This is a short post about the Leica M6. I'm not going to talk about all of the special editions or how the rangefinder flares, although there's another post about that (soon), or how using an M6 can make you into a better photographer, no, I'm interested only in the technical aspects.
I can officially announce that no further replacement metering circuits will be available for the M6. I think we had our last one a few months ago that is it.
The Leica M6 is really just like a Leica M4 really, which is just like a Leica M2 but the Leica M2 doesn't have the attractive yet somewhat fragile yet jaunty rewind knob. I did own a Leica M4-P once, which was one of the later production batches. The windows were flush and it had a single flash PC port on the rear, just like an M6. This was a transition model and it was just waiting for a magical ingredient - a light meter.
Other than the CL, the M5 was Leica's only other foray into built-in metering. Unfortunately, the additional gubbins required to make it all work required a larger body and that resulted in what some would say was an ungainly appearance. This is because to make the analogue metering work, a galvanometer had to be fitted to the side, with the meter needle on top, along with a bridge circuit board (the electronics were rudimentary)and there were additional mechanical components and even an illuminator mirror that had to be added to the top plate with a small window. The main prism also had to be made horribly more complex in order to display the meter needle in the viewfinder, then, you have complex linkages to the speed dial and a way of adjusting the ASA. Also, why, oh why, oh why did they go for mercury cells? Even the CL - mercury battery. Minolta version with very sophisticated metering - silver oxide!
Make no mistake though. Remove those additional components and you're left with something that looks just like an M4 in an oversized body. What people really wanted was a Leica with metering that looked just like an M4. Although Leica made thousands of cameras each year, they just didn't have the resources of the Japanese, who really set out to standardise and mass produce quality cameras. Economies of scale and access to the burgeoning electronic sector allowed the Japanese camera manufacturers to innovate into micro electronics. A case in point, a year before the M6 was released, Nikon released the FA. It had a matrix metering system controlled by a CPU, top speed of 1/4000th sec with flash sync at 1/250th sec. Four years later, the F4 had I believe four CPU's several servo motors, an advanced metering and AF system that even knew which way you were holding the camera, along with a counterbalanced square shutter with dual curtains, incapable of light leaks.
To be fair, Leica had invented autofocus in the 1970's but didn't think anyone would want to use it, so sold the patent! Just like I regret selling my old Leica with the serial number 1980 but I digress!
Leica got to work, with somewhat limited resources. I have seen some Leica research articles regarding metering and although I could find no circuit diagrams, their block diagrams described an advanced circuit that later would be used in the R6. In order to accommodate the quantities that Leica would need, the whole metering circuit was constructed on a ceramic hybrid circuit board that was soldered directly to the flex. Later, fully encapsulated modules would be made. The problem with the ceramic hybrid main boards is that the surface mount components are not soldered but glued using silver loaded epoxy. In addition, on close inspection, there seems to be a screen printed overlay of black ink. This I think may be screen printed resistors. This is problematic because over time, the material can start to drop off. Replacing the 'off delay' components, which can cause problems can be difficult with these hybrid boards.
Generally, light meters should be reliable long term. The voltages and currents employed are exceedingly low, meaning that no heat is generated and components are operated well below their rated maximum. Silicone photodiodes are also more reliable than the older CdS cells and much more so than the photovoltaic cells as used on the old Leicameters. The electronics of an M6 are primarily transistors, op-amps, diodes, resistors and capacitors, the latter being the only one to degrade in any great way. Then you have ageing of soldered joints and conductive glued joints, battery corrosion, moisture corrosion and general oxidation. Using modern components and materials it should be possible to improve upon yesterday's technology.
When we couldn't get hold of rangefinder prism components, we decided to buy into high vacuum technology so that we could fix them ourselves. This will eventually lead us to remanufacturing entire M3 prisms. When we couldn't get brightline masks, we reverse engineered them and made our own. You can't complain really. The first M6 was 1984 and circuit boards have only just become unavailable. That's nearly 40 years but these cameras have at least another 40 years life in them. For that reason I fully intend to redesign the metering and produce replacement circuit boards. I will make an announcement when the time comes. For the time being we will continue in our efforts to repair them. To book an M6 or any other Leica rangefinder repair, even the M4-P, visit our price list page.
