Metraphot Lightmeter Repair

Mr_Flibble

Mr_Flibble

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I picked up a cheap little Metraphot 2 lightmeter. Of course it was completely dead as it probably spent 40 years exposed to light in a display case at a Good Will store somewhere.

Well, because I'm bored and I really like the look of this compact little meter I've decided to bring it back to life.

Disassembling one isn't too hard. Just with 4 screws you can take the top off. Then two more screws hold the wiring to the Selenium element in place. I hooked up my trusty volt-meter and it was as I suspected the Selenium cell is as dead as a doornail. Yes, I cleaned the contacts...no go.

metraphot01.jpg


When adding a little current to the mix from an old TI calculator solar cell the meter jumps to the other side. So at least that part still works fine.

The choices;
Either get a new selenium cell, which has to be custom cut and will cost at least 18 GBP. Or rip one out of a working lightmeter and cut it to size...hmmm...
Or do some creative soldering and use a modern solar cell and variable resistor instead.

I've already got a resistor which cost me a whole 60 Eurocent at a local electronics shop.
All I'm looking for now is a small enough solar cell or cells (up to 1" x 1/3") . Calculators seem to be my best bet. I'll have to go hunt one down.

If I can't find one I'm getting some Micro Cells from Budgetronics which go for about 1.95 Euro a piece.

Wish me luck 😉
 
You could try a photoresistor (also called a CdS cell). They were commonly used in the sixties/early seventies' cameras. They display linear (light vs. current) from around 5EV through to 15EV. These cost a few dollars in hobby electronic shops like Radio Shack.
 
I would still need a powersource for the CdS to work. I guess there might be a bit of room for a hearing aid battery, as the photoresistors aren't that big.

I'll look into that if I can't find any solar cells that work/fit, thanks.
 
After only a few days the hunt for a solar cell came to an end. I acquired a novelty business card dispenser with in-built calculator for the meagre sum of 2 euro. And after ripping out the cell I found that it is only a fraction of a millimeter too long to fit in the location of the original selenium cell. Nothing a sharp pocket knife can't fix.

Tomorrow I'll see if I can solder it all up, and hopefully fit it back inside the casing together with that little variable resistor. And then to check it against a couple of other light meters.
 
Here is the luckless calculator (from rosewoodstock)

metraphot02.jpg


First thing on the agenda, make the solar cell fit in the little plastic frame that held the selenium cell...two seconds with a scalpel gave me this

metraphot03.jpg
You can see I've shortened the negative connector as I wouldn't be using it or the resistor on the back of it.

So after a little bit creative cutting and bending and soldering I ended up with this
metraphot04.jpg


metraphot05.jpg


Next up was the variable resistor....I seem to have made a slight miscalculation in the resistance of this little gem...the maximum 5 Kilo Ohm doesn't seem to offer enough.

metraphot06.jpg

Of course I figure this out after wrecking the thing (grinding down the sides to make it fit and soldering up the connections

d'oh!

Off to the electronics shop tomorrow for a tougher resistor.
 
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This is a great approach, good luck with it, as solar-powered calculators are so ubiquitous.

But are you sure the response from the cell will be linear?
 
According to an article on Tiger's Lair it doesn't matter what kind of solar cell you use, as long as it responds swiftly to light.

It's Tom's site that gave me the idea to try to repair/convert this meter in the first place. More specifically: this article.
Of course I should've memorized the bit about using a 100 Kilo-Ohms resistor.

I don't know any other hobby shops in the area selling these resistors....unless...Hmm I just recalled there's a shop selling RC aircraft not two blocks from here. It's worth a try.
 
Finally managed to sort out some other little potentiometers from a friend of mine. I'll see if I can get it all wired up when I get home.

I'll be using the meter in the Konica S2 Auto and an old GE meter for calibration
 
Well darn,

The black honeycomb structure in the little frame that holds the solar cell blocks too much light for the meter to work properly at the lower light intensities.
My solution: Put the cell in front of the little frame, directly up against the clear plastic shield and adjust the resistor appropriately.

We'll see how that works out later today.
 
Better finish this article up now.

I had some trouble rewiring it all (soldering came lose from the solar cell and it was a heck of a fight to get it to stay on again).
unfortunately with this new set up I could no longer use the clips that normally hold on the diffuser (and are used for checking the unit)

I picked the highest value potentiometer of the bunch a friend supplied me with, which was 100K. hooked it back up
normal_metraphot07.jpg


Stuck it all together in the body
normal_metraphot08.jpg


Calibrated the meter to best of my ability with my other meters, but because it's such a small Solar cell there are indeed some linearity problems at low light levels. Under f/4 (with 100 ASA) the values are about half a stop off.

normal_metraphot09.jpg

But in normal daylight/overcast it was perfectly usable, not to mention that it looks rather cool on my Leica IIIc 😉
 
Thanks Steve,

I'm sorry I couldn't save the metal clips that hold the incident panel, even though I didn't get one with the meter. More light does fall on the solar panel without them though.

Without considering all the problems with supply of parts the repair was actually quite easy to do. If it had been more then a single circuit I would've been in trouble 🙂
 
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