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M_currie
Tinkerer
Username: M_currie
Post Number: 17
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 10, 2006 - 08:09 pm: | 
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I hope there are some light meter gurus out there to help me with this one:
I have an old Sekonic Micro Leader meter. No model number known, but it's a CdS meter that uses a single mercury battery. It has worked flawlessly for many years, and amazingly after over 18 years the battery still tests at the correct voltage.
Unfortunately when I went to use it tonight, although the test button showed the correct readin g on the meter, when I pushed the meter button it pinned the meter. So here are my questions:
1. All circuitry looks good, and it's dead simple anyway. Am I right in assuming that this means the CdS cell has packed up?
2. Assuming the answer to # 1 is "yes," does anybody know of a source for a replacement cell?
3. Since the circuitry is essentially just a battery, a CdS cell, and a meter with a single resistor in the test circuit, might it be possible, if the answer to # 2 is "yes," to add resistance in the main circuit to recalibrate it for a silver oxide cell? |
Wernerjb
Tinkerer
Username: Wernerjb
Post Number: 41
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 11, 2006 - 06:06 am: |
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Well, I am not the meter guru you are calling out for, but I found that in each one of those automatic devices illuminating people's front doors when sb. approaches them there is a light sensitive cell (LDR). I do not know whether they are still CdS cells or represent a more recent technology, but I have successfully used for replacement purposes in in 70's cameras, W. |
Roy
Tinkerer
Username: Roy
Post Number: 10
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 11, 2006 - 09:39 am: |
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If you have an AVO/VOM meter, testing the Cds cell is easy. Unsolder one lead from the cell and connect your meter across it and set it to read Ohms, then shine a light at the cell, if the Ohm meter doesn't respond then the cell is useless.
Hope this helps. |
Hovaness
Tinkerer
Username: Hovaness
Post Number: 14
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 12, 2006 - 07:27 pm: |
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Radio Shack sells generic CDS cells inexpensively. I don't know how accurate they would be, but you might be able to screen a bunch of them to find a suitable replacement. |
Roy
Tinkerer
Username: Roy
Post Number: 11
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 13, 2006 - 08:33 am: |
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In a meter used for photographic purposes the Cds cell is normally capable of driving the meter needle off scale and a resistance is placed in series to calibrate the instrument. It also covers against manufacturing 'spread' i.e. variations in cell sensitivity. The best way, if the existing cell is U/S, is to obtain one from any supply house that can be made to physically fit than replace the resistor with a small variable resistor and then re-calibrate. |
M_currie
Tinkerer
Username: M_currie
Post Number: 18
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 14, 2006 - 02:46 pm: |
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Thanks to all who responded. Neither of my local Radio Shacks had any CdS cells. I wasn't able to find a small enough photocell in my own junk boxes to use, but decided to test the one in there anyway. When I went to probe at a soldered connection, it fell apart. Turns out corrosion had just eaten off a couple of connections. A little resoldering and it's back on line, at least until the apparently eternal mercury cell finally dies. When and if that ever happens, I'll tackle the recalibration issue.
Useful to note that on a simple bridge circuit, as this apparently is, an open or disconnected CdS cell will pin the meter. Now I just have to get my courage up to tackle the meter in my Voigtlander Ultramatic CS, which does the same thing. |
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