Nikon F4 Autofocus works only one direction - the other direction is slow/sluggish

tilder · Nov 1, 2024

Hi,

new forum member here

I bought an F4 cheap and I think I need to fix the autofocus. I got it working nicely from the start, it was grumpy but it slowly started to operate well. But then after a couple of days the focus worked in only one direction. Going from far away to closer works, but the other way is either slow, sluggish or not working at all. It varies, but I haven't figured out what makes it work/not work.

Does anyone recognise this problem? Is this a problem with the motors (are there separate motors for each direction or just that the motor work well in only one direction)?

Any tips on this is greatly appreciated

Chan Tran · Nov 1, 2024

I don't know. My F4 AF works OK but I have an N2002 that definitely focuses only in one direction. If I set the lens to infinity then have it AF it would work fine. But if I set the lens to a distance closer than the subject it will not move.
On your F4 does the focus confirm light work correctly? If it's the motor or motor cicuitry does it work with an AF-S lens?

tilder · Nov 1, 2024

Chan Tran said:
I don't know. My F4 AF works OK but I have an N2002 that definitely focuses only in one direction. If I set the lens to infinity then have it AF it would work fine. But if I set the lens to a distance closer than the subject it will not move.
On your F4 does the focus confirm light work correctly?

Yes, the lights work correctly. Sometimes you can hear that it tries to move but it is just not moving, or it moves very slowly. Same as with your N2002 - works from infinity to closer, but not the other way.

neilt3 · Nov 1, 2024

Does it behave the same way with lenses with built in AF drives or just the screwdriver types ?

If it's both types , it's not the in body AF motor .

Sometimes the AF module gets dirt on it and prevents proper functioning.
With the mirror raised , used a blower bulb to blow any dust etc out of the mirror chamber .
The AF module being in the bottom of it .

tilder · Nov 1, 2024

neilt3 said:
Does it behave the same way with lenses with built in AF drives or just the screwdriver types ?

If it's both types , it's not the in body AF motor .

Sometimes the AF module gets dirt on it and prevents proper functioning.
With the mirror raised , used a blower bulb to blow any dust etc out of the mirror chamber .
The AF module being in the bottom of it .

Thanks, I haven't tried with a lens with an internal AF motor, that's a good idea! I only have AF lenses without motors, but will look for one with. Will check the mirror chamber too.

It's my impression that the camera knows it needs to move the focuser ring in a certain direction, but it can't. The frustrating thing is that it works sometimes, and then it slowly gets more and more sluggish before it doesn't work at all. Then, suddenly it works for a brief moment a couple of weeks later.

forest bagger · Nov 2, 2024

tilder said:
The frustrating thing is that it works sometimes, and then it slowly gets more and more sluggish before it doesn't work at all. Then, suddenly it works for a brief moment a couple of weeks later.

I think the commutator in the AF-motor is worn, maybe by 10,000 turns or by salty water?

tilder · Nov 2, 2024

forest bagger said:
I think the commutator in the AF-motor is worn, maybe by 10,000 turns or by salty water?

Hard to say, no indication of salt water, but the camera is probably well used, most owners were press photographers.

Is it possible to replace the motor, meaning, are there replacements that would work for this old camera? There are cheap F4s around, so it's probably a matter of buying another one, but it's cool to have a project

Andreas Thaler · Nov 2, 2024

tilder said:
Is it possible to replace the motor, meaning, are there replacements that would work for this old camera? There are cheap F4s around, so it's probably a matter of buying another one, but it's cool to have a project

This should be feasible.

Nikon F4: Mission Aperture Lever - two ways to fix the aperture problem

{Moderator note 12 Jan 25: five threads on the F4/F4S aperture lever repair have been merged into the present thread. You may encounter one or two dead links to 'other' threads. The posts they pointed to, are now all part of this thread, which mostly still follows the journey in a logical and...

www.photrio.com

tilder · Nov 2, 2024

Andreas Thaler said:
This should be feasible.

View attachment 382447

Nikon F4: Mission Aperture Lever - two ways to fix the aperture problem

{Moderator note 12 Jan 25: five threads on the F4/F4S aperture lever repair have been merged into the present thread. You may encounter one or two dead links to 'other' threads. The posts they pointed to, are now all part of this thread, which mostly still follows the journey in a logical and...

www.photrio.com

Thanks! I’ve seen your posts on the aperture lever regrease and I’m considering following it to do both the regrease and service on the AF motor if possible. Ebay has F4s for parts very cheap and that’s the option go to for I think.

Andreas Thaler · Nov 2, 2024

AF coupler area in the mirror box:

Nikon F4 Repair Article

Article from the Society of Photo-Technologists Journal

learncamerarepair.com

Please ignore the arrow.

For more photos of the interior, see my various posts on the Nikon F4, F4S and F4E.

As far as I know, the AF cannot be adjusted without Nikon software. See the SPT repair article (link above) and the Nikon Service Manual for the F4:

Nikon F4 Repair Manual

Nikon F4 factory repair manual

learncamerarepair.com

Chan Tran · Nov 2, 2024

I think it has to do with the worn mechanical linkage from the motor to the screw driver. The motor commutator won't make it more problem of turning one way than the other. But a mechanical linkage can be worn in such a way that there is binding in one way but not the other.

tilder · Nov 2, 2024

Chan Tran said:
I think it has to do with the worn mechanical linkage from the motor to the screw driver. The motor commutator won't make it more problem of turning one way than the other. But a mechanical linkage can be worn in such a way that there is binding in one way but not the other.

Thanks, these are very helpful suggestions. I will look for a F4 replacement and when I have one, I will repair this one to the best of my abilities.

Andreas Thaler · Nov 2, 2024

tilder said:
Thanks, these are very helpful suggestions. I will look for a F4 replacement and when I have one, I will repair this one to the best of my abilities.

Take a look at how to correctly couple the mirror box and the back, it's a procedure. The F4 is docile when disassembled, but not when assembled.

This war horse requires an orderly sequence of disassembly and assembly

Andreas Thaler · Nov 2, 2024

Chan Tran said:
I think it has to do with the worn mechanical linkage from the motor to the screw driver. The motor commutator won't make it more problem of turning one way than the other. But a mechanical linkage can be worn in such a way that there is binding in one way but not the other.

The F4 is designed for this kind of stress (tested shutter count = 150,000 exposures1 ~ 4200 film cartridges of 36 exposures), so how could such severe wear occur?

At best, I think it could be dirt or a problem with the electronic control of the AF. These cameras were designed for continuous use.

1

The Shutter Unit in Nikon F4 - Part III

Andreas Thaler · Nov 2, 2024

tilder said:
Thanks! I’ve seen your posts on the aperture lever regrease and I’m considering following it to do both the regrease and service on the AF motor if possible. Ebay has F4s for parts very cheap and that’s the option go to for I think.

Good luck!

Show us how you're getting on with some photos

Chan Tran · Nov 2, 2024

Andreas Thaler said:
The F4 is designed for this kind of stress (tested shutter count = 150,000 exposures1 ~ 4200 film cartridges of 36 exposures), so how could such severe wear occur?

At best, I think it could be dirt or a problem with the electronic control of the AF. These cameras were designed for continuous use.

1
The Shutter Unit in Nikon F4 - Part III

how do we know how many shutter actuations that F4 has?

forest bagger · Nov 3, 2024

Chan Tran said:
The motor commutator won't make it more problem of turning one way than the other. But a mechanical linkage can be worn in such a way that there is binding in one way but not the other.

I encountered some motors where the sliding contact springs had gotten weak.
And some AF-motors have a slip clutch in its gear - I don't know if the F4 has one?

4season · Nov 3, 2024

If no obvious mechanical issues are found, I'd trace the motor wiring and see if there are burly MOSFET devices connected to it, check and replace as needed. It's common to encounter circuits like this:

Motor controller IC -> Power MOSFET -> Motor

Those MOSFETs can take a beating, and you might find additional protection for those devices in the form of zener diodes, which would serve to dampen inductive surge currents from the motor. And if the MOSFET is failing, I'd replace any associated zeners while I was at it.

Edit: And if you haven't already done so, I'd try another lens, and if the problem only occurs with a particular lens, then it may be the lens itself which needs servicing.

tilder · Nov 9, 2024

Here is a video that shows the different speeds for both directions. Sometimes it's better that this, and other times it doesn't move at all in the slow direction. From infinity to closer it's always working.

Andreas Thaler · Nov 9, 2024

tilder said:
Here is a video that shows the different speeds for both directions. Sometimes it's better that this, and other times it doesn't move at all in the slow direction. From infinity to closer it's always working.

The video can thus only be viewed with a Google account.

tilder · Nov 9, 2024

Andreas Thaler said:
The video can thus only be viewed with a Google account.

Oops - I'm trying to figure out how to share it - I don't know if links work, but maybe try this one:
https://www.dropbox.com/scl/fi/1nqi...ey=n7ln2o5u2df2b83lox4zjbbrf&st=ttepeiyp&dl=0

forest bagger · Nov 9, 2024

In this video the AF 50/1.8 Nikkor seems to have a damaged focus thread.
I think you should try this with an other lens!

tilder · Nov 9, 2024

forest bagger said:
In this video the AF 50/1.8 Nikkor seems to have a damaged focus thread.
I think you should try this with an other lens!

Thanks, that might be the case, but it's the same symptom here with my 85mm f1.8 lens. This is as good as it gets, it's usually more sluggish than this:
https://www.dropbox.com/scl/fi/0jbn...ey=fat3qfhb1slp73xrqo8r2orqr&st=ct26b71d&dl=0

forest bagger · Nov 9, 2024

Then there is no mechanical defect, but something wrong with the differentiating electronic of the phase detect autofocus system.

Nikon F4 Autofocus works only one direction - the other direction is slow/sluggish

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