Olympus OM-1 CLA and meter circuit upgrade

[MattKing]

You could always get the MR9, but a much simpler hack is to just adjust the ASA downward until the meter matches a known standard, note how much (1/3, 1/2, 2/3, 1stop ...) and apply that correction every time you load film with a different ASA. You can also do this with an exposure compensation setting, but I cannot recall if the OM-1 had this sort of thing.

The OM-1 metering circuit is non-linear when the voltage is incorrect.
This will give you the results that a broken clock gives you, except as it will be right at only one light level, it will be half as right as the clock.

 

[chuckroast]

The OM-1 metering circuit is non-linear when the voltage is incorrect.
This will give you the results that a broken clock gives you, except as it will be right at only one light level, it will be half as right as the clock.

Is it really THAT nonlinear. Not being snarky, just genuinely curious what your testing has shown.

Many meter circuits are balanced bridge designs that mostly shouldn't care a lot about a little overvoltage. (Undervoltage can be an issue because it can run the diodes in the bridge into a kind of nonlinear part of their transfer curve.)

And all meters using a CdS cell are somewhat nonlinear across bright to dark because of the cell itself.

But we're talking about an 11% voltage difference. That notionally falls within the "for all practical purposes no big deal" category if you adjust accordingly.

I would note that I have not actually examined the OM-1 metering circuit and there are certainly ways to design such a thing as to be highly sensitive to minor voltage variations.

I would also note that I have personally recalibrated LunaPros for higher voltage and they worked pretty well thereafter in both the high- and low sensitivity ranges, but of course the LPs were never perfectly linear in the first place.

I also have an M5 that DAG recalibrated for new batteries and meter is pretty much bang on, at least so far as I have used it.

 

[ic-racer]

I would note that I have not actually examined the OM-1 metering circuit

^^ This ^^

 

[MattKing]

Is it really THAT nonlinear. Not being snarky, just genuinely curious what your testing has shown.

My experimentation was done a long time ago.
IIRC, John Hermanson of Zuiko.com/Camtech used to have more detail on his OM repair website about why one could not use an alkaline or other 1.55V battery in the OM-1, and I believe that my opinion on this is heavily influenced by him.
I used to have to deal with this back when mercury batteries were still available, but customers were trying to put the wrong batteries in anyways. And not just in OM-1s.
FWIW, I've owned two different OM-1s over the years - the first for several years when mercury batteries were common, and the second for a few years when they weren't.
With respect to the latter, I used both adapters with a built in diode to drop the voltage from a silver-oxide battery, and brass adapters that worked with zinc-air hearing aid batteries.

 

[ic-racer]

Findings from an OM-1 I repaired and the schematic for those that don't know what we are talking about.

 

[chuckroast]

Findings from an OM-1 I repaired and the schematic for those that don't know what we are talking about.

Interesting. Since - other than the CdS cells, everything is a resistor (which are linear in behavior), the only explanation I can think of would be that the cells themselves are extremely voltage sensitive. Not something I've seen in other meters, but your graph tells the story.

Thanks for sharing this.

 

[jgoody]

Had the MR9 with an OM1N worked fine (I think the CRIS) -- eventually the shutter failed on the camera and was unrepairable -- got an OM2 as it was all I could find, which takes available batteries.

 

[Chan Tran]

Findings from an OM-1 I repaired and the schematic for those that don't know what we are talking about. View attachment 380400

I don't have an OM-1 so I don't know but changing shutter speed, apeture and film speed what moves? The needle of the galvanometer moves or the + and - sign scale moves? If the gavanometer moves then there much be at least 1 variable resistor that moves by those settings. I don't see no such things in the diagram.

 

[mrosenlof]

My two OM-1n are close enough with an MR-9 adapter and silver cell.

 

[wiltw]

Is it really THAT nonlinear. Not being snarky, just genuinely curious what your testing has shown.

Many meter circuits are balanced bridge designs that mostly shouldn't care a lot about a little overvoltage. (Undervoltage can be an issue because it can run the diodes in the bridge into a kind of nonlinear part of their transfer curve.)

And all meters using a CdS cell are somewhat nonlinear across bright to dark because of the cell itself.
But we're talking about an 11% voltage difference. That notionally falls within the "for all practical purposes no big deal" category if you adjust accordingly.

Decades ago, while mercuric oxide batteries were still available, I did a test with my OM-1n using mercury cell and alkaline cell. I discovered that the amount of error in the meter reading caused with the 1.5V alkaline in place of the 1.35V mercury cell was DIFFERENT at different LIGHT LEVELS. ...no single adjustment would work for a given battery Voltage (assuming the alkaline cell Voltage for the day was constant) when the light levels in which you were shooting was varying continuously. I bought an MR-9 in view of that observation.