Reviving Spot-O-matic photo cell

Check for corroded connections to the cell. In cameras; many times the variable resistor is bad on cds meters; and the cell is blamed. On "solar cell type" meters; in about 4 out of 5 meters I have gotten for nothing were made working by removing the greenish crud between the cell's back and the spring contact. Suspect dirty contacts first; before the cell. This is a racket in the repair business. A cds cell should vary in resistance with light; and can be measured with a ohmmeter. It might also have a diode detector; in which the current varies with the light.

Thanks Kelly for your quick reply to my problem and offering a solution based on your hands on experience. Is it difficult to open up the Spot-O-matic to clean the corroded connection(s)? I'm guessing that the base plate pops off. If not, how do you get to the meter's innards? BTW, about a year ago, I asked the same question about reviving my Spot-O-matic on Ilford's equivalent to photo.net's bulletin board. When I got no reply that would solve my problem, I gave up and was ready to chuck the Spot-O-matic out. Thank goodness I didn't. Yesterday, I found photonet's message board for the first time, when I asked Google's search engine for information re my EPOI Analite II meter. It was a pleasant and surprising discovery to see so many related problems and proposed solutions. I'll certainly read and use photo.net's bboard for any other photo information or problem.

I'm eager to try your suggestion, and with fingers crossed, expect to enjoy using that gem of a meter once again. It's the best enlarger meter I've ever used and it computes the exposure time as well as the contrast of graded paper (or filter for variable contrast paper) so easily.

Thanks again for your help.

Les Gediman

I'm not familar with the "Spot-O-matic" but I do own (what could be called an antique by today's 50 years rule for technological items) a Gossen Majosix, a selenium cell spot darkroom photometer that Gossen produced from 1953 until the early 1960s when it got replaced by the CdS controlled Labosix--- it works amazingly well with the exception that I need to add in a factor as it seems that the papers I'm mainly using, as almost all contemporary papers, are significantly faster than those it was designed for.

Selenium cells do deteroriate over time, either due to constant exposure to light sources or to moisture. They tend to be glued into the systems to protect them from the elements. They, however, can be replaced. Many people have even gotten quite satisfactory results by using cut sheets of solar cell material.

CdS cells too deteriorate over time but their decline is less spectacular. They are not the problem as they tend to be easy to replace but their driver cicuits often use electronic components that are nearly impossible to service and even the older units with discrete components, which would lend themselves to repair, tend to use transistors that are long out of production and hardly available.

When I saw "Spot-o-Matic" I remembered mine. It was before photo cells in those meters. They had a bacl lit translucent spot in an opaque white surround. The objext was to adjust a variable neutral density filter betwee the lmp and the spot until the spot disappeared. The human eye senses differences between adjacent brightnesses to about 1%, which made the meter fairly accurate if you used it right. There was a little cone over which you were supposed to sight to keep the viewing angle constant. It was good for setting shadow exposures, but not much else.

Pat Gainer

"They had a bacl lit translucent spot in an opaque white surround. The objext was to adjust a variable neutral density filter betwee the lmp and the spot until the spot disappeared. "

Sounds a bit like my Deutometer

The other extreme of complexity (and cost) doing almost exactly the same thing--- and to no higher accuracy, just a bit easier calibration/zeroing--- is the "Ansel Adams approved" SEI Photometer See: http://www.kcbx.net/~mhd/2photo/film/expose/sei.htm for a good article on it.

Thanks for comments from Pat. G. and Ed. Z for his suggestions and nicely illustrated comments. Here is where I am in my attempt to restore my Arkay CDs Master Spot-O-Matic: Removing the bottom plate fully exposed all components. There was no sign of corrosion, however I resoldered each fused connection, in case a cold soldered joint existed. The photo cell is RCA 4403 7D. I've called RCA to see if it's still available or has a replacement. They haven't returned my calls. BTW, there are no transistors in this circuit.

I'm sorry that my first letter didn't state that the is a condition when the neon light goes out while the power switch is turned on. By moving the adjustable sensitivity rotary scale, the neon light stay on from 0-30 and goes off from 31-38. It acts more like an on/off switch, rather than a photo cell that is influenced by threshold light levels.

The instruction booklet mentions if meter is exposed to too much, or too litle light for long period of time, it needs to be re-aged. Arkay recommended exposing the sensor to a fluorescent light source (few inches to 1 foot) for over 48 hrs. This is to correct "memory" of cell, and age it. I've tried this several times in the past without any luck in reviving photo cell's activity.