Zeiss Ikon Contax Camera Repair

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The IIIa Lightmeter Problem

There is a story going around through various web sites that there is a simple fix for a IIIa light meter that doesn't work. According to these experts the problem is oxidation of the electrical contacts to the photocell and that all one needs to do is to remove the cell, take a blade and scrape the contacts. This is supposed to restore the meter to normal operation.

The fact is that about one of every five IIIa camera I receive for servicing has damage to the cell contacts caused by scraping. Zeiss understood corrosion, and in most of their light meters the cell contacts were gold plated to eliminate the possibility of corrosion and to ensure long term good electrical contact. In its last production meters Zeiss dispensed with mechanical cell contacts completely and changed to using permanent soldered connections. There is no need to scrape the contacts. Corrosion is the problem, but it is corrosion to the cell itself, and not to the electrical contacts. The following illustration of the ideal selenium photocell shows what I mean by this.

• C is a transparent thin layer of metal that is deposited over the selenium layer (B) that is deposited over the steel sell support (A). The selenium layer is made up of small crystals. In order to collect the electricity from all of them, a conductor must contact each crystal. The only way to accomplish this is to use a contactor surface that is so thin that light can pass through it. Over a long time (around 20 years) this layer oxidizes and looses its ability to conduct electricity. The result is cell failure.
• (E) and (D) are raised electrically conductive surfaces that are applied to the front and back of the cell. These are typically made of epoxy filled with silver dust. These are the surfaces the meter electrical contacts compress against when the cell is installed in the meter.
• Another problem that can effect the cell over time is that the selenium layer (B) is a special form of selenium called "metallic" selenium. Selenium can have several crystal forms, some of which are electrically unconductive and which will not generate electricity when exposed to light. It is possible for the selenium in a cell to begin to change its crystal form over time. This conversion process can be triggered by the cell being exposed to excessively high or low temperatures. Once started the conversion process will proceed and cannot be stopped. In most cases its progress is slow.

The cells in Contax IIIa cameras are all old. While some may respond to light, the meter cannot possibly be accurate or linear with an old cell in it. The only way to restore the meter to like new operation is to have a new cell installed in it. There is absolutely no substitute for this. When I replace a cell I use the Zeiss method to solder it in place to completely eliminate the friction type contact source of unreliability.

There are almost no workers who work on the Contax IIIa light meter. I am happy to work on them and am expert in restoring meters to full operation, even those which have been written off by other experts.

If your IIIa meter has a problem and you want it made right correctly please write to me.

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