Spotmatic SP meter -- 24 hours on Camera-Fix list and no responses

Okay, I posted first on Camera Fix, thinking those guys, who

specialize in repair, might be able to help.

 

I just replaced the battery in my Spottie after leaving the meter

switched on for a couple weeks by accident. Now, the meter needle

moves, but has many times as much response from shutter speed changes

as from aperture changes. The meter worked correctly last time I used

the camera prior to this situation.

 

It occurs to me to wonder if leaving the meter on (with camera in the

everready case, so in the dark) could have damaged the CdS cell in the

prism. I wouldn't think so, on 1.6 or less volts from a silver oxide

cell. Hopefully not, since I don't at all like the idea of trying to

replace that (or the prism) and this body is perfectly functional

otherwise, despite no service beyond periodic battery replacement over

at least the past 25 years.

 

However, I don't understand what else could cause this odd meter

behavior. As I understand the meter in this camera, the resistance of

the CdS cell (which gets less as the light level increases) is

compared to that selected by a switch in the shutter/film speed dial

via a bridge, and the difference results in a current through the

actual meter which moves the needle to the plus or minus side. Could

bad contacts in the switch under the speed dial do this? It's been

dead on the 8 speed for a long time, I thought that was a feature (to

alert the operator with his eye in the finder to a too-slow shutter),

but the meter operates at 4, 2, and 1, if the light is bright enough.

 

Am I going to need to get the soldering iron and dig around under the

top cover?

I don't know the mechanical details of this camera but it sounds like its not the CdS cell if the shutter and aperture are causing different readings. Normally, there is some sort of variable resistor built into the circuit which you use to balance the resistance between the cell by using the meter needle. If could be that the contacts in this variable resistor have become dirty and require cleaning.

 

<p>This is quite a common problem with the Nikon CdS meters I am familiar with (Nikon F, F2, Nikkormats) <a href="http://hometown.aol.com/drwyn/myhomepage/">Robert Decker's webpage</a> has some notes on the Nikon meters that I used as a guide to fix a Nikon F Photomic FTn meter, which should give you a better idea of what I'm talking about.

sorry - balance the current flowing from the power cell. E.g. if resistance is high in the CdS photoresisitor, it takes less resistance in the variable resistor to provide a given current to flow through the meter needle, which is really just an (micro!) amp meter.

Is the battery providing too high a voltage? Remember, one side of the bridge is the shutter dial which is connected to fixed increment resistors of one form or another. The other side is the CdS cell which changes its resistance according to the light falling on it and the voltage across it.

DOnald,I'd reccomend you email eric (pentaxrepairs at aol dot com)

 

He is an expert in these repairs and may help you.

I also have the reapir manuals in pdf if you need them

Another good forum that's very repair oriented is: http://www.kyphoto.com/classics/forum/ I can usually get enlightened responses from members of that group.

Mike, I think that's what I said -- the current through the CdS cell produces a voltage, which is applied to one leg of the bridge. The current through the resistance selected by the speed dial produces another voltage, which is applied to the other leg of the bridge; it's the *difference* that produces the micro-ampere class current that moves the needle. That's what makes the Spotmatics more or less inedpendent of battery voltage -- with a higher voltage, you'll get a larger deflection of the needle when exposure is set too high or too low, but your center null will always be in the same light as long as there's enough voltage to make the needle move (though if the battery is almost dead, the needle might not move visibly from center).

 

Thanks, Pablo, I'll try e-mailing Eric. Hopefully this will be something simple and silly like "leave the meter on and lens cap off for two hours to regenerate the CdS cell" rather than something annoying and expensive like "you'll have to replace the prism assembly with one carrying a good CdS cell". I'm perfectly willing to open the top cover and try cleaning the conacts under the speed dial -- I'd be very happy indeed if that was all it takes to fix it. I might yet be looking for those manuals...

 

Gary, I'll try the Camera Repair Forum if I don't get anything useful from Eric.