Bolex Light Meter Repair

[Don H Marks]

I got the light meter (top of picture) with a camera many years ago. It works well with the common zinc cells. However, it is an older model and it is missing the trim screw and protective clear plastic over the rotating scale. 

I recently picked up a newer one (bottom of picture) listed as 'parts only.'

I was going to just steal its clear plastic cover for the other meter, but maybe it can be fixed. 

Edited March 31 by Don H Marks

[Don H Marks]

If one has never used these meters before, the manual (available on line) shows how to use it. Make sure you are using it correctly. The knob on the top has two settings: always ON and intermittent with pressing only. 

Setting the ASA is not intuitive. You need to pull up and rotate the wheel to set the ASA. 

The scale on the meter is calibrated to already account for light loss through the REX prism. For non-Rex cameras, add one DIN to the film speed, or 1/3 stop on the ASA scale. 

The circuit design calls for a mercury cell. One of the common mercury cell substitues is needed. I use zinc cells but there are other options, including small voltage regulators. 

 

The meter in question here was nearly off the scale when doing the battery test with a zinc cell. Time to open it up and see what is going on. 

 

Edited March 31 by Don H Marks

[Don H Marks]

To get to the insides, the rotating wheel needs to come off. 

First unscrew the trim screw and then a flat blade screw driver can remove the screw that holds the wheel together. 

It is spring loaded and requires 3 hands to get it back together. Record the order that the parts come off. 

Trim screw, clear protector, fixing screw, ASA/DIN HOLE plate, fancy washer, ASA NUMBER disk, fancy washer (same as the other), spring, plastic wheel. 

With the plastic wheel off, the case is held together with two long brass screws. The cold shoe probably does not need to be removed. 

 

[Don H Marks]

I found a single trimmer potentiometer. It was wrapped in black tape and stuffed under the meter face. They provided long enough wires that it can be pulled out and unwrapped without tearing the wires off. 

 

[Don H Marks]

I was able to use the trimmer potentiometer to get it to read very well across its range with a calibrated light source. I used K=14.03. After getting it set to a good spot, the battery check now appropriately landed in the correct red zone. 

[Don H Marks]

Taking it apart more than shown might involve removing the whole galvenometer movement. Something I'd rather not do. 

When re-assembling, the plastic wheel and the ASA disk are the only things that should rotate. The two fancy washers should be locked on the center shaft so they don't rotate. The screw on the shaft should lock the ASA window in place. It should not be hard to get this back to the same place. Since the rotating wheel locks in place at 1/2 stop intervals, the alignment of the ASA window's pointer with the correct line on the outer ring should be easy. If you are off by 1/2 stop it should be obviously crooked. 

Edited March 31 by Don H Marks