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Scott
Tinkerer
Username: Scott
Post Number: 37
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 07, 2009 - 09:45 pm: | 
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I have a Vivitar 45 exposure meter that seems to be way off when I put in a new SR44 battery. I assume it's because the meter was made to use a 1.35v mercury cell.
I'm hoping I can calibrate it to read reasonably accurately using an SR44 cell. When pushing the battery-check switch, the needle goes way beyond the yellow dot.
Inside, and there are three variable resistors on a circuit board attached to the actual CdS cell. I see only one tiny (non-variable) resistor. It is soldered between the battery and the actual meter coil, color-coded gold-red-black-red.
So should I change or add another resistor? Or adjust the variable resistors? |
That_swede
Tinkerer
Username: That_swede
Post Number: 1
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 09, 2009 - 11:11 am: |
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Hello Scott, I stumbled upon your post by accident. I'm fiddling with a couple of old minolta SRT's which are well known for having the mercury battery issue. Bizzarely one of the meters I'm using to help calibrate it happens to be a Vivitar 45 meter that I've owned from new and has not had a new battery for years. It tests OK so I gave it no thought. After seeing your post I stuck it on a meter and yep 1.35v - mercury. After splitting the case I was surprised at what I saw. I'm not fully sure I understand for sure how this meter is designed but it apparent that one of the variable resistors is for setting the battery test level - should be obvious when you trace the leads. The LDR itself has three leads. I'm guessing, but I dont know for sure that there are actually two LDR's in the package (one low level one high?) hence the two other variable resistors. I would emphasise that I am not an expert. To cut to the chase, assuming that you have managed to resist the temptation to adjust anything this looks like a prime candidate for inserting a signal diode to drop the voltage (inserted in the positive line to the switch looks prime candidate). Have a look at the following links http://www.mi-fo.de/forum/index.php?showtopic=23259&st=0&p=241986&#entry241986 (converting the SRT to Silver Oxide Cells) Another link takes you to a pdf which is something of a mini thesis but confirms (you have to hunt through the text as its not on the graphs) that a 1N5711 diode is probably OK. http://www.buhla.de/Foto/batt-adapt-US.pdf Strewth - that's a very long winded way of saying use a forward biased diode to drop the voltage and use a silver oxide cell. Just my opinion but at least its reversible! |
That_swede
Tinkerer
Username: That_swede
Post Number: 2
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 09, 2009 - 11:18 am: |
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ps Or use a zinc air hearing aid battery (correct voltage but they have a very limited life) - sorry should have stated that first but for my own purposes I have already dismissed it as a solution. |
Scott
Tinkerer
Username: Scott
Post Number: 38
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 09, 2009 - 09:54 pm: |
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No, I was afraid to try to adjust anything. Those three things on the circuit board-- are they variable resistors or potentiometers?
I did a bit more searching and saw a few tips, among which some that said that once you put a diode into to circuit, then you can disregard the battery-check reading, because that will permanently be made incorrect.
Also, all the tips I read say that the diode should be put into the circut between the battery negative treminal and the rest of the circuit (with the cathode towards the battery). Is that right?
I could not find a 1N5711, but they sold me some 1N60 diodes which they told me were almost the same. |
Nickon51
Tinkerer
Username: Nickon51
Post Number: 63
Registered: 05-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 10, 2009 - 07:13 am: |
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A 1N60 is a germanium diode. Germanium was a popular semiconductor before the silcon era. The only thing it has in common with a 1N5711 is that the voltage drop across a forward biased juction is about 0.2v. That voltage varies more than the 1N5711 does with varying load and temperature. The 1N5711 is a silicon schottky barrier diode. |
That_swede
Tinkerer
Username: That_swede
Post Number: 3
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 10, 2009 - 02:33 pm: |
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Hi Scott, I think Nickon51 is correct with his comments re the diode. I dont have the time or energy for a rather cheap light meter to investigate much deeper to see what current range we are talking about for this application but I doubt that the 1N5711 would be an unreasonable choice. I found a comment elsewhere which seems to confer with what I suspected that these meters uses two CdS cells, but since they are in one capsule its not that obvious other than the number of connections. I've now checked out the accuracy of my own meter and its not very good. I'd obtain a zinc air cell (675 I think it is) first to see that the meter is accurate before worrying about that diode. If it is not accurate then I think it's open season on those pots fit a silver oxide cell and twiddle away - you'll soon see which sets battery check level and which is high EV and which low EV. These are a pretty simple device. Obviously you will need a good meter or camera as cross reference. If you do go down the diode route then you will probably have to stick it in positive line as there are two connections hooked on to the negative terminal already, one of which is a resistor. It makes no difference - its just a voltage drop - it will also be obvious if you have it the wrong way round. |
M_currie
Tinkerer
Username: M_currie
Post Number: 178
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 10, 2009 - 05:23 pm: |
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I think I'd take That Swede's second choice first, and suggest that if you have a meter with three variable resistors in it, it's likely there will be a way to recalibrate it.
So here's what I'd do. First of all, I'd get a silver-oxide battery of the right size. Skip alkalines because they vary too much. Silver oxides are just a wee bit higher voltage than Alkalines, but they stay there.
Then, I'd take a digital camera with macro feature, and make very careful shots of the position of all the variable resistors, or mark them, or both, so that you can put it back the way it was. And then, I'd just experiment. Set the meter to some ASA like 100, aim at a fairly repeatable light source, and take a reading. Try one resistor and see what effect it has. There's a likelihood that they are all interdependent, but that one will set the battery check level, and the others will correct the readings at high and low levels. If you can get an accurate reading in the middle range of light and the middle range of film speed, you'll have a useful meter and won't have to worry about odd batteries. If it doesn't work, put the resistors back the way they were. |
