Selenium meters

[John Farrell]

Lightmeter repair (for a Zenit E) from Tom Piel. The same idea may work for other selenium meters.

tomtiger.home.xs4all.nl/index.html?

[retina]

My experience is mostly with the Kodak Retinas of course, since they are the cameras I commonly repair. The faults with the selenium meters on those particular cameras where the meter is completely unresponsive to light are caused in about ninety percent of cases by the meter movement being open-circuit. The wire in the coil of the meter is extremely fine, and a break develops somewhere.

In the remaining cases where the meter is completely unresponsive the fault has been a meter movement that has jumped off the pivots, and is therefore jammed, a poor connection between the selenium cell and the spring-loaded contact at the rear of the cell, or a broken solder connection.

When I repaired Weston Master meters forty years ago, meter movements in the dead meters coming for repair also often had an open-circuit movement coil. The break was often in the outside winding, and it was sometimes possible to pull the winding free by one turn and re-solder the wire.

Weston Masters with low sensitivity were usually dealt with by replacing the selenium cell, and cells with too high an output were dealt with by masking some of the cell area with a neutral-density filter.

Some generations of Weston-Master were more prone to problems with the cell, something to do with porosity in the coating allowing moisture to degrade the cell I think I was told.

Regards, Chris

[truls]

There was a discussion at Flickr, a user claiming selenium meters never dies... It is the internal Construction and wire setup that broke Down, making a repair possible.

[retina]

Well, it would be nice if things never died, but that isn't the way the world works.

Selenium meters don't have many parts. The Selenium cell degrades over time and they lose their sensitivity to light, or at least their useful output. Apparently they are sensitive to moisture from what I was told.

The connections between the selenium cell and the movement are prone to problems, broken solder joints, corroded contacts etc.

The movement itself can suffer from physical damage from the meter having been dropped, this can pop a movement right off the pivots. Not uncommonly, tiny particles of metal are gathered up by the magnet from who knows where and these jam the movement.

Of course, a common cause of failure is simply that the coil on the movement goes open-circuit. The wire is incredibly fine, and it just breaks somewhere.

Probably other things I have overlooked...

Regards, Chris

[Stephen]

The cells themselves should last a very long time, but......they are made of iron, layered with selenium, and the electric contacts cause issues. The iron is usually connected by soldering, and the selenium via a pressure contact or soldering, after removal of the protective varnish. The surface under the outer varnish is a special clear varnish that is electrically conductive, to which contact is made.

The soldering involves flux, which breaks down the varnish a bit, and moisture can get under the varnish, rusting the iron, or corroding the selenium.

Better makers used inert flux, or a micro weld process, and after the joints were made, re-coated the lot with new varnish.
Weston cells can be clean on the rim to restore the contact.

Russian cells are a bit patchy in quality, but fairly reliable, it is more usual for the meter movement to be faulty. Again flux is the problem, the wire ends where soldered may still have flux eating away at the solder or wire.

Also a little known point, the solder has a finite life, estimated by the GPO Dollis Hill lab to be about 30 years at minimum.

Solder is only a cover for a good joint, a dab of solder on wires will not last well. So many older cameras are past the sell by date for the solder, it may be oxidising or breaking up. Some purer solders last much longer, and it depends on the flux as well.

Stephen

[rickoleson]

I've been struck by the durability of General Electric light meters: I've had several of both the 8DW58 (yes, that's really what they called it - catchy name, eh?) from the 1940s and the PR-1 from the early 50s, and every one seems to still be in working order no matter how bad it looks. I have not had the same experience with Westons of similar age.

[genazzano]

Can't say I have had any success restoring a selenium meter. My Photoscop worked though weakly for a while until a wire broke. It was dated 1934 inside. My Electro Bewi and Dejur Professional both work very well and match well the readings in both my VC meter as well as the Nikon F3 meter. They are 78 and 67 years old respectively. I would like to get the Photoscop going again if possible.



David

[philbirch]

I picked this little darling up from West Yorkshire Cameras. Works lovely!

[genazzano]

So, what's the final advice on resoldering broken wires on selenium cell? I want to get the 1934 Photoscop meter operating again if possible.



David

[Stephen]

The first thing is to find out if the meter movements is OK. Power it with a low voltage source that is current limited if possible.

A lab bench power supply is best, set to about .5volts and limit the current to minimum. You can measure the resistance of the coil, and assuming most selenium cells make 2 volts max, calculate the current flow. For say, 20 ohm, then .1 amp would flow at 2volt or at.5 then .025 amp will flow.

It would seem that you can just apply a voltage, and not bother to restrict the current, but the coils may have shorted turns, and as the power is applied burn out.

As long as the coil moves and returns then all is OK. This assumes that the bearings and hairspring are all OK.

Examine the cell, and put a volt meter in contact with the back, or lead, or spring contact. On the front there should be an area of solder or a pad that makes contact to the front. Touch the lead to the pad or solder and light falling on the cell will cause a voltage to register on the meter.

If both the cell and meter work, then re-solder the leads, or arrange a spring contact. Use a 145oc type solder (low melt), or 60/40 lead tin, do not use modern lead free, it melts too high. Flux must be inert electrical type.

There are silver and carbon conducting glues that could be used to make contact, but they are not easy to use without experience.

You may be able to spring contacts against the cell onto the contacts already there.

Stephen.

[Stephen]

From the picture and others on the net it appears the Photoscop has plated on front contact in the form of a spiders web. There must have been contact point, maybe solder, but it may be the surface of the web makes contact to the rim, which would be insulted from contact with the back. The rim may screw on to give pressure or have sprung rim, etc.
The meter would contact the back and to the rim, this is only a suggestion, I would need to see the parts in good photos to check.
But the solder less contact was favored by early users of selenium cells.
Stephen.

[lesdmess]

The selenium meter in my 1959 Baldamatic I is dead on accurate and the rest of the camera itself is fully functional. Likely hardly used and kept in the case in the box all those years.