Fixing Mamiya CdS Meter

[brofkand]

I have a Mamiya m645 CdS prism I bought from KEH that was described as having an inoperative meter. I saw this as a challenge, not a hindrance.

I guess it's because I'm a guy. I like buying broken things cheaply and fixing them. But I don't like to do it without knowing what I'm up against first.

Does anyone have any tips on fixing this meter? CdS photoresistors are pretty cheap at Radio Shack, but I've not yet opened the thing so I have no idea what it even looks like, if it is a standard CdS photocell or something else. Is it even worth fixing it? Should I just spend the $70 or so and buy one with a working meter?

Thanks for any help!

 

[Curt]

I've seen them too at a reasonable price with a broken or nonworking meters and wondered what it would take to get one working again. Maybe a short or fault or a component burned out etc., I fell asleep that day in electronics class when they went over oscilloscope but it may be worth looking at. If nothing else you will have a Mamiya CdS finder with a broken meter.

I'd like to add, ask the question, what is the most likely component, part, thing to go out in a prism meter.

 

[brofkand]

That's exactly my thinking Curt. I have a finder for my camera whatever happens. I may or may not have a meter, but I'll still have a finder.

If I can save $50 by putting in a new $2 photoresistor by myself, why not? But I doubt it's anywhere near that simple. Hopefully someone here on APUG will have done something similar to this before that I can at least start with.

 

[archphoto]

Try to make a scematic for your self first: it makes thinking about the repair a lot easier.

Check the battery compartment and check if the moving part of the meter with the needle att to it still is in its propper place.
(If it has a meter in it......)
CdS cells seldom fail, so the trouble could be somewhere else.

If you have the diagram made up and still don't know you can send me a PM.
Most of the time these circuits are pretty simple in design.

Good luck,
Peter

 

[John Koehrer]

CDS cells do age & become less responsive or die over time.

 

[brofkand]

John,

That's news to me. I had always thought CdS cells were pretty stable, it's selenium cells that give out over time.

Does anyone have (or know where I can get) a service manual for this prism? I tried taking it apart today, and afraid I'd lose a screw or try to pull something I shouldn't, closed it back up without making any progress. I feel confident if I have a good idea of what I'm up against I can do it; I've done much more difficult things than this with a soldering iron and eyeglass screwdriver before.

 

[John Koehrer]

Selenium cells don't usually die. The contact between cell & frame typically go through corrosion, tarnish or even shock. Sometimes the connections are soldered to the cell. I've never been sucessful in resoldering them.

Sometimes a CDS meter can be brought back to life by reheating all the connections with a soldering iron, it may be the failure is just a defective joint.

I've got an old Selenium GE meter from the early days where their warranty states that the technology of the cell itself is so new they don't know how long they will last.

 

[brofkand]

I'm probably going to give up on this project, at least until I get another prism that works.

I don't want to possibly destroy my only prism to fix it while I don't have a backup.

Hopefully I'll have the "stones" to fix it one day!