Building a new light meter for the Bolex D8L

Building a new light meter for the Bolex D8L

The components of cameras age and lead to a loss of functionality, but some cameras, namely those from Bolex, Leitz or Beaulieu, to name just a few, seem to be built mechanically for eternity and can be made working again with a bit of manual skills.

Some time ago I bought a nice Bolex D8L on ebay, with 3 clean Kern lenses, but the exposure meter was defective. Nothing moved, and the first suspicion concerned the selenium cell, which had apparently died after 60 years.

So I took the cover off the light meter housing to measure it. With the lighting of my desk lamp, 275mV arrived at the measuring instrument, and the selenium cell reacted promptly to changes in exposure. The cell was obviously still in order, the fault was in the camera’s moving-coil instrument, infinity resistance — that was the cause.

Unfortunately, there are no repair documents for the camera and certainly not for the exposure meter, but Dom Jaeger has excellent maintenance instructions for the D8L, about CLA of the mechanics. Of course, it also needed an overhaul, the camera ran quite sluggishly.

So, doing just CLA and using my external Gossen Lunasix (which is almost as big as the D8L) for exposure metering, that would have been the most simple solution. Doesn’t feel like the right approach though for a camera that already has a TTL measurement instead of the usual honeycomb grid and is therefore actually rather modern.

The decision was therefore made quickly: Both CLA and a new light meter were needed. It should also get higher sensitivity than maxing out at 80 ASA, and it should work easier than the cumbersome setting via index numbers.

Exposure meters on old cameras are simple: battery, CdS cell, potentiometer, small battery, moving coil instrument — no transistor or op-amp needed. While researching I found circuit diagrams for the Minolta SRT 101, the Rolleiflex SL35 and the Leica M5. The Minolta is still in my photo bag, it measures reliably, with two CdS cells connected in series to compensate for high contrasts. I like spot metering on a camera better, because then you know what you’re measuring, but it doesn’t matter, the Minolta has been measuring reliably for 45 years. And it should also work with a CdS cell.

Circuit diagram of the Minolta SRT light meter that served as the basis. The on/off switch at the top left can be removed, as can SW1 and SW2 and one of the CdS cells. I also ditched the battery check functionality. First attempts with the CdS cell with a PDV P8103, for R1 a 20k trimpot, for “Fixed Res” also, more detailed explanation in the text

To my great delight, “” in Langen had new, small moving-coil instruments for the Rolleiflex SL35 as an original replacement, each for 6 euros, and after modifying the carrier plate they fitted perfectly. But more on that later.

Let’s go. “The optimist does not believe that everything will go well, but that not everything will go wrong.” Loosely based on Friedrich Schiller.

The camera is dismantled according to the Cinetinker instructions, the gummy parts cleaned in naphta with a soft (tooth)brush, so far so good. In order to get to the selenium cell, however, the camera has to be completely dismantled: For this, the film path, shutter blades, and the gear block have to be removed, which is a bit of a jigsaw puzzle. You eventually get to a thin metal cover plate, behind which the entire mechanism for the release, single-frame switching, release lock and the swiveling of the holder for the measuring cell with all kinds of levers and especially springs is hidden. They tend to easily jump out of place, so be prepared or some searching…

What is solved electronically today can be done fully mechanically here, very impressive. The mechanism is of course resinified, so take out all the individual parts and carefully clean them, it’s a good thing that there are pictures of the Cinetinker for assembly!

The selenium cell is now removed from the holding lever, the surface is cleaned and a thin Pertinax plate is glued on for insulation. The CdS photoresistor is then attached to this, in my case one from the craft box, resistance in darkness around 80k, in light (under my desk lamp) around 1k, only the physical size was decisive though. Type designation? None!

Camera front panel, blade with mounted CdS cell
mounted cell

A PDVP 8103 from Radioshack would probably also be suitable, both in terms of dimensions and electrical properties, but the scatter in the resistance values is so large that you have to adjust it with trim pots anyway. In the past, the components were selected, today it is already difficult to get CdS cells at all (Ordinance on Hazardous Substances…).

The installation of the new measuring mechanism requires processing of the black carrier plate, which of course needs to be unscrewed beforehand. The old measuring instrument needs to be removed afterwards, too. With a Dremel tool and a small sanding disc, a circular indentation is quickly milled out, leaving some material on the bottom – as insulation, so to speak. The new measuring mechanism is fixed with double-sided adhesive tape. Test with the housing cover – fits!

Milled cover plate for the new measuring mechanism
Measuring mechanism put in place

Solder four fine wires, in my case black and yellow for the coil, red and white for the measuring cell. Highly flexible and thin wire is a must, so that the measuring lever in particular remains easy to move later and the thin cover plate for the trigger mechanism can be placed completely flat again.

cable routing

Then a recess has to be milled out of the carrier plate with the Dremel so that the wires can be routed to the viewfinder window. Seal with black silicone or Plastidip!

Assemble the release mechanism so that all the levers and springs are in the right place, then take a deep breath and put the cover plate on. It must lie completely flat, otherwise the individual parts of the mechanism will slip out of their position and jam or block. Usually you need a few attempts until everything is in place and fits, you can then secure the cover with adhesive tape, but this has to be removed completely and with all the adhesive residue before assembly, because otherwise the focal flange will no longer be correct.

I recommend reassembling the film path, the shutter blades and its controls, this fixes the cover plate and reduces the risk of everything falling apart again when the adhesive tape is removed. When everything is screwed back together, check the functions, swivel the blade in and see whether it swings back completely when you press the trigger. Are trigger and trigger lock intact, does everything run smoothly? If so, congratulations, you’ve cleared the first hurdle.

Break! Gladly with a glass of red wine and classical music to reward yourself for the first success and to cure the aching knees from spending an hour searching for a tiny sping on the floor. We’ll continue in the next few days.

Before further assembly, a notch must be filed for the cable passage into the camera interior at the viewfinder window between the shaft for the viewfinder mechanism (neat, a lens is moved back and forth with a cable like the variator on a zoom lens!) and the housing. Carefully remove all metal dust and shavings, otherwise the shutter blade or so will definitely get stuck afterwards…!

When fitting the assembly, seal the gap between the film path and the camera housing with black silicone or Plastidip, as well as the cable entry into the housing interior, otherwise there is a risk of light coming in, although you can’t really see the fine gap. You can save yourself from that disappointment.

If you have done everything right up to this point, you can screw on the front panel. To do this, the pin for adjusting the shutter blade has to be juggled into the groove of the brass gear wheel, which is a bit fiddly. When everything is assembled, do a first functional test. The camera has to run quietly, the frame rate has to be adjustable, the variable shutter has to be adjustable, and the measuring cell has to be able to be swiveled in and retracted quickly and completely when the shutter release is pressed.

cable routing
Cables in heat shrink tubing so that the governor runs unhindered

The four wires are now on the underside of the viewfinder cover (the sheet metal with the slope) inside, they are bundled with a piece of shrink tubing and routed to the rear. This is important so that the governor does not get obstructed by a wire. It makes sense to glue the shrink tubing in place. The trimpots find their place on the incline of the viewfinder housing, again insulated with a small Pertinax plate. There’s just enough space. According to the circuit, both with 20k, but this can vary considerably depending on the resistance values of the CdS element. You have to try it, pure empiricism, see below. The adjustment is the second hurdle.

A PDVP 8104 has a higher dark resistance, see its data sheet.

Mounting the trimpots

I adjusted the red reference needle of the light meter with the wheel so that the two black triangles are exactly on top of each other. It is then located at the lowest point at the bottom edge, for the zero adjustment of the original measuring mechanism. The setting path, starting from an open aperture, is as long as possible and therefore more precise. In principle, other film sensitivities can also be set by turning, if you get a suitable film, for example the Cine 40 BW, see below. I want to shoot Foma R100, so I made my settings accordingly.

With a fresh 1.55 V silver oxide cell (SR44), the comparison is now carried out using a hand-held exposure meter or a correctly measuring photo camera as a reference and, for example, white, gray and black panels under constant lighting. Iteratively, first the light reference with pot 1 behind the CdS element, then the dark reference with pot 2 parallel to the measuring instrument. That changes the light balance again, so next round. After a few (several) runs it is correct, the spring tension on the measuring mechanism and thus the deflection of the pointer must be adjusted if necessary. can also be adjusted (the red lever on the instrument, zero adjustment). This changes the zero position of the instrument, but also the response of the pointer and its deflection. If you have a reference light source for a professional comparison, you can count yourself lucky, but it also works as described.

Ultimately, it is an adjustment with several variables, which leaves exposure times greater than 1/40 sec out. The procedure may take a long time, but the proverbial patience of an angel helps, as with any other hobby. If you’re very unlucky and the adjustment doesn’t work, trimmers with different resistance values have to be soldered in, and then it starts all over again.

It’s annoying, but cheer up, Rome wasn’t built in a day either…!

When the measurements are finally correct, install the cover plate and the axle for the meter counter, and check the function again. Reinstall the claw with the spring, then the cover plate. Attention, the spring-loaded roller tends to fall out. The axle for the take-up spool must be easy to move, otherwise the tension on the film will be too bg and registration will be horrible. Gets easily forgotten due to all the joy about the successful restoration!

The battery holder for the SR 44 silver oxide cell is then placed on the cover for the footage counter. Is also glued on, this just abojt fits, so that the film transport is not hindered. Shorten and solder the cables: Done!

Connected battery holder

Insert Fomapan R100, shoot test film. Should work, and will work! Final Hallelujah!

The D8L now has a TTL tracking exposure meter that covers 100 ASA film regardless of the focal length (!) without the hassle of transferring index values. In my opinion, a higher film sensitivity makes no sense, but would be possible with the appropriate adjustment. You probably already need a neutral density filter when the sun is shining. There is also no faster material, at most you could push the Foma. The Cine 40 BW could be measured, for this the red reference needle in the light meter would have to be adjusted (try it out or calibrate with a hand lens and 40 ASA).

You can’t see anything of the modernization from the outside, but what counts is what’s on the inside. A switch for the light meter could be retrofitted on the top of the camera via a milled rectangle, but it remains uncertain whether this will be light-tight. I didn’t do it, the current with the retracted measuring cell could not be measured with my ammeter in the 200 microampere position, so the LR 44 should last a very long time.

And if that’s not enough for you, you can disassemble, file, mill, reshape the claw like a fine-mechanical master forge and even upgrade the camera for DS8. Friedemann Wachsmuth demonstrated it.

Components: PVDP 8103, 8104 photoresistor, SR 44 (Renata) battery holder, trimmer potentiometer, preferably an assortment from Amazon, Bojack potentiometer KIT, measuring mechanism from Rolleiflex from “”, sold in a set of 5, shrink tubing, thin colored wire, Total cost about 15$ or Euro. I still have four gauges…

Good luck and good light!

Thomas Fährenkemper

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