A new 139 fault

There's not many faults on the 139 I haven't previously seen but I did find a new one recently. The camera had very inconsistent shutter speeds with variations of up to half a stop in either direction. The shutter speeds are often out of specification, maybe 50% of the time, but I've never before seen a 139 with such variations.

The problem was caused by a contaminated shutter magnet so the solution was quite simple - clean the magnet. It's something I do anyway if I service a 139.

The shutter magnet is not to be confused with the release magnet, which can also become contaminated but the symptoms are different. Here's a picture of the shutter magnet. The white plastic arm has a metal plate at the top which contacts the magnet face. This plate and the magnet face are the parts that need to be cleaned.

The more usual symptom of a contaminated shutter magnet is the shutter staying open due to the plastic arm staying permanently stuck to the magnet.

An unusual fault

Thought I'd document a fault I saw recently as it's an unusual one.

The symptoms are that, when a manual shutter speed is selected and the camera not cocked, when the shutter release button is pressed, the LED in the viewfinder that indicates the selected shutter speed will come on immediately but the LED showing the shutter speed required will not come on until after a period equal to the selected shutter speed. And if B is selected, it will never come on.

The cause was a faulty transfer switch - the first type which is a sealed micro switch. Normally, when these fail the camera won't release the shutter as the transfer switch goes open circuit and the processor never gets the signal to say the shutter is cocked. In this case, the switch went short circuit so the processor thought the shutter was cocked even though it wasn't. Consequently, the processor would activate the shutter magnet for the selected shutter speed and, for reasons I don't understand but it's probably just part of the programming, the meter is not activated and the LED showing the suggested shutter speed isn't lit.

This is only unusual in the way the transfer switch failed. I've never seen one fail this way before. And, because the camera otherwise worked normally, there was little to indicate what the issue was.

Does your 139 over expose?

If your 139 has never been serviced or had its meter adjusted, chances are it will over expose.

Almost every 139 I've seen has had a degree of over exposure. Usually between a half and one stop but sometimes more. Recently I had a 139 that overexposed by a couple of stops and decided to investigate further. This lead to me discovering that the meter sensor had become cloudy. Checking the sensors in some other cameras I found nearly all of them were cloudy to some extent and this seems to be the reason why 139s generally over expose. Looking closely at the sensors, there is a glass filter fitted over the sensor and the cloudiness is under the glass. It's impossible to remove. The glass filter is very thin and is stuck down and any attempt to remove it results in its destruction. Under the glass is a clear compound - could be silicone or something similar. It seems it's this which is going cloudy.

Here's just three I picked at random from my collection of donor cameras. You'll see two have cracked filters also although I don't think that's a problem.

To compare, here's a clear one.

In a few cameras where the cloudiness has been quite bad, I've replaced the sensors but I only had ones removed from donor cameras and they were also cloudy to some extent. What I needed was a new replacement and, amazingly, I seem to have found one.

The sensor I found is mechanically identical and has the same glass filter on it. The specification says it's sensitive to the visible spectrum with the filter removing an extended sensitivity into the IR region. As I had no specification for the original, the only way to find out if the new sensor was a good replacement was to try it. I removed a sensor from a working camera, having first done some exposure checks, then replaced it with the new one. Repeating the exposure checks gave almost identical results but probably half a stop less exposure which would be about correct as the original sensor I removed was cloudy.

For anyone who wants to replace their own sensor, the replacement I found is part number VTB8440BH from Excelitas Technologies. I bought mine from RS-Online (UK).

Edit: Couldn't resist adding this picture of a sensor removed from a 159. The 159 clearly also suffers the same problem. This one appears to have flowers growing in it and it was over exposing by one stop.

And in case anyone is wondering if the flash meter sensor also has the same problem, it seems it doesn't. The flash meter sensor appears to be the same, or a similar, sensor but there is no filter on it and the ones I've checked are perfectly clear.

UPDATE: It seems the new sensor is now out of production. DigiKey still has stock but it won't last. Buy while you can. 

https://www.digikey.co.uk/en/products/detail/excelitas-technologies/VTB8440BH/5885859

Some Zeiss/Contax lenses under expose by half a stop!

This is a continuation of a previous post on how the Contax lenses communicate to the camera body in which I mentioned I was doing some investigation into some questions I had and also some forum posts that suggested some lenses had compatibility issues with some Contax bodies.

To deal with the second issue first - I've not been able to duplicate any of the problems I've seen mentioned that might suggest some incompatibility between some lenses and bodies. I suspect these problems were due to individual lenses or bodies that had one or more faults.

I have, however, determined that the information passed from the lens to the body differs between lenses and that means, in some cases, certain lenses will cause a half stop under exposure.

The questions I had were raised by references in a few different Contax body service manuals to a feature called 'theta compensation'. This is something that appeared to relate only to some lenses. I initially tried to discover what theta compensation was but have been unable to find anything. Even an email to Zeiss was unproductive with Zeiss saying all the information was passed to Yashica. So I took a different approach and started to look at individual lenses that appeared to have theta compensation to see if I could find a difference between them and other lenses.

I was able to identify two lenses in my collection that have theta compensation. I was able to do this because those lenses have a different tab that couples to the AE/MM switch on the body. The AE/MM switch on the body is usually referred to as the 'theta switch' in the service manuals and it has three positions. The first, normal, position indicates an AE lens is fitted, the third position indicates an MM lens is fitted, but there is a second, middle, position that indicates an MM lens with theta compensation is fitted. So lenses that have theta compensation have a narrow tab that only moves the switch to the middle position.

Here's a couple of pictures of two of my lenses that show the difference. I've outlined the tag on the lens in red and the switch lever in green so their shapes and positions are more clear. The first is a 85mm f/2.8 MM lens. Note the tag is pushing the switch all the way to the end of its travel.

The second is a 50mm f/1.7 MM lens. Note the tag on the lens is notched and it only pushes the switch half way.

Besides the 50mm f/1.7 MM, I also found the 18mm f/4 MM had the same shaped tag so that is the second lens I have that has the feature.

To try and ascertain if there were any differences between lenses with and without theta compensation, I set up a test rig using the mirror box from a 139. The mirror box has the lens mount attached and the various couplings to the lens and has a potentiometer (variable resistor) directly coupled to the aperture stop down ring of the lens mount. By measuring the resistance of the potentiometer with various lenses attached and at all apertures, I was able to plot graphs of resistance against the number of stops the lens was stopped down. Here's a picture of the rig and the results.

The X axis is the number of stops the lens is stopped down. The Y axis is the measured resistance.

What is clear from this is there are two distinct groups of results with one group shifted by half a stop from the other. Two of the lenses in the group shifted to the right are the 50mm f/1.7 MM and the 18mm f/4 MM which had previously been identified as having theta compensation. Surprisingly, the third lens in this group is a 50mm f/1.7 AE lens. This is surprising as the AE lenses were designed and manufactured before MM cameras were brought out and before there was any method of detecting that such a lens had been fitted to the body. As a check that this wasn't just a fault of the individual lens, I tested a second 50mm f/1.7 AE lens and got identical results.

The second test I did was to check the position of the maximum aperture lever for each lens. In the 139 the lever is coupled mechanically to the aperture display so there's nothing that could be measured other than physical position but it was clear that the lenses that exhibited the half stop shift in the aperture stop down lever, also had a half stop shift in the position of the maximum aperture lever.

Here's the inside view of the maximum aperture lever (arrowed). You can see the black marks on the chassis where I've recorded the position for different lenses.

My first thought was that these two half stop shifts would cancel each other out so there would be no effect on the metering but it's not that simple. In the 139, for instance, the maximum aperture lever plays no part in the metering, it is only used to display the correct aperture in the viewfinder. The metering is done purely from the potentiometer coupled to the aperture stop down ring. So, not unsurprisingly, lenses that have theta compensation under expose by half a stop when used on a 139. This is easily confirmed by metering a scene with a lens with theta compensation then again with a lens that doesn't. The half stop difference in the meter reading is quite obvious.

With other MM bodies I thought the exposure error might not happen but I tested an RX, RXII and a 167 and it does happen and in all metering modes.

Conclusions

In the 167MT service manual there are some tables that show the aperture associated with each position of the aperture stop down lever and the maximum aperture lever. The tables have two different values, with a half stop difference, for lenses with and without theta compensation. This effectively confirms my findings that the physical position of these levers differ on lenses with theta compensation. Or, put another way, any particular position represents a different value on lenses with theta compensation to ones that don't.

You would think, therefore, that the MM bodies would carry out a different exposure calculation depending on the position of the AE/MM switch. I have tried manually changing the position of the switch but it makes no difference to the meter reading. Either my expectations are inaccurate, or the bodies don't have the theta compensation feature fully implemented.

Clearly I'm not able to test every combination of lens and body in every metering mode - I don't have examples or the time - and it may be that the half stop change in metering only affects a few lenses, but the phenomenon definitely exists. Whether it's a problem is for individuals to decide. Does half a stop error in metering matter? In the days when I used to use transparency film, I would have said it does.

If anyone has anything to add to this I would be very interested to hear from them. If you have seen the issue with particular lenses then which ones and with which bodies and in what metering mode? Also, if you find any lenses that display the wrong aperture in the viewfinder when other lenses are OK, I would be interested in the same information. And, finally, if anyone has any idea what theta compensation is (specifically related to the lenses under discussion), please let me know.

(Edit)

There was one other test I did which I failed to mention above, probably because it produced no useful information at the time. However, on reflection, the fact it showed no difference between lenses with theta compensation and those without is relevant. The test was to attach the lenses to my A7, digital camera, and check for differences in meter readings. The A7 has no connections to the lens and simply measures the light passing through it. The result was there is no difference in the light transmitted between lenses with, or without, theta compensation. This is significant because, if the lenses with theta compensation did transmit a half stop more light, the apparent half stop under exposure would be correct and make all the above rather inane.

(Edit 2)

I recently discovered the 50/1.7 MM lens, when attached to an RTS body, will cause the aperture readout in the viewfinder to be half a stop out. So, for example, with the lens set to f/5.6 the aperture display will show the aperture as between f/4 and f/5.6.