Tech Question, re Nikon FTN CdS Cell Resistance Measurements

Ive got 2 FTN Finders here, both working but both have gone non-linear. They'll both read perfect in the mid ranges. But as soon as I throw a test wall at it down at 1/8 sec and f/2.8 (ASA 200), It's off by a stop or more, tested against my Sekonic 308. On the other end if if throw a 1/250 or 500 f/11 at it, It's off a stop and a half at that end. The typical first suspicion is that one of the 2 cells has gone sour. Since they're wired in parallel, I'm going to have to de-solder one leg on all 4 cells and set up a test light source and use my multimeter to find any 2 that match the best. Logical. Test each one with the same light source, and repeat with no light. But I don't know what the ohmeter readings are supposed to be on these cells. I thought about going on Digikey and studying data sheet on CdS cells to get an idea. But these Nikon cells may very well have a built-in neutral density filter or a lens. Sure would be nice to know the official Nikon resistance data on these cells Thank you.

I’d pick up a couple parts FTN finders, that way you get them as they came from the factory. I put a generic cds in a Pentax finder and can’t get low readings to work. But parts bodies give me good cells almost all the time.

I got hold of a service manual, which was of some small amount of help. Still pretty much on my own, I desoldered one leg each on all 4 cells (2 each), and measured them. One was OBVIOUSLY off. The other 3 were not much of a match. So I took he closest of 2, for the better or worse of it, Being after dark and having only the incandescent light on my work bench and a wall in a darker corner of the room, the results were not hopeful. I need to wait till daylight tomorrow and meter the street in the full sun, and a white wall in my shaded garage. Both readings are at the extremes of the range, and both similar color temperatures. Right now, the yellow of a light bulb is not really ideal. Oh well. This finder is for my F with motor drive. It's already a big piece of beef. If no luck, I'll just put the plain prism finder back on it and use my Sekonic 308

Edit: Forgot to mention, according to the service manual, the factory apparently called for ND filter inserts to "tune" any mis-match at the factory in the 2 CdS cells. Although I saw no such inserts. At least where I logically reasoned they would be, if there were any.

Well here's the final verdict. Walking around the yard metering things at 200 ASA, I can meter the gray asphalt road in full sun at 1/500 and f/11 and the FTN finder really wants me to close down another 1/2 stop at most. And walking around metering differnt things in the partial sun or shade, it's all pretty much dead on the money, Then walked into my garage with the garage door open, the back wall of the garage shows 1/8 sec and F/2.8 on the Sekonic. But the camera really wants me to open up another 1/2 stop. So I believe I can go ahead and call it good and finish buttoning up the finder, and go ahead and leave it on the camera. Not as accurate as my other F or my F2, but considering the age of it, still usable. If I go out and leave the Sekonic at home, I believe I can still get by alright. I'll just leave the top leather off till these Wein cells go dead. Then I'll just get the rings off them and put 357's in there, and pop the top cover back off and recalibrate a hair for 357 batteries. So I suppose all in all it's a semi success. Not perfect, but likely good enough. The other junk finder is now missing a CdS cell. So I believe I'll stick the "bad" cell in it and go out and play and see how it does. Always a small chance that one could turn out better than this one. If not, I guess I'm pretty happy. Before, I had 2 worthless finders. The moral is ], metering indoors at night by the light of an incandescent bulb is never going to lead to accuracy with any kind of light meter.

To cap it off... for kicks I put the "bad" CdS cell in the parts finder, just to see how it would do. Looks like I did indeed pick the best 2 out of 4. Because now, that parts finder meter is a mile off anywhere. Neither the high end or the low end is anywhere near usable. I paid $29.84 including tax and shipping on ebay for it. So I suppose I started off with what comes out of the chicken and still managed to make a chicken salad sandwich out of it. And still have all the other parts in fine condition to keep, to boot. Who knows? Maybe 2 extra-nice Nikon FTN CdS meter cells will come along some day, looking for a home.

Oh! I didn’t realize one was already a parts finder.

Remember, I dealt with some of those issues in this thread. BTW that finder from the thread still works perfect with the replacement cells.