Gossen Luna Pro Repair Question

[HT Finley]

It's been years since I worked on the gray Luna Pros. I used to know everything about them. Now I can't remember the simplest thing. So here's my question: Does the meter switch turn the power on and off, or does it just untrap the needle for a reading? That is, is the meter "always on" and require a case to cut the flow of power?

[Pieter]

https://www.butkus.org/chinon/flashes_meters/gossen_luna-pro/gossen_luna-pro.htm

[HT Finley]

Thanks. I've already seen that. As far as I can see it says nothing about the "switch" being anything more than to un-trap the needle so it can swing for a reading. It gives no hint that it actually opens and closes the circuit. Many meters are always-on and depend on closing the case or putting on the lens cap to run the photocell resistance up, efferctively stopping battery drain.

[Pieter]

That might be a Gossen design "feature." My DigiSix is always on.

[sharktooth]

I don't know the answer, but you could jury-rig a circuit with a multimeter to measure current. You'd have to take the battery out and hook up some external leads to the battery and battery connections, and use the multimeter to measure current flow when trigger is depressed and undepressed. A big pain in the butt to do, but at least you'd be sure.

[BrianShaw]

According to a schematic I've seen, the high-low switch puts no power on the circuit when in the neutral position... only when in the low or high position.

EDIT: Here it is

https://learncamerarepair.com/product.php?product=465&category=2&secondary=37

[HT Finley]

Thank you Brian. Thr schematic is so small on screen I can't get a good look at it. But I am prone to take your word. The meter is off when the switch is in neutral, which is what I'd hoped. I just finished cleaning and calibration of it but didn't go so far as removing the circuit board from the rear case half. And you can't tell if there is an actual switch that breaks the circuit in neutral. This meter has no case. And if it can cut off, I prefer it with no case. Those Luna-Pro cases add bulk and they are awkward. You have to bend them backward to keep them out of the field of view of the meter cell. And since this meter is a beater anyway, I can just keep it in my pocket and whip it out and meter with one hand. I have a really nice Luna-Pro in a very nice case that I also had installed a 2.7V v oltage regulator in for two 357 batteries. I just hate the case. Too fat for your pocket and a nuisance. This junker meter with no case will be perfect for X-ray film with my 8X10 Horseman I modified to reduce weight, and the contractor's tripod. Nice to know I won't be burning up batteries if it just sits on the table with no case, or walking around in the sun.

[BrianShaw]

Download the articles and the schematics are a lot easier to read.

I agree about the case.

[HT Finley]

Got it calibrated perfectly for 357 batteries that had some use on them and were down to about 1.45-1.47V. Since it has no voltage regulation, it will swing between about 1.3v when the batteries are near discard, and the 1.55 when they are new. I figure that gives me a 1/2 stop swing, or a wee bit more, either way. But X-ray film isn't Kodachrome. This should do nicely. Not bad for a free meter given me by a friend who didn't like a little grunge under the dial. I can't get the center button unscrewed so I can clean it, but who cares? I'm sure it'll get even more nasty as a dependable junker in the pocket. Couldn't get te battery test down from above the red zone because the pot was full clockwise. So what? In this meter, red zone will simply mean time to get new batteries. Just like red zone on a tube tester.

[nitroplait]

I have good experience with these zinc-air batteries (https://smile.amazon.de/-/en/gp/product/B07ZG3F85V/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1) when substituting mercury cells in meters/cameras.

When I measured the voltage depletion profile I could see it goes pretty quickly from 1.42V down to 1.36V and steadily stays around there until it takes a rapid deep dive.

I am currently successfully using them (with appropriate adapters to fit) in a Leica Meter and Pentax Spotmeter III - and other PX625/PX675 powered cameras.

There is 4-6 month usage in them after activation. They should be removed when not in use or depleted.

[sharktooth]

I have good experience with these zinc-air batteries (https://smile.amazon.de/-/en/gp/product/B07ZG3F85V/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1) when substituting mercury cells in meters/cameras.

When I measured the voltage depletion profile I could see it goes pretty quickly from 1.42V down to 1.36V and steadily stays around there until it takes a rapid deep dive.

I am currently successfully using them (with appropriate adapters to fit) in a Leica Meter and Pentax Spotmeter III - and other PX625/PX675 powered cameras.

There is 4-6 month usage in them after activation. They should be removed when not in use or depleted.

If you remove the battery after use and tape over the vent holes, will that prolong the life of the zinc-air batteries? I've started to do that, but I'm wondering if I'm just wasting my time.

[BrianShaw]

Got it calibrated perfectly for 357 batteries that had some use on them and were down to about 1.45-1.47V. Since it has no voltage regulation, it will swing between about 1.3v when the batteries are near discard, and the 1.55 when they are new. I figure that gives me a 1/2 stop swing, or a wee bit more, either way. But X-ray film isn't Kodachrome. This should do nicely. Not bad for a free meter given me by a friend who didn't like a little grunge under the dial. I can't get the center button unscrewed so I can clean it, but who cares? I'm sure it'll get even more nasty as a dependable junker in the pocket. Couldn't get te battery test down from above the red zone because the pot was full clockwise. So what? In this meter, red zone will simply mean time to get new batteries. Just like red zone on a tube tester.
Why not move the red battery test marking?

[HT Finley]

Why not move the red battery test marking?

No need. When I first started checking out the meter a few days ago, I noticed all the readings were about 1 stop over-exposed. And the battery check was about a quarter of the way up into the red zone. I checked the batteries on the multimeter and they were about 1.25V. So I knew they were on ther way out. I stuck in some fresh ones and they were blazing hot at about 1.67V. The battery test was way over the red zone. So I plucked the 357's out of my Nikon F and took the ring off them I had put on. The were about 1.45V. Not too hot, not too cold. So I calibrated the meter off them and got it as perfect as possible. In the process I could tell the meter cell was still very linear, expecially considering its age. I was able to calibrate nicely all the way down fto EV-2 all the way up to EV 15 on ASA 50, which is about where I expect to end up on my stash of Fuji HR-T green, after those tests. Pretty darn good. Here is a photo of the meter in the battery test position. It's high, but the pot is pegged. Who cares? Now I know, red zone means get new batteries, and new batteries will be so blazing hot I'll underepose a bit. But they settle down pretty soon for the long haul, right about my calibration work was, and I'll be fine. All-in-all, I never had an old meter so linear from bottom to top. I got every penny of my money's worth. Today I am finishing up the adapter plate I've been 3D printing to mount my big Horseman on the contractor's tripod. That project is turning out BEAUTIFUL. I'll post about it. Got 1 more hour of printer time before a spacer is finished and I can call it done.
Edit: I'm using a Sekonic L-308S as my calibration standard.

[nitroplait]

If you remove the battery after use and tape over the vent holes, will that prolong the life of the zinc-air batteries? I've started to do that, but I'm wondering if I'm just wasting my time.
I honestly haven't bothered to test, but I guess that is a possibility.
I just take them out and put them in a plastic box - in pairs if activated as such. I have a multimeter handy and check the voltage before I reload them.
If used lightly I think they are mostly fine for up to 6 month. Less if used heavily. At less than $2 for 6 pcs I am not too concerned about cost.
Of course, if you use $$$ Wein cells, that is an entirely different matter, but then - why would you?

[HT Finley]

There was a fellow on another site, I don't recall his handle, but lived up in NY State up there in Adirondacks somewhere I recall. Anyway he said he had been having success with using zinc-air cells in his Nikkormat FTN by painting over all but one of the air holes. Said they worked and lasted for years. Of course those little bitty meter movements in camera viewfinders have practically zilch of corrent draw. So it was probably enough capacity to run them before the battery could get enough air to make more power. I have my doubts as to whether that idea would work on a Luna-Pro because the meter movement and coils are so much bigger, to where I question that a battery that had been that starved of air like that could make juice fast enough to keep up.

[sharktooth]

I'm in the habit of removing all batteries from any camera once I've finished using it. I've got waaaay to many cameras, and there's too much risk that the batteries would leak and cause damage before I got around to using it again, if left in the device. My thought process was that if I'm taking the battery out anyway, I might as well tape up the vent holes, and just remove the tape the next time I needed to use that camera/meter. Those zinc-air batteries already come with a nice stick on vent tab, so I'm just reusing that. I still check the voltage before I put them in the device. I was just wondering if plugging the vent holes stopped the chemical process, or if the process continues on as soon as the process is started with initial vent uncovering, and can't be stopped.

I agree that the zinc-air batteries are cheap, but there's no need to waste them if sticking a tab on would prolong storage life.

[Bob Salomon]

I'm in the habit of removing all batteries from any camera once I've finished using it. I've got waaaay to many cameras, and there's too much risk that the batteries would leak and cause damage before I got around to using it again, if left in the device. My thought process was that if I'm taking the battery out anyway, I might as well tape up the vent holes, and just remove the tape the next time I needed to use that camera/meter. Those zinc-air batteries already come with a nice stick on vent tab, so I'm just reusing that. I still check the voltage before I put them in the device. I was just wondering if plugging the vent holes stopped the chemical process, or if the process continues on as soon as the process is started with initial vent uncovering, and can't be stopped.

I agree that the zinc-air batteries are cheap, but there's no need to waste them if sticking a tab on would prolong storage life.

Most, if not all batteries, out gas as a by product. The vent holes let those gasses escape from the cell. If you block the vents various things can happen. The body of the cell can expand. The worst would be gas building up to a point where the cell could explode!
Ever see an AA lithium cell explode? I have, it could take your hand off. That cell exploded as the factory blocked the vents to demonstrate to my sales force what happens with blocked vents.

[HT Finley]

As a final addendum on thee Luna Pros, here's one thing I've learned. On the low range in a room with only a 60 watt equivalent CFL bulb, or in a dimly lit daytime room with no lights on, the behavior is the same. You will notice under these circumstances that the meter will give 2 readings depending on how you use it. The best way is to flip the switch and get your reading pretty quickly. Holding the switch down will allow the meter to rise above where you want to be. Don't believe me?. Flip it again and watch it fall back. Flip it again quickly and it gives the same reading as the first time. It's when you hold the switch down and give it time to keep rising that you get into trouble. Flip it a couple more times--on/off pretty fast and you see it prefers to hang around at the lower reading, before you held the switch down, giving it time to rise. THAT is your true reading, which is a full stop lower than when you gave it time to keep rising. The meter wasn't made to be a low-light meter with yellow lighting. All calibration procedures need to be made with varying brightnesses of daylight. It's not a color temperature meter. It likes daylight color temperatures. That's how it should be calibrated. I've sat here comparing it to my D7100 Nikon. This thing is on the button if you remember a simple fact. A CdS cell has a terrible ability to tell color temperature, and it has a memory. If you go outside on a sunny day and flip the switch to the low range accidentally, you may as well go back inside and give it time to recover, because it ain't going to work right till it gets over being blinded like that. Kind of like your own eyes. Properly calibrated for 357's are fine, if you don't calibrate on brand new batteries. You'll never get Kodachrome accuracy out of it. But back in the day I don't recall ANYTHING that gave Kodachrome accuracy outside of the Kodak labaratory up in Rochester. That's all they had to do all day was fiddle with fancy instruments. If we can get 1/2 stop accuracy anywhere, we're pretty fortunate.