Hi Everyone,
(TLDR need to identify and lower the resistance in a semi functional camera without breaking the damn thing)

Odd little electronics project here that is being complicated by the fact I know ****-all about circuitry and electrical engineering.

I bought a 1970’s camera in exceptional condition except the light meter overexposes (the camera thinks it’s darker than it really is). Now it isn’t a huge problem because film has a fair amount of latitude (especially when it comes to overexposure) but it’s pretty damn annoying and I’d like to take a crack at fixing it.

The circuit itself is very rudimentary with two fixed resistors and a CdS cell that has a lower resistance when exposed to light. With the camera overexposing that means there is too much resistance in the circuit. I found the original service manual wiring diagram and a small snippet from an old repair guide that says you can calibrate the meter by swapping out the fixed resistor. This is where I run into problems because I have no damn clue what the current resistor is, which one to swap, and what to swap it out for. The marking bands are faded so visual identification has been difficult. I also have a multimeter but I’m not sure how to test and if doing so would brick the electronics in an otherwise mostly functional 45-year-old camera.

1) What’s the best way to identify these resistors?
2) What should I swap them out for?
3) How stupid of an idea is this?



May be able get a clearer picture of the resistor

 

RS will alter the current through the meter , lower value will make the meter read higher, and vice versa. Ideally we need to see the resistor.

 

The resistor that you should replace with a lower value is RS. To measure the value of it, first remove the battery so that your multimeter does not damage the galvanometer. You will have to experiment to get the right value. Replace it with a resistor that has about 10% less resistance for a start. A metal film resistor with a power rating of 1/10 watt or higher will do the job.
Good luck.
Keith

 

Stupid question but can’t you just select an offset film speed, for example tell it you’re using ASA 400 when you’re actually using 200?

Also, be certain your battery is good and the right voltage. My camera from that era would respond more slowly and give low readings if the battery was weak.

CdS cells age more than resistors tend to (which is not at all). You might consider replacing the cell.

 

Thanks for the responses!

You are correct Wayneh! Unfortunately with the age of the camera the ASA only goes up to 800. The meter is consistently off by 2.5-3 stops, so if I compensate mechanically I'm stuck with using 100 ASA film or lower. My preferred speed is 400/800 for interior work.

Been through a number of batteries and voltages trying to compensate and eliminate any possibility of that being the issue.

Replacing the CdS is intriguing though, how hard would it be to find something that would fit and have the correct response? In theory I can pop the front element off and get to the sensor without too much trouble.

Dodgy and Kieth: I did manage to get the meter on the resistors and it still worked when I put it back together! One measured 2k the other at 3.5k. Does that sound right? Wanna make sure I'm not screwing things up. I'm assuming the one with the lower resistance is Rs, the service manual is a little opaque on the subject.

 

If the calibration process is to change the resistor value, then the resistor must be removable. So I suggest carefully removing it and then measuring the value. But before that make sure that the window in front of the photo sensor i s clean A dirty window could also be a problem.

 

I think RS is the bottom resistor in your photo. I would not recommend replacing the CdS sensor. You would not find a direct replacement with the same value and linearity. A dirty window would definitely cause over exposure.

 

KW is right. And now it is also clear that it is very important to assure that the battery contacts are clean and not adding any voltage drop. And examining the photo in post#1, it looks like at least one end of the resistors could be unsoldered so that the value could be measured. And for experimenting with different values, short leads soldered onto chip resistors could be a useful option. But you will need a good magnifier, sharp tweezers, a steady hand, and a fine-point soldering iron.

 

Ideally if you can get a better picture of the Resistors would be better, you measured them in circuit which will give a false reading, ..

 

The sensor window looks pretty clear from the outside. Given the fact that the lens itself is dust free on the interior elements I'd bet the sensor window is in similar shape. That said it could be something like thoriated glass which can yellow over the years so I'll see if I can pop off the front element and get a better look.

Here are some macro shots of the resistors. I doubt I'd be able to get much closer but I'll keep experimenting with other lenses to see if I can get something clearer.

 

Ignore the previous photos, these are much cleaner.

 

Ignore the previous photos, these are much cleaner.

I wonder if all that rosin, or whatever the debris is, might be contributing to your issue. You might consider a careful cleaning.

 

Hello,

The tubular part on the left looks like a vintage capacitor.
It has a color code that can be seen on this picture:

The picture is taken from this page with more color codes:
https://www.instructables.com/From-Resistors-to-ICs-Color-Codes/

Bertus

 

That picture is much clearer. I can see now that the red wire is connected to the resistor on the right, so the other onethe left is definitely RS. Yes, you do need to disconnect one end of it to measure its value accurately. I think the colors indicate that it is 34 Kohms with 10% tolerance, but I could be wrong.
.

 

It certainly looks like i should not be hard to unsolder one end of the resistors so that the resistance can be measured. And they look like metal film resistors.

 

Looks like 34K to me, you could replace it 33K or 30K.

 

34k, Awesome! I'll see if i can track down some under that range.
My borrowed soldering iron is too bulky to get in there without demolishing it. I'll have to track down something else. I'll report back once I get something worked out.

 

Well none of my friends have anything small enough and none of the local repair shops will take a swing at it, so it looks like I'm buying a new soldering iron.
Can I get away with something cheap with a fine point, or will that cause more harm then good?

 

Hello,

Where are you located?
I have an ersa and a weller solder iron.
Those are available in several price classes.

Bertus

 

Can I get away with something cheap with a fine point, or will that cause more harm then good?

The topic of soldering irons comes up a lot around here. Here are just a couple threads.

https://forum.allaboutcircuits.com/threads/comparing-irons-for-a-beginner.105414
https://forum.allaboutcircuits.com/threads/which-soldering-station-would-you-advise.157645