Modifying an Olympus OM-1 to use 1.55v silver oxide cells

[Jimskelton]

This topic was discussed in an earlier post, though the OM-1 that I worked on was the original model which had resistor values that were quite different from the specs:

So the values I came up that worked would likely not work well with later OM-1s that use resistor values as pictured. I got ahold of an OM-1 that had the following resistor values which are close to specs:

R1: 6.87K (same)
R2: 11.4K (15% higher)
R3: 14.6K (slightly higher)

The resistor values which I found gave accurate meter readings while using the 1.55v silver oxide cell from EV2 to EV17 are as follows:

R1: 3.0K
R2: 6.7K
R3: 15.5K

These values should be closer to what would typically be needed to modify the OM-1 to use 1.55v silver oxide cells. R3 may be a problem to purchase as standard resistors come in either 15K or 16K. It may be feasible to combine 2 smaller resistors which will fit on the board (1/6 watt) in parallel to achieve 15.5K (such as 16K and 510K).

The circuit board is quite simple and standard 1/4 watt resistors are used which are easy to desolder and replace with new ones.

The prior discussion can be found at https://www.photrio.com/forum/threa...-to-operate-with-silver-oxide-battery.196350/ so read it before suggesting that a voltage limiting diode is the solution.

 

[BHuij]

Very cool!

I've been really happy with mine after just tossing in a schottky diode and then recalibrating the cam on the galvanometer, but I suspect your approach gives greater linearity at the extreme ends of the EV range.

 

[ic-racer]

Nice work on the resistor substitutions!


Looks like there are plenty of solutions for the OM-1 . This is mine; add a voltage regulator (top photo).

I actually did not do it because, most of my other cameras use zinc cells. It would be a big inconvenience to have an oddball camera that does not use zinc cells.

For example I replaced the cds cells in my Nikon FTn meter and did what you did with external jumper wires. But I calibrated it for the zinc cells (bottom photo).