing electronic components.

It's just a shame because you think you can troubleshoot on the circuit boards, some of which are still discretely assembled.

It seems like everything is right in front of you (which it is not, since parts of the circuitry are usually built deep into the camera.)

But how do you actually want to go about it, with what technical documents? Is the circuit diagram helpful when voltage information is usually missing and ICs are black boxes?

It's a real shame.
 
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forest bagger

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Andreas Thaler said:
But maybe someone can recognize typical circuit parts?
Click to expand...
I recognize an electrolytic capacitor of 1.5 µF (# 134) which is bridged by a resistor of 2.2 MOhm.
I could imagine that this resistor is broken, so that the capacitor doesn't unload within the intended time.
 
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Andreas Thaler

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Today the EM surprised me by switching off the light meter perfectly after about 25 seconds of inactivity. It doesn't matter what aperture is set and how bright it is.

I'm starting to get a bit scared of my cameras, they're obviously healing themselves

The winding lever swings back a little listlessly, maybe I'll take a look at that.
 

Exx-on

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Thanks for the super in depth write up and images for your EM teardown and cleanup!

I have a Nikon EM that I’ve inherited from my Grandad that has seemingly got a dim/flickering LED which I’ve been trying to get to the bottom of. It also seems to be a bit temperamental in terms of the light meter working and beeper indicating over/under exposure.

I may use this as a guide to try and see what’s going on. It had a cracked battery compartment housing that I’ve since repaired however that doesn’t seem to have fully fixed the issue, so may manually inject power after somewhat of a strip down to see if this fixes the issue.

She’s in mint condition!
 

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Andreas Thaler

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Thanks!

Congratulations and welcome to the workshop of the fearnoughts!
 
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