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Poum
Tinkerer
Username: Poum
Post Number: 1
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 12, 2007 - 03:28 pm: | 
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Hello,
My OM2n has a weird behavior. It works fine in manual mode, at all speeds. It works fine in auto mode, for speeds greater than 1/60 (maybe 1/125). For slower speeds (for example, 1/30 or 1/15 indicated in the viewfinder), the shutter opens for a very long time, ten or twelve seconds, as if the cell wasn't receiving any more light. I tried this with a roll, with a white paper instead and without any film in it (the cell reads the amount of light on the film).
If I open the rear door while the shutter is open, that is if the cells receive a great amount of light, then the shutter will close immediately. It looks like the cells provide a very low signal, far lower than it might be (like an old, used CdS cell). But in this case, why does it work at high speeds? Is there a cell for high light and the second one for low light? Or are there two circuits, and the low light electronical circuit is defective?
Thanks for your help,
Guillaume |
Poum
Tinkerer
Username: Poum
Post Number: 3
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 15, 2007 - 02:02 am: |
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I made some others tests yesterday; generally the first shot after leaving the camera for a long time (10 minutes or more) is ok. Then it starts to fail.
I cleaned the electromagnet that frees the shutter, but this made no change (the entire electronics circuit in the mirror box appears very clean).
Sounds like an electronical problem, but despite the fact that I'm an electronics engineer, I'm much more familiar with springs and bolts in our beloved cameras. :o)
The fact that it works after a few minutes, let me think a condenser might be out of service. I found the circuit diagram on the Olympus hardware resource page (http://olympus.dementia.org/Hardware/) but if anyone has an idea, I'd be really grateful.
Thanks
Guillaume. |
Dmax
Tinkerer
Username: Dmax
Post Number: 5
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 20, 2007 - 05:11 pm: |
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Poum,
Firstly, I am not an electronics engineer, and so I may not have the correct vocabulary. Secondly, my prior experience with fixing an OM 2n has been many years ago. It had the same problem as you indicate. Caveat: I ended up destroying the camera and cannibalizing it for parts, since I was going at the task blindly! So all I can offer are general ideas from that misadventure. The OM2 has two independent metering circuits. The CDS governs only the manual setting as well as giving a "preview" of what occurs during an auto exposure. On auto, the two gallium arsenide cells sitting on the floor of the mirror box are what govern the actual exposure in an auto setting. I do seem to recall that at the time that I was working on the camera that I realized that there was indeed a high light value circuit (higher EVs) and a low-light circuit. I recall this because I had a really ***tty time getting the correct crossover point to work. A defective resistor perhaps? You may also want to check the condition of the pot below the prism (it's a linear strip along which an arm slides as you adjust aperture and shutter speed.) I hope this bit of information helps. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 440
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 20, 2007 - 07:32 pm: |
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This sounds like a failing capacitor (condenser), but I'm not an electrical guy either. I do know that the OM2 is a capacitor based system, in which power through the light cell charges a capacitor which becomes the shutter timing device in AUTO mode... so it would make sense that if the capacitor won't take a charge the shutter will not get the signal to close. |
Poum
Tinkerer
Username: Poum
Post Number: 4
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, November 29, 2007 - 01:59 pm: |
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Thank you Dmax and Rick for your answers. Yes it has two electronic circuits, and the one for real time exposure is based on capacitors. I will try to have the two condensers health checked... Hoping they are standard and can be replaced easily.
I made a few tests outside the house last week-end and it worked fine; it seems it only fails in low light conditions. |
Thuggins
Tinkerer
Username: Thuggins
Post Number: 15
Registered: 12-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, January 05, 2008 - 06:29 pm: |
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This may be off base, because I can't remember where I read it. But I seem to recall that the OTF metering only kicked in at lower shutter speeds - right around 1/60 or 1/30 second and less. At faster speeds it uses the reading from the upper (CDS) cell, even in auto mode. |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 480
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, January 06, 2008 - 08:53 am: |
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You read wrong. OTF metering controls all AUTO speeds in the OM2 and other OM automatics. That's the reason for the white checkerboard on the opening curtain, so that OTF metering will be accurate when the first curtain is still covering the film. |
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