success! stubborn spotmatic battery cover is off

[fish4570]

cover and threads cleaned of corrosion. could it be the meter will work on this old sp1000 with application of a fresh battery?

i bought the body (cheaply) with the understanding the owner could not remove the cover, and didn't know if the meter would work. a little break-free lube and a 3/16ths screwdriver got it off with no trouble. the bottom contact looks shiny new. it was the cover that was corroded. the old battery reads 40D, i think ...

now then, what battery?

[cooltouch]

This site might help you out:

http://www.photoethnography.com/ClassicCameras/index-frameset.html?batteries.html~mainFrame

Read up on the PX400, which is the battery meant for the Spotmatic. According to the above site, the Spotmatic has a bridge circuit for its meter, so it can tolerate the modern replacements for the original 1.35v mercury cell. Modern replacements are listed.

I was gonna suggest that you try the 1.4v 675 hearing aid battery, which is what I use to power the meters on my old Canons. Advantage is that they're cheap. But then I looked up the 675's dimensions. It is the exact same diameter as the PX400, but it is 1.8mm taller. So, probably a no-go.

[fish4570]

yes, the old battery DOES say 400, not 40D. thanks for the link.

[ryan s]

I had to remove a stuck battery door from a...I want to say...ME Super.

Take the bottom plate off, soak it in vinegar...then "uncover" the holes next to the coin slot and put a spanner/tweezers in them and twist.

That is, of course, when the coin slot is stripped...it should also work with corroded threads

[fish4570]

already off. just took a little break-free and moderate torque from a 7-inch screwdriver ...

[peterqd]

The correct replacement battery, with the required insulating collar, is an Energizer 387S silver oxide cell. However the voltage output of this is 1.55v instead of the 1.35v of the old PX-400 mercury cell. This makes the needle over-react and causes pictures to be under-exposed by about 2 stops if you centre the needle.

The Spotmatic meter does indeed have a "bridge" circuit of sorts, but this doesn't automatically mean that higher voltage has no effect. The meter is designed so that the needle is centred when the exposure is correct, and this requires a small current, which IS affected by the voltage. In a proper "Wheatstone bridge" circuit used by more modern cameras, the current passing through the meter is zero when the exposure is correct, so the voltage is not critical.

One way round this is to set the ASA setting on the camera to 2 stops slower to compensate. Or you can use a Wein-Cell MRB-400 or hearing-aid zinc-air cell with correct 1.35v output, but they are expensive and short-lived, about 3 months. Once exposed to air, they continually produce power until they're exhausted, whether or not you use the camera.

Alternatively, you can modify the camera to compensate for the higher voltage. There are two ways - the easier way is to insert a diode into the battery wire in the base of the camera. This works fine while the battery is fresh and the temperature is normal. When the battery tires, or in extreme hot or cold, the voltage reduction effect is altered in a non-linear way, so the camera meter becomes unreliable. The other way is to insert two new resistors in the meter circuit to reduce the current in the meter to waht it should be. This means more work removing the top cover, but it's much more reliable. This thread explains how:
http://forum.mflenses.com/international-teamwork-fixing-spotmatics-t7047.html

[fish4570]

thank you, peterqd. your advice was clearly and concisely presented. i no longer have the fine-motor skills required for either of the electrical cures, but one of my sons is an electrician. perhaps he could give it a shot.

[peterqd]

[fish4570]

wow. even a schematic. thanks again.

[Kamerer]

[peterqd]

[erreoa]