Selenium meters

[truls]

There are different opinions of the lifespan of selenium meters. Some claim they have devices more than 50 years old, still fully working. My light meter still works (Sekonic), it has to be more than 30 years old. What factor makes the selenium meter die? Is it exposing to light, moisture, or could inaccurate sealing in production reduce the life of the cell. Are there anyone who can replace a selenium meter, or is it a simple do it yourself.

[philbirch]

Almost everything you say in your post can stop selenium meters working. Often it is just the solder joint in the cell.

Other factors creep in too. There is a meter, probably a milliamp meter. These are more likely to be faulty. I have taken a few meters apart that have no life in them only to read a healthy 0.6v +/- coming from the cell.

I have a 1950's Zeiss Ikophot, in mint condition that is dead accurate yet the Weston Master III is wildly out.

Unfortunately selenium cells for light meters appear to be made for the meter. The voltage output must be balanced with the meter its self. Usually achieved by masking. I have thought of building my own. Perhaps I'll think again.

[Stephen]

There is a technical reason they are unreliable, the back plate is iron, not steel, and is difficult to reliably solder to. Some designs like Weston did not solder for this reason, they used sprung contacts, but over the years these fail too, but can be cleaned.
The other problem is the solder, solder has a life, about 40 years minimum, maybe up to 60. The older radios etc often have solder failures, but are unnoticed as the wire is wrapped around a post or tag, and still makes contact.
But on the selenium cell it is a blob contact, and when the solder goes it falls off.
It may be possible to re-solder, but the iron may have corroded as well, making the solder not take again.
Stephen

[Deleted]

Yep. Bad solder joints are the primary reason selenium meters quit working.

[philbirch]

Stephen, I didn't know they were iron, but that explains why re-soldering doesn't work. Would it be possible to grind off the outer layer and re-solder? Using a Dremel or something?

[dps3006]

Interesting about the iron plate. The Kiev 4 in my avtar is a 1959 model and the meter in it is still very active and ballpark accurate. I hear that I can still get a new cell from Russia if needed. The problem is getting the old one out and installing the new which is a chore. I recently bought a Wesson III which was sticking when I got it. When I opened it up I was sweating at the delicate mechanicals inside. It appears to have jumped it's pivot point during transit. I popped it back into alignment and it now works (very close to a Gossen Luna Pro that I have).

[Stephen]

It needs a bit of skill to re-solder, the iron must be cleaned, and a Dremel with a diamond spot end grindstone would do it, or a dental burr, but the work must be confined to the original connection area exactly.

The re-soldering must be done with Lead/Tin 60/40 traditional solder, not modern pure tin. The modern stuff melts a bit too high.

Despite keeping the temperature lower it needs a really hot, clean and pre-tinned soldering iron, a smaller low wattage type would "dwell" to long to bring the solder to flow.

The flux to use is any electrical paste type, or an acid type like Phosphoric acid, as long as the acid is washed away, although phosphoric cannot harm the iron as it is a rust proofer. Phosphoric is obtainable from Model railway suppliers in the UK in Carrs brand.

The risk with to much heat for too long is the breakdown of the selenium coating. There is no G/tee that it will work, as corrosion may have affected the plate too much.

Stephen.

[philbirch]

Thanks Stephen. I may have a go at a couple of old meters. Nothing to lose.

[shortstop]

Some selenium lightmeters are better than others. The lightmeter of my retina IIIC for example works very well after so many decades. Also in low light.

[Stephen]

Quite often the back plate of iron( may be low carbon mild steel)is contacted by a wiper contact that simply bears on the plate back.
The selenium is covered by a conducting metallised layer, usually with a soldered contact on a more heavily plated area.
There is a non heat contact method, carbon conducting paint. It can have a fine wire embedded in the paint to get the contact. I have never tried the method.

Stephen.

[philbirch]

Jul 22, 2014 12:36:14 GMT -5 shortstop said:

Some selenium lightmeters are better than others. The lightmeter of my retina IIIC for example works very well after so many decades. Also in low light.

Yes the light meters on my Continas work perfectly of a similar vintage. And the Ikophot. Hmmm the Zeiss meters were good. Who made them? Metrawatt?

[truls]

I would say the lifespan indeed could be 50 years and more, but no definitive answer. Stephen and Phil's method seems complicated for a simle camera collector. Interesting our new member "dps3006" mention it is possible to get replacement cells from russia. It could have been fun dismantling a Fed with a faulty meter, I assume those russians are more easy to open up and put together. My Fed 5C has an incorrect meter. May be I will donate this for the cause of repair. I found an interesting link to someone fixing selenium meters. Selenium meter easy fix.

May be the meter just needs some tlc.

[dps3006]

New Selenium Cells

This same gentleman CLA'd a Fed 2 of mine. It purrs like a kitten and the collapsible Industar lens is razor sharp

[truls]

Jul 22, 2014 18:53:34 GMT -5 dps3006 said:

New Selenium Cells

This same gentleman CLA'd a Fed 2 of mine. It purrs like a kitten and the collapsible Industar lens is razor sharp

Interesting, and the prices are human! I might send him one of my russians. He also repair Leica II and III. The link is bookmarked.

[lloydy]

www.google.co.uk/search?q=epoxy+solder&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&gfe_rd=cr&ei=H7jfU6G3G-3H8ge4i4LYBQ&gws_rd=ssl#q=conductive+epoxy&rls=org.mozilla:en-US:official&tbm=shop&spd=16462548455755483611

Conductive epoxy resin might work ?