Nikon F Photomic Meter Doesn't Work

[Ariston]

I have an ugly photomic finder with a meter that works perfectly, and a beautiful photomic finder that doesn't work at all. Are there any common, easy-to-fix, issues I can check with the non-working finder that might bring it back to life? I've already tried taking a hammer to it.

 

[DonW]

youtube is your friend

 

[Huss]

I have a photomic meter for the F2 that works, but is inconsistent. Sometimes it seems accurate, other times way off. I think it is the DP1 head.

 

[shutterfinger]

Some info available here: http://arcticwolfs.net/
More info/service manuals for your finder(s) here: https://learncamerarepair.com/productlist.php?category=2&secondary=8

 

[guangong]

As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. A plain meterless prism makes a well-balanced F that is a delight to shoot with. Try it.

 

[Ariston]

As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. A plain meterless prism makes a well-balanced F that is a delight to shoot with. Try it.

I agree! I would love to. The prices are insane for the F prisms.

 

[Huss]

As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. .

What part is that?

 

[Kino]

Probably the ring resistor...

 

[Huss]

Probably the ring resistor...

Sover Wong has that part. He has had new ones made.

 

[benjiboy]

Sover Wong has that part. He has had new ones made.

Sover Wong has the ring resisters he indeed has them made but he only uses them to service Nikon F's and F2s he doesn't sell them.

 

[BradS]

Sover Wong has the ring resisters he indeed has them made but he only uses them to service Nikon F's and F2s he doesn't sell them.

I thought that Sover only works on the F2 body and finders?

 

[Pentode]

As I understand from what technicians have told me, the same little part wears out, Therefore, no replacement parts. A plain meterless prism makes a well-balanced F that is a delight to shoot with. Try it.

With all due respect, aren't we getting a little ahead of ourselves here?
Before just assuming the part is worn out, it would probably be worthwhile to give the OP a chance to research those links and see if he can fix it. Some ring resistors do, in fact, wear out but others just get oxidized and dirty.
Let's not write the meter off before anyone's even opened it up....

 

[RLangham]

I have a photomic meter for the F2 that works, but is inconsistent. Sometimes it seems accurate, other times way off. I think it is the DP1 head.

If it is called a Photomic it is the DP1. It has a needle for exposure indication, doesn't it? And takes pre-Ai lenses? At any rate it should say on the bottom of the finder housing towards the front, I think.

There are names for every combination of standard finder and the F2 body. F2 Photomic is DP-1. Mine is F2SB, indicating DP-3, with the improved three segment display and backlit exposure information indicators.

 

[BradS]

...but from the title, the OP has a Nikon F - not an F2.
I was under the impression that virtually none of the light meters in the old F finders worked accurately anymore....seems like the CdS cells have all wandered off into the non-useful area.

..just my impression though.

 

[reddesert]

My Nikon F metered prism works okay and seems to be accurate (tweaking the ISO less than a stop to adjust for modern batteries). I have had it for over 30 years and used it only sporadically recently, so it has been kept clean and the ring resistor hasn't been abused.

I don't think CdS photoresistors "wear out" with age. It is claimed that CdS photo-resistors could die from corrosion that causes the contacts to go bad, but this would be externally induced damage, while some of the age effects on selenium cells are claimed to be damage from the internally generated current. (CdS photoresistors, of course, don't generate their own current.)

 

[RLangham]

My Nikon F metered prism works okay and seems to be accurate (tweaking the ISO less than a stop to adjust for modern batteries). I have had it for over 30 years and used it only sporadically recently, so it has been kept clean and the ring resistor hasn't been abused.

I don't think CdS photoresistors "wear out" with age. It is claimed that CdS photo-resistors could die from corrosion that causes the contacts to go bad, but this would be externally induced damage, while some of the age effects on selenium cells are claimed to be damage from the internally generated current. (CdS photoresistors, of course, don't generate their own current.)

Plenty of Minolta srT meter perfectly, and they date to roughly the same era as the photomic T or TN finders. 60's CdS cells are often as not in fine shape if the rest of the camera is.

 

[BradS]

....
I don't think CdS photoresistors "wear out" with age. It is claimed that CdS photo-resistors could die from corrosion that causes the contacts to go bad, but this would be externally induced damage, while some of the age effects on selenium cells are claimed to be damage from the internally generated current. (CdS photoresistors, of course, don't generate their own current.)

What makes the meter in the DP-1 and DP-11 or Spotmatic for example go non-linear (eg off by 2 stops in bright light and of by 4 or 5 stops in dim light )? I've always been under the impression that it was the CdS cells...it does not happen to light meters with SBC cells and it does not happen to all meters with CdS cells. It certainly seems as though some CdS cells deteriorate with age

 

[BradS]

Plenty of Minolta srT meter perfectly, and they date to roughly the same era as the photomic T or TN finders. 60's CdS cells are often as not in fine shape if the rest of the camera is.

I've had a few SRT cameras...three SRT-101, a very nice SRT102, and two SRT202. None of the three SRT-101 meters were sufficiently linear to be useful - even with a regulated 1.35vdc power supply. The others all seemed to work perfectly. The meter in the SRT-102 was of the 'CLC' variety...it seemed to have its own ideas about subject matter...but was usually perfectly usable. I gave the SRT-102 to my youngest son with a 58mm f/1.4 MC Rokkor-PF and a 135mm f/2.6 Tele-RokkorPF...he loves it.

Haceta Head, Oregon by Brad S, on Flickr

 

[reddesert]

What makes the meter in the DP-1 and DP-11 or Spotmatic for example go non-linear (eg off by 2 stops in bright light and of by 4 or 5 stops in dim light )? I've always been under the impression that it was the CdS cells...it does not happen to light meters with SBC cells.

First, to keep this really on-topic, I found this schematic of the FTn meter, which might be useful to the OP:
http://www.peterrussellphotography.com/nbr/Nikon_F_Photomic_FTn_meter_repair_instructions.pdf

A couple of things to note:
- The circuit is extremely simple. It's a voltage divider with the CdS photoresistors in series with a fixed 41 ohm resistor and then the meter+trimpot in parallel with the main pot (the infamous variable resistor). The circuit must be designed and trimmed for whatever the CdS resistance vs illumination curve is, over a wide range. It's impressive that such a simple circuit works.
- The currents involved are tiny. CdS cells have a resistance up to over a megohm in a relatively dark scene. The author of that schematic says the meter is centered with a current of 16 micro-amps.

What this means is that any extra resistance could really mess things up. For example, corrosion at the CdS cell terminals, or extra resistance at the wiper of the potentiometer (which seems more likely). That extra resistance could easily make the circuit non-linear.

SBC cells are silicon photo-diodes, not photoresistors, so a circuit with SBCs would be completely different. I have no doubt that SBCs are superior, but they're also a lot newer. A major reason for going to SBCs (as I understand it) was that they are a little more sensitive and a lot faster to respond to changes in light.

 

[BradS]

First, to keep this really on-topic, I found this schematic of the FTn meter, which might be useful to the OP:
http://www.peterrussellphotography.com/nbr/Nikon_F_Photomic_FTn_meter_repair_instructions.pdf

A couple of things to note:
- The circuit is extremely simple. It's a voltage divider with the CdS photoresistors in series with a fixed 41 ohm resistor and then the meter+trimpot in parallel with the main pot (the infamous variable resistor). The circuit must be designed and trimmed for whatever the CdS resistance vs illumination curve is, over a wide range. It's impressive that such a simple circuit works.
- The currents involved are tiny. CdS cells have a resistance up to over a megohm in a relatively dark scene. The author of that schematic says the meter is centered with a current of 16 micro-amps.

What this means is that any extra resistance could really mess things up. For example, corrosion at the CdS cell terminals, or extra resistance at the wiper of the potentiometer (which seems more likely). That extra resistance could easily make the circuit non-linear.

SBC cells are silicon photo-diodes, not photoresistors, so a circuit with SBCs would be completely different. I have no doubt that SBCs are superior, but they're also a lot newer. A major reason for going to SBCs (as I understand it) was that they are a little more sensitive and a lot faster to respond to changes in light.

I do not see how adding resistance can possibly make the circuit respond such that the meter is off by 2 stops in bright light and off 5 stops in dim light - a situation I have seen in at least four different DP-1 meters and several Spotmatic meters.


EDIT:....ah, wait...l think I get it now...
The CdS cell's response is non-linear with a region that is linear enough to be useful. So, adding some small resistance to the circuit could push the CdS cell up into the highly non-linear area of it's response curve !!!
Thanks for helping me think this through. It has bugged me for a very long time.

 

[Bikerider]

I thought that Sover only works on the F2 body and finders?

I do believe that he is working on a limited number of 'F' models. But as he has enough work keeping F2's in working order I think the servicing of an 'F' Photomic head will be well down the pecking order.

 

[BradS]

....ah, wait...l think I get it now...
The CdS cell's response is non-linear with a region that is linear enough to be useful. So, adding some small resistance to the circuit could push the CdS cell up into the highly non-linear area of it's response curve !!!
Thanks for helping me think this through. It has bugged me for a very long time.

No. That does not make sense....unless the response of the CdS cell depends upon current as well as light.
Hmmm...I'm back to thinking that some CdS cells somehow deteriorate with age.

 

[reddesert]

I do not see how adding resistance can possibly make the circuit respond such that the meter is off by 2 stops in bright light and off 5 stops in dim light - a situation I have seen in at least four different DP-1 meters and several Spotmatic meters.


EDIT:....ah, wait...l think I get it now...
The CdS cell's response is non-linear with a region that is linear enough to be useful. So, adding some small resistance to the circuit could push the CdS cell up into the highly non-linear area of it's response curve !!!
Thanks for helping me think this through. It has bugged me for a very long time.

What I would say is, there's nothing really linear in this circuit, except the main potentiometer, which is presumably a linear pot. The CdS photoresistor is unlikely to have a perfect logarithmic dependence on the light intensity, but the circuit has extra elements to allow trimming to balance it at low and high light levels. I haven't worked it all out yet, but for an example, look at this photoresistor datasheet: https://cdn.sparkfun.com/datasheets/Sensors/LightImaging/SEN-09088.pdf
This isn't the CdS cell in a Nikon F, of course, but most CdS cells have sort of similar behavior. You can see that as the illumination increases from 1 to 100 lux, the resistance drops from about 60 to 3 Kohms. The response is a straight line on a log-log plot, but it doesn't have a slope of 1 (100 times more light = 20 times less resistance). And this is over 100x in light, which is only a range of 7 f-stops.

The meter circuit has to arrange balancing the current through the meter+trimpot with the current through the main pot, which may be linear with the position of the aperture and shutter rings, but without looking at the mechanical linkage I don't know.