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Wednesday, 13 February 2019

Is your 139 locked up? This may help.

The Contax 139 is an electronically controlled mechanical camera. 'Mechanical' in that all the physical operations are driven directly by operating the film advance lever or by springs that are tensioned by the same action. Those springs subsequently drive the mirror up, rotate the shutter main shaft, drive the shutter curtains and return the mirror back to its normal position - in that order. Without any electronic control, all the steps in the sequence will still happen if manually triggered. The electronics just introduce a delay between the release of the first and second curtains by blocking the release of the second curtain with an electro-magnet and so controlling the period the shutter is open for.

Sometimes the sequence might not complete, or maybe fails to start, leaving the camera in a blocked condition where it's not possible to operate the film advance lever and the shutter release does nothing. Here's some tips on identifying where the problem could be and what might be done about it.

The first step is to identify if the sequence has started and, if so, at what point it has stalled. This is done by inspecting the position of the mirror and the shutter curtains. Check if the mirror is up or down. Also check if the shutter is open or closed and, if closed, whether it is still cocked or not. The following two pictures show the shutter in both cocked and released condition looking at the back of the shutter.

Shutter cocked
Shutter released
Possible scenarios:

1. Mirror is down and shutter is still cocked.
The sequence hasn't started. Watch the data back LED located at the bottom left of the shutter (visible in above pictures) and press the shutter release. If the LED flashes, the most likely fault is the release magnet is stuck. The release magnet is a permanent magnet which holds the mirror mechanism in its cocked condition. To release the mirror, an electro-magnet, wound around the permanent magnet and in opposition to it, is energised so cancelling the permanent magnet field and releasing the mechanism. The face of the magnet can become contaminated causing the release lever to stick to it even when the magnetic field is cancelled. The solution is to clean the magnet and release lever faces. There is more information on this on my DIY page. If the data back LED doesn't flash, the release signal is not getting through to the magnet driver circuit. Most likely cause is the transfer switch but there could be other reasons. More investigation is required.

2. Mirror is up and shutter is still cocked.
When the mirror reaches its up position, the mirror mechanism triggers the shutter. See the bottom of the page for a description of the couplings between the mirror mechanism and the shutter. With care, it is possible to manually trigger the shutter.

Triggering shutter
If the shutter triggers, the shutter is not at fault and the problem is with the mirror mechanism. Most likely cause is the lever of the mirror mechanism that contacts the shutter release lever is seized due to corrosion of the snap rings holding the lever onto its pivot.

Corroded snap rings
If the shutter doesn't release even when manually moving the release lever, there is a fault with the shutter. It's possible something is blocking the shutter, such as a piece of broken film, so it's worth trying to blow any debris out of the shutter with a can of compressed air. Other than that, the shutter needs removing for investigation.

3. Mirror is up and shutter is open.
The first shutter curtain opens immediately the shutter is triggered. The second curtain is held by the shutter magnet. If the second curtain fails to close the magnet may be contaminated and the lever that releases the curtain is stuck to it. This is easy to check visually. See the 'Triggering shutter' picture above. The white plastic lever that is held in position by the magnet is clearly visible. In the picture, the lever is in its released position. If the magnet has released the lever but the second curtain doesn't close, there is a fault with the shutter. See previous scenario.

4. Mirror is up and shutter is closed but not cocked.
In this instance, the first thing to check is if the shutter has fully closed. Compare the shutter with the 'Shutter released' picture above. There may only be a millimetre of difference in the position of the shutter blades but that is enough to cause a problem. If the curtain isn't fully closed, it may be pushed upwards by pushing against one of the rivets. This may close the curtain and release the mirror. If this happens, the problem is likely to be the second curtain brake which can become seized, or very tight, and prevent the curtain from fully closing. There is information on my DIY page about this issue. If the shutter is fully closed but the mirror hasn't returned then the problem is likely to be with the mirror mechanism again as in 2 above.

5. Mirror is down and shutter is not cocked.
The sequence has completed normally.

Mirror box/shutter couplings

These pictures of the shutter and mirror box show the two points of contact between them. The green arrows show the coupling used to trigger the shutter and the red arrows show the coupling used to lower the mirror.



Saturday, 9 February 2019

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).